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Defines the difference between scholarly, reputable, and non-reputable resources.

Before completing your post, review the Week 1: Biomes & Diversity (Links to an external site.)

interactive video, which is designed to assist you in understanding more about this week’s topic and to help you organize your initial post.  Also, watch the video “Choosing Articles and Revising Searches (Links to an external site.),”

which defines the difference between scholarly, reputable, and non-reputable resources.

As you have learned in the readings, extinction is a natural selection process and humans are often responsible for accelerating this process. In your initial post this week, address whether or not we as humans should be concerned with the extinction rate, and justify your decision. Additionally, discuss whether or not humans should strive to preserve representative samples of all biomes on the planet, and if so, how that might be accomplished.

Your initial post should be at least 300 words in length. Utilize at least two scholarly or reputable resources and your textbook to support your claims. Cite your sources in APA format. Quoted text should constitute no more than ten percent of your post. 


Learning Objectives

After studying this chapter, you should be able to:

• Identify the different terrestrial and aquatic biome types that cover the planet and explain why they might differ in terms of biodiversity and species richness.

• Describe how energy produced through photosynthesis forms the basis for most life on the planet and how this energy flows through different trophic levels in an ecosystem.

• Explain how nutrients, such as nitrogen and phosphorous, cycle within ecosystems and how human activities are altering the flow and location of these nutrients, often with unintended consequences.

• Understand the difference in life history strategy between different organisms, including those between r-selected and K-selected species.

• Explain the concepts of niche, limiting factor, keystone species, and trophic cascades and how these relate to the functioning of ecosystems and the species within them.

• Discuss how interactions between different species in an ecosystem (such as predators and their prey) result in evolutionary changes in these organisms and how ecosystem change and succession over time alters the balance of species present in a given location.

• Describe how toxic substances like mercury can find their way into natural environments far from any source and impact wildlife populations in that area.

Ecosystems 1

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1. Which biome would be expected to have the warmest and wettest conditions? a. Coniferous forest b. Desert c. Tropical forest d. Temperate grassland 2. The major sources of human emissions of the pollutant mercury are a. disposal of thermometers and hospital waste. b. car and truck exhaust. c coal burning and gold mining. d. agriculture and cattle ranching. 3. Which of the following is NOT an example of an important biogeochemical cycle? a. The water cycle b. The phosphorous cycle c. The solar cycle d. The carbon cycle 4. The population biology concept that refers to the maximum number of organisms that a

given environment can support is a. survival rate. b. reproductive rate. c. K-selection. d. carrying capacity. 5. When a top predator is removed from an ecosystem it can have dramatic impacts on the

entire food web. These impacts are referred to as a. biomagnification. b. bioaccumulation. c. trophic cascades. d. photosynthesis. 6. Which of the following is NOT an example of an avoidance/escape feature used to deter

predators from attacking prey? a. A panda feeding only on bamboo b. Fish swimming in a school c. Wildebeests moving in a herd d. A moth with false eye spots on its hind wings 7. Because mercury tends to accumulate in an animal’s tissue, we would expect what kinds

of organisms to carry the highest amounts of this toxin? a. Long-lived predators b. Primary producers c. short-lived predators d. Detritivores

Answers 1. c. tropical forest. The answer can be found in section 1.1. 2. c. coal burning and gold mining. The answer can be found in section 1.2. 3. c. the solar cycle. The answer can be found in section 1.3. 4. d. carrying capacity. The answer can be found in section 1.4. 5. c. trophic cascades. The answer can be found in section 1.5. 6. a. a panda bear feeding only on bamboo. The answer can be found in section 1.6. 7. a. long-lived predators. The answer can be found in section 1.7.

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Introduction Anyone who has spent time outdoors in a favorite patch of forest or other natural setting might gain an appreciation for the complexity of life present in these ecosystems. Though silent and invisible to us, trees and other plants are busy converting sunlight into stored energy through photosynthesis. Birds, insects, and other creatures are on the move searching for food or themselves ending up as food for other organisms. some of these ecosystems seem little changed over time while others might undergo rapid and dramatic transformation over the course of only a few years. For example, mature forests might change little from year to year, while shallow lakes gradually fill with sediment and slowly morph into swamps. such changes to ecosystems move ecologists to investigate the mechanisms that maintain stability in some systems like the forest, yet promote change in others like the lake. Indeed, ecology is the study of all natural systems, including the forest and the lake.

Ecologists study the relationships between living organisms and the physical environment. For example, in an ecosystem, plants compete with one another for sunlight, and some ani- mals eat plants, while others eat plant eaters. Ecologists studying such an ecosystem might ask questions, such as: What mineral qualities of the soil nourish this particular community of plants? And, how does competition and predation among all the billions of soil microor- ganisms affect the nutrient qualities of the soil? such queries help guide researchers as they examine the interactions that occur within a particular ecosystem. Furthermore, the knowl- edge gained from such research helps environmental scientists study the impacts of human actions on the environment, such as the damage done to a salt marsh by an oil spill or the impact of air pollution on trees and plants.

This chapter will explore ecology as the study of change and stasis, balance and imbalance, life and death in all natural systems—rainforests, tundra, grasslands, deserts, rivers, and oceans that constitute our world. It begins with a review of the concept of biomes, major ecological communities like forests, deserts, tundra, and oceans. While biomes differ dramatically in terms of climate and the variety of life present, they all are generally powered by solar energy. The second section examines how energy enters and flows through different trophic levels in an ecosystem. The third section considers how nutrients, such as nitrogen and phosphorous, are cycled within ecosystems. This is followed by an overview of population biology, the study of how different organisms grow and reproduce in different ways. The fifth section intro- duces the concepts of niche, limiting factor, keystone species, and trophic cascades, and how they impact the functioning of various ecosystems. section 1.6 reviews evolution and natural selection and how these processes alter the species composition of ecosystems over time. All of these topics will provide you with a basic foundation in ecology and environmental science needed to understand the subjects presented in later chapters. To illustrate how the topics covered in this chapter connect, the final section presents a case history of how mercury con- tamination is affecting wildlife and ecosystems the world over.

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seCTION 1.1 earTh’s BIOmes

1.1 Earth’s Biomes The diversity of life on Earth is vast. Yet ecologists have found that areas on different continents that share similar climate conditions tend to have similar ecosystem structures and functions. As a result, ecologists use the concept of a biome to classify large areas of the planet into a small number of similar units. Biomes include both terrestrial (land-based) and aquatic (in water) communities. Biomes display huge differences in the number or diversity of species present and how these species interact with one another. The following section, which has been excerpted from habitable Planet: a systems approach to environmental science by Annenberg Learner, discusses the different types of biomes and how they are classified. It will help you gain an appre- ciation for the incredible variety of ecosystems and natural conditions on the planet, and how conditions shape the diversity of life found in each area.

The reading points out that scientists have determined that a handful of factors—namely tem- perature, availability of moisture, abundance of light, and availability of nutrients—are the key influences on the number and variety of organisms in a given ecosystem. Generally speaking, tropical regions with their warm temperatures, abundance of moisture, and relatively constant levels of daylight have the highest number and diversity of organisms. Indeed, tropical, moist for- est ecosystems make up the terrestrial biome with the highest productivity and diversity of life. In contrast, polar regions with their frigid temperatures, low moisture conditions, and months of the year with little or no natural light tend to have the lowest levels of productivity and diversity. Scientists study all types of biomes in order to learn about the life cycle and optimal conditions within different types of climates.

By Annenberg Learner Geography has a profound impact on ecosystems because global circulation patterns and cli- mate zones set basic physical conditions for the organisms that inhabit a given area. The most important factors are temperature ranges, moisture availability, light, and nutrient availabil- ity, which together determine what types of life are most likely to flourish in specific regions and what environmental challenges they will face.

earth is divided into distinct climate zones that are created by global circulation patterns. The tropics are the warmest, wettest regions of the globe, while subtropical high-pressure zones create dry zones at about 308 latitude north and south. Temperatures and precipitation are lowest at the poles. These conditions create biomes—broad geographic zones whose plants and animals are adapted to different climate patterns. since temperature and precipitation vary by latitude, earth’s major terrestrial biomes are broad zones that stretch around the globe. Each biome contains many ecosystems (smaller communities) made up of organisms adapted for life in their specific settings.

Land biomes are typically named for their characteristic types of vegetation, which in turn influence what kinds of animals will live there. soil characteristics also vary from one biome to another, depending on local climate and geology. [. . .]

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Figure 1.1: Global biomes

earth’s major biomes result primarily from differences in climate. each biome contains many ecosystems made up of species adapted for life in their specific biome.

Adapted from U.S. Department of Agriculture Natural Resources Conservation Service. Retrieved from /wps/portal/nrcs/detail/soils/use/worldsoils/?cid=nrcs142p2_054013

Aquatic biomes (marine and freshwater) cover three-quarters of the Earth’s surface and include rivers, lakes, coral reefs, estuaries, and open ocean. Oceans account for almost all of this area. Large bodies of water (oceans and lakes) are stratified into layers: surface waters are warmest and contain most of the available light, but depend on mixing to bring up nutri- ents from deeper levels. The distribution of temperature, light, and nutrients set broad condi- tions for life in aquatic biomes in much the same way that climate and soils do for land biomes.

marine and freshwater biomes change daily or seasonally. For example, in the intertidal zone where the oceans and land meet, areas are submerged and exposed as the tide moves in and out. During the winter months lakes and ponds can freeze over, and wetlands that are covered with water in late winter and spring can dry out during the summer months.

There are important differences between marine and freshwater biomes. The oceans occupy large continuous areas, while freshwater habitats vary in size from small ponds to lakes cov- ering thousands of square kilometers. As a result, organisms that live in isolated and tem- porary freshwater environments must be adapted to a wide range of conditions and able to disperse between habitats when their conditions change or disappear.


Tropic of Capricorn

Tropic of Cancer

30° S

30° N

Tropical forest Temperate deciduous forest

Savanna Temperate grassland

Desert Coniferous forest

Chaparral Tundra (arctic and alpine)

Oceans Polar and high- mountain ice

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Biomes and Biodiversity since biomes represent consistent sets of condi- tions for life, they will support similar kinds of organisms wherever they exist, although the spe- cies in the communities in different places may not be taxonomically [the science of classifying animals] related. For example, large areas of Africa, Austra- lia, south america, and India are covered by savan- nas (grasslands with scattered trees). The various grasses, shrubs, and trees that grow on savannas all are generally adapted to hot climates with distinct rainy and dry seasons and periodic fires, although they may also have characteristics that make them well-suited to specific conditions in the areas where they appear.

species are not uniformly spread among earth’s biomes. Tropical areas generally have more plant and animal biodiversity [the diversity of animal and plant life in a region] than high latitudes, measured in species richness (the total number of species present). This pattern, known as the latitudinal bio- diversity gradient, exists in marine, freshwater, and terrestrial ecosystems in both hemispheres. [. . .]

Why is biodiversity distributed in this way? Ecolo- gists have proposed a number of explanations:

• higher productivity in the tropics allows for more species;

• The tropics were not severely affected by glaciation and thus have had more time for species to develop and adapt;

• Environments are more stable and predictable in the tropics, with fairly constant temperatures and rainfall levels year-round;

• more predators and pathogens limit competition in the tropics, which allows more species to coexist; and

• Disturbances occur in the tropics at frequencies that promote high successional diversity.

Consider This Recall that as part of the scientific method scientists regularly formulate and test hypotheses about how the world works. Now, note the language used in the pre- vious paragraph about how “evidence is strongest . . . ” for one proposition over the others. What does this tell you about the scientific method and the kind of lan- guage and terminology used by scientists to describe the natural world?

. luoman/iStock/Thinkstock

Tropical rainforests produce their own moisture. Scientists believe that as these ecosystems are cleared through deforestation—as shown here in the Amazon—there is a threshold beyond which they will no longer produce enough moisture to sustain themselves. The result could be conversion of rainforests to drier savannas.

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Of these hypotheses, evidence is strongest for the proposition that a stable, predict- able environment over time tends to pro- duce larger numbers of species. For exam- ple, both tropical ecosystems on land and deep sea marine ecosystems—which are subject to much less physical fluctuation than other marine ecosystems, such as estuaries—have high species diversity. Predators that seek out specific target species may also play a role in maintain- ing species richness in the tropics.

Excerpted from Weeks, S., Moorcoft, P.R. (2007). Unit 4: Ecosystems. The habitable Planet: a systems approach to environmental science. Retrieved from =0. Used with permission of Annenberg Learner.

1.2 Energy Flows Through Ecosystems Despite the incredible range of conditions that characterize the ecosystems found in differ- ent biomes, they all have something in common. With few exceptions, Earth’s ecosystems are powered by solar energy. Primary producers such as plants and algae use sunlight in a process known as photosynthesis to convert carbon dioxide and water into glucose (sugars). Glucose represents a form of stored energy that is used by plants for their own growth and maintenance. Other organisms can then consume this plant material and use it as a source of energy. Animals, in turn, can eat the organisms that ate the plants in order to acquire energy. Ecologists use the concept of trophic levels to study how energy moves through ecosystems. The following selection adapted from habitable Planet: a systems approach to environmental science, by Annenberg Learner, explains how energy flows through ecosystems and discusses the impact on the environment. It will introduce you to the critical concept of primary productivity, the basis for almost all life on the planet.

Trophic levels can be best visualized as a series of steps, with the base made up of large amounts of primary producers such as plants and algae. These primary producers have the unique abil- ity to transform solar energy from the sun into stored energy in the form of sugars through the process of photosynthesis. Animals that feed on primary producers are known as primary con- sumers. An example of a primary consumer is a rabbit that eats grass and then utilizes much of the energy stored in the grass for its own growth and bodily functions. In order to sustain the rabbit there must be a huge amount of available grass for it to eat. This is why the trophic level comprised of primary producers is the largest. However, the animals that eat rabbits and other primary consumers are fewer in number than rabbits, so their step is smaller than the one below it that represents plants and algae.

Biomes and Biodiversity since biomes represent consistent sets of condi- tions for life, they will support similar kinds of organisms wherever they exist, although the spe- cies in the communities in different places may not be taxonomically [the science of classifying animals] related. For example, large areas of Africa, Austra- lia, south america, and India are covered by savan- nas (grasslands with scattered trees). The various grasses, shrubs, and trees that grow on savannas all are generally adapted to hot climates with distinct rainy and dry seasons and periodic fires, although they may also have characteristics that make them well-suited to specific conditions in the areas where they appear.

species are not uniformly spread among earth’s biomes. Tropical areas generally have more plant and animal biodiversity [the diversity of animal and plant life in a region] than high latitudes, measured in species richness (the total number of species present). This pattern, known as the latitudinal bio- diversity gradient, exists in marine, freshwater, and terrestrial ecosystems in both hemispheres. [. . .]

Why is biodiversity distributed in this way? Ecolo- gists have proposed a number of explanations:

• higher productivity in the tropics allows for more species;

• The tropics were not severely affected by glaciation and thus have had more time for species to develop and adapt;

• Environments are more stable and predictable in the tropics, with fairly constant temperatures and rainfall levels year-round;

• more predators and pathogens limit competition in the tropics, which allows more species to coexist; and

• Disturbances occur in the tropics at frequencies that promote high successional diversity.

Consider This Recall that as part of the scientific method scientists regularly formulate and test hypotheses about how the world works. Now, note the language used in the pre- vious paragraph about how “evidence is strongest . . . ” for one proposition over the others. What does this tell you about the scientific method and the kind of lan- guage and terminology used by scientists to describe the natural world?

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Ecologists study dynamics between and among trophic levels as well as the concept of primary productivity to figure out how much energy is available to support the organisms within a par- ticular ecosystem. For example, net primary productivity is the amount of energy available as plant matter for primary consumers, or the amount left over after plants use some of the energy from photosynthesis for themselves. The last section made clear that tropical, moist forests are the most productive of terrestrial biomes. That’s the same as saying that tropical forests have the highest net primary productivity (NPP). Since it is the NPP of an ecosystem that supports all life at higher trophic levels, the high NPP in tropical forests helps explain the abundance and diversity of life in these ecosystems.

One note of clarification regarding the following discussion on bioaccumulation. Bioaccumula- tion describes an increase, or an accumulation, of a specific pollutant or toxin in an organism over time. Many toxic substances, such as mercury, are what are known as lipophilic, or having the tendency to dissolve in fat. Human emissions of mercury from activities like coal burning and gold mining tend to end up in water bodies. Fish in those water bodies might ingest small amounts of that mercury, and over time this substance can build up or bioaccumulate in their tissue. When another organism at a higher trophic level, such as a bear or a fish-eating bird, ingests large num- bers of fish, the mercury contained in the fish is transferred higher up the food chain. This process is known as biomagnification, an increase in the concentration of a pollutant as you move higher up the food chain. The case history section at the end of this chapter discusses some of the unex- pected and troubling ways in which mercury is bioaccumulating in individual organisms, biomag- nifying in many food chains, and wreaking havoc on wildlife populations.

By Annenberg Learner Ecosystems maintain themselves by cycling energy and nutrients obtained from external sources. At the first trophic level, primary producers (plants, algae, and some bacteria) use solar energy to produce organic plant material through photosynthesis. herbivores—animals that feed solely on plants—make up the second trophic level. Predators that eat herbivores comprise the third trophic level; if larger predators are present, they represent still higher trophic levels. Organisms that feed at several trophic levels (for example, grizzly bears that eat berries and salmon) are classified at the highest of the trophic levels at which they feed. Decomposers, which include bacteria, fungi, molds, worms, and insects, break down wastes and dead organisms and return nutrients to the soil.

On average about 10 percent of net energy production at one trophic level is passed on to the next level. Processes that reduce the energy transferred between trophic levels include respiration, growth and reproduction, defecation, and nonpredatory death (organisms that die but are not eaten by consumers). The nutritional quality of material that is consumed also influences how efficiently energy is transferred, because consumers can convert high-quality food sources into new living tissue more efficiently than low-quality food sources.

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Figure 1.2: Trophic levels

Energy enters an ecosystem through an external source (the sun) and flows through the progressive trophic levels of a food chain. On average, about 10 percent of the net energy produced at one trophic level is passed on to the next level; the rest is lost as heat energy.

The low rate of energy transfer between trophic levels makes decomposers gen- erally more important than producers in terms of energy flow. Decomposers pro- cess large amounts of organic material and return nutrients to the ecosystem in inorganic form, which are then taken up again by primary producers. Energy is not recycled during decomposition, but rather is released, mostly as heat (this is what makes compost piles and fresh gar- den mulch warm). [. . .]

Consider This It’s estimated that only about 10 percent of net energy production at one trophic level is passed to the next level. This pattern can be used to support an argument for reduc- ing meat consumption by humans and adopting a more plant-based diet. From an ecological standpoint (that is, ignoring ethical or other considerations) why might this argument make sense?

Solar energy

Higher level predator

Predators (animals that feed on herbivores)

Herbivores (animals that feed on plants)

Heat lost

Primary producers (plants, algae, and some bacteria)



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Gross and Net Primary Productivity in Ecosystems An ecosystem’s gross primary productivity (GPP) is the total amount of organic matter that it produces through photosynthesis. Net primary productivity (NPP) describes the amount of energy that remains available for plant growth after subtracting the fraction that plants use for respiration. Productivity in land ecosystems generally rises with temperature up to about 308C, after which it declines, and is positively correlated [related] with moisture. On land primary productivity thus is highest in warm, wet zones in the tropics where tropical forest biomes are located. In contrast, desert scrub ecosystems have the lowest productivity because their climates are extremely hot and dry.

In the oceans, light and nutrients are important controlling factors for productivity. [. . .] [L]ight penetrates only into the uppermost level of the oceans, so photosynthesis occurs in surface and near-surface waters. marine primary productivity is high near coastlines and other areas where upwelling brings nutrients to the surface, promoting plankton blooms. Runoff from land is also a source of nutrients in estuaries and along the continental shelves. Among aquatic ecosystems, algal beds and coral reefs have the highest net primary produc- tion, while the lowest rates occur in the open due to a lack of nutrients in the illuminated surface layers.

how many trophic levels can an ecosystem support? The answer depends on several factors, including the amount of energy entering the ecosystem, energy loss between trophic levels, and the form, structure, and physiology [functioning] of organisms at each level. At higher trophic levels, predators generally are physically larger and are able to utilize a fraction of the energy that was produced at the level beneath them, so they have to forage over increasingly large areas to meet their caloric needs.

Because of these energy losses, most terrestrial ecosystems have no more than five trophic levels, and marine ecosystems generally have no more than seven. [. . .]

Food Webs and Bioaccumulation The simplest way to describe the flux of energy through ecosystems is as a food chain in which energy passes from one trophic level to the next, without factoring in more complex relationships between individual species. some very simple ecosystems may consist of a food chain with only a few trophic levels. For example, the ecosystem of the remote wind-swept Taylor Valley in antarctica consists mainly of bacteria and algae that are eaten by nematode worms. more commonly, however, producers and consumers are connected in intricate food webs with some consumers feeding at several trophic levels.

An important consequence of the loss of energy between trophic levels is that contaminants collect in animal tissues—a process called bioaccumulation. As contaminants bioaccumulate up the food web, organisms at higher trophic levels can be threatened even if the pollutant is introduced to the environment in very small quantities.

The insecticide DDT, which was widely used in the United states from the 1940s through the 1960s, is a famous case of bioaccumulation. DDT built up in eagles and other raptors to levels high enough to affect their reproduction, causing the birds to lay thin-shelled eggs that

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broke in their nests. Fortunately, populations have rebounded over several decades since the pesticide was banned in the United states. however, problems persist in some developing countries where toxic bioaccumulating pesticides are still used.

Figure 1.3: Food web

This food web demonstrates how contaminants introduced at lower trophic levels can bioaccumulate and affect species at higher trophic levels, as was the case with eagles and raptors in the 1940s.

Bioaccumulation can threaten humans as well as animals. For example, in the United states many federal and state agencies currently warn consumers to avoid or limit their consump- tion of large predatory fish that contain high levels of mercury, such as shark, swordfish, tile- fish, and king mackerel, to avoid risking neurological damage and birth defects.

Excerpted from Weeks, S., Moorcoft, P.R. (2007). Unit 4: Ecosystems. The habitable Planet: a systems approach to environmental science. Retrieved from =0. Used with permission of Annenberg Learner.

Bald Eagles Salmon


Rabbits Snakes



Grass and Crops

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Apply Your Knowledge environmental scientist g. Tyler miller (1902–1988) once wrote:

Three hundred trout are needed to support one man for a year. The trout, in turn, must consume 90,000 frogs, that must consume 27 million grasshoppers that live off 1,000 tons of grass (American Chemist, 1971).

This quote illustrates the concept of energy transfer between different trophic levels. For this activity you should do the following:

Think about an animal that resides at the highest trophic level or at the top of the food web.

make a list of the kinds of food this animal eats. after that, list the kinds of food eaten by the animals further down the food chain until you get to the lowest trophic level or the bottom of the food chain. A simple Internet search like “what do grasshoppers eat” should turn up enough information.

Create a sketch or diagram of what this food chain might look like as trophic levels (similar to Figure 1.2) or as a food web (similar to Figure 1.3).

1.3 Nutrient Cycling in Ecosystems The previous section described how energy tends to flow through ecosystems—entering as sun- light and leaving as heat. In contrast, nutrients such as carbon, nitrogen, and phosphorous tend to cycle in ecosystems. Ecologists who study this cycle have learned that the same molecule of carbon that is used by a tree outside your window for photosynthesis may have been exhaled by a human or animal thousands of years ago. Ecologists also closely study the hydrologic (water) cycle, and the following excerpt from The habitable Planet: a systems approach to environ- mental science by Annenberg Learner explains why ecologists must have an appre-ciation for how water and nutrients cycle. Indeed, the study of nutrient cycling through various ecosystems has made ecologists aware that pollution or contaminants released in one part of an ecosystem can show up elsewhere in undesirable ways.

The interesting fact about nutrient and water cycles is that we are talking about the same material cycling over time. In other words, due to conservation of matter—matter can be trans- formed and combined in different ways but cannot be created, nor destroyed—nutrient and water cycles are working with a fixed amount of material. Water can be transformed to ice or mist; it can end up in the ocean only to come down later as rain and soak into the ground to become groundwater; but it is always water and there is only so much of it. Likewise, a carbon molecule could be absorbed from the atmosphere by a plant during photosynthesis, transferred to a rabbit that eats the leaves of the plant, transferred to a fox that eats the rabbit, and then returned to the atmosphere when the fox exhales carbon dioxide.

This principle of conservation of matter is sometimes described by ecologists as “there is no away.” When we burn fossil fuels that contain mostly carbon (such as coal or oil), we are mov- ing that carbon from one place, where it had been buried for millions of years, to another. When

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we mine phosphate deposits to make fertilizer and some of that fertilizer runs off into streams, we are moving that phosphorous from one place to another, but it does not go away. This basic concept of conservation of matter will be a central theme of much of the material in upcoming chapters. We’ll see that when we release nitrogen and phosphorous from agriculture, carbon dioxide from fossil fuel combustion, or toxic chemicals from manufacturing, these things don’t just “go away.” Instead, they can alter ecosystems and organisms in distant locations in unin- tended ways. A basic understanding of how nutrients cycle through ecosystems will help you see how that is possible.

By Annenberg Learner Along with energy, water and several other chemical elements cycle through ecosystems and influence the rates at which organisms grow and reproduce. about 10 major nutrients and six trace nutrients are essential to all animals and plants, while others play important roles for selected species.. The most important biogeochemical cycles [movement of matter, such as nitrogen, between living and non-living components of an ecosystem] affecting ecosystem health are the water, carbon, nitrogen, and phosphorus cycles.

As noted earlier, most of the Earth’s area that is covered by water is ocean. In terms of volume, the oceans dominate further still: nearly all of Earth’s water inventory is contained in the oceans (about 97 percent) or in ice caps and glaciers (about 2 percent), with the rest divided among groundwater, lakes, rivers, streams, soils, and the atmosphere. In addition, water moves very quickly through land ecosystems. These two factors mean that water’s residence time in land ecosystems is generally short, on average one or two months as soil moisture, weeks or months in shallow groundwater, or up to six months as snow cover.

But land ecosystems process a lot of water: almost two-thirds of the water that falls on land as precipitation annually is transpired [conversion of water to water vapor through plant tissue] back into the atmosphere by plants, with the rest flowing into rivers and then to the oceans. Because cycling of water is central to the functioning of land ecosystems, changes that affect the hydrologic cycle are likely to have significant impacts on land ecosystems. [. . .]

Both land and ocean ecosystems are important sinks for carbon, which is taken up by plants and algae during photosynthesis and fixed as plant tissue. [. . .]

Carbon cycles relatively quickly through land and surface-ocean ecosystems, but may remain locked up in the deep oceans or in sediments for thousands of years. The average residence time that a molecule of carbon spends in a terrestrial ecosystem is about 17.5 years, although this varies widely depending on the type of ecosystem: carbon can be held in old-growth for- ests for hundreds of years, but its residence time in heavily grazed ecosystems where plants and soils are repeatedly turned over may be as short as a few months.

Nitrogen and Phosphorous Cycles Nitrogen and phosphorus are two of the most essential mineral nutrients for all types of eco- systems and often limit growth if they are not available in sufficient quantities. (This is why the basic ingredients in plant fertilizer are nitrogen, phosphorus, and potassium, commonly abbreviated as NPK.) [. . .]

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Because atmospheric nitrogen (N2) is inert [does not react chemically] and cannot be used directly by most organisms, microorganisms that convert it into usable forms of nitrogen play central roles in the nitrogen cycle. so-called nitrogen-fixing bacteria take inert nitrogen (N2) from the atmosphere and convert it to ammonia (Nh4) nitrate (NO3) and another nitrogen compounds, which in turn are taken up by plants. some of these bacteria live in mutualistic relationships [an interaction between two species that benefits both] on the roots of plants, mainly legumes (peas and beans), and provide nitrogen directly to the plants; farmers often plant these crops to restore nitrogen to depleted soils. At the back end of the cycle, decom- posers break down dead organisms and wastes, converting organic materials to inorganic nutrients. Other bacteria carry out denitrification, breaking down nitrate to gain oxygen and returning gaseous nitrogen to the atmosphere.

Figure 1.4: The nitrogen cycle

Nitrogen circulates from the environment to living organisms and back to the environment. This cycle involves nitrogen-fixing bacteria that convert nitrogen into forms usable by living organisms, and denitrifying bacteria, which break down nitrogen compounds and return gaseous nitrogen to the atmosphere.

Atmospheric Nitrogen

Plant proteins


Decaying organic matter

Ammonia (NH4), nitrate (NO3) and other nitrogen compounds

“ammonification” and “nitrification”

Decaying organic matter


Denitrifying bacteria


Denitrifying bacteria


Nitrogen-fixing bacteria


Nitrogen-fixing bacteria


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human activities, including fossil fuel combustion, cultivation of nitrogen-fixing crops, and ris- ing use of nitrogen fertilizer, are altering the natural nitrogen cycle. Together these activities add roughly as much nitrogen to terrestrial ecosystems each year as the amount fixed by natu- ral processes; in other words, anthropogenic [human-caused] inputs are doubling annual nitro- gen fixation in land ecosystems. The main effect of this extra nitrogen is over-fertilization of aquatic ecosystems. Excess nitrogen promotes algal blooms, which then deplete oxygen from the water when the algae die and decompose. [. . .] Additionally, airborne nitrogen emissions from fossil fuel combustion promote the formation of ground-level ozone, particulate emis- sions, and acid rain [forms of pollution discussed in Chapter 9]. [. . .]

Phosphorus, the other major plant nutri- ent, does not have a gaseous phase like carbon or nitrogen. As a result it cycles more slowly through the biosphere. most phosphorus in soils occurs in forms that organisms cannot use directly, such as calcium and iron phosphate. Usable forms (mainly orthophosphate, or PO4) are pro- duced mainly by decomposition [disinte- gration] of organic material, with a small contribution from weathering [breaking down] of rocks.

Excessive phosphorus can also contrib- ute to over-fertilization and eutrophica- tion [excessive growth of algae] of rivers and lakes. human activities that increase phosphorus concentrations in natural ecosystems include fertilizer use, discharges from wastewater treatment plants, and use of phosphate detergents. [. . .]

Excerpted from Weeks, S., Moorcoft, P.R. (2007). Unit 4: Ecosystems. The habitable Planet: a systems approach to environmental science. Retrieved from Used with permission of Annenberg Learner.

1.4 Population Biology The excerpted selection below from The habitable Planet: a systems approach to envi- ronmental science by Annenberg Learner explains that different organisms grow and reproduce in very different ways. Some organisms are characterized by very short life spans but high rates of reproduction, whereas others have long life spans and low rates of reproduction. The difference depends on environmental conditions in a particular area and the organism’s life history strategy—such as how fast it develops, age of sexual maturity, and number of offspring. Ecologists study differences in life history strategies to better determine how to manage a spe- cies. For example, an insect pest with high rates of reproduction will require one approach to management whereas an endangered mammal with low rates of reproduction will require a different approach.

Consider This As pointed out above, human activities add roughly as much nitrogen to terres- trial (land-based) ecosystems as natural processes. If this nitrogen is being added to terrestrial ecosystems, why is it that the main effect is over-fertilization of aquatic (water-based) ecosystems? how does the nitrogen get from land to water, and since nitrogen acts as a fertilizer why is this a bad thing?

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Since ecosystems vary greatly from each other, organisms must be able to adapt to the conditions they face in order to survive. Ecologists refer to organisms that reproduce quickly as r-selected and note that these kinds of species are found in areas that are relatively unstable, such as flood plains. K-selected species, in contrast, reproduce more slowly and are found in more stable eco- systems such as old-growth forests. Ecologists and resource managers can use their understand- ing of an organism’s population biology, such as the degree to which it is r-selected or K-selected, to try to manage a given population. This approach can be utilized for commercial purposes, such as in the management of wild fish populations.

By Annenberg Learner Every organism in an ecosystem divides its energy among three competing goals: growing, surviving, and reproducing. Ecologists refer to an organism’s allocation of energy among these three ends throughout its lifetime as its life history strategy. There are tradeoffs between these functions: for example, an organism that spends much of its energy on reproduction early in life will have lower growth and survival rates, and thus a lower reproductive level later in life. An optimal life history strategy maximizes the organism’s contribution to population growth.

Understanding how the environment shapes organisms’ life histories is a major question in ecology. Compare the conditions for survival in an unstable area, such as a flood plain near a river that frequently overflows its banks, to those in a stable environment, such as a remote old-growth forest. On the flood plain, there is a higher chance of being killed early in life, so the organisms that mature and reproduce earlier will be most likely to survive and add to population growth. Producing many offspring increases the chance that some will sur- vive. Conversely, organisms in the forest will mature later and have lower early reproductive

rates. This allows them to put more energy into growth and competition for resources.

Ecologists refer to organisms at the first of these two extremes (those adapted to unstable environments) as r-selected [able to reproduce quickly]. These organisms live in settings where popu- lation levels are well below the maxi- mum number that the environment can support—the carrying capacity—so their numbers are growing exponen- tially at the maximum rate at which that population can increase if resources are not limited (often abbreviated as r). The other extreme, organisms adapted to stable environments, are termed K-selected because they live in environ- ments in which the number of individu- als is at or near the environment’s car- rying capacity (often abbreviated as K).

Anup Shah/Digital Vision/Thinkstock

As wildlife populations grow toward their carrying capacity, competition for limited resources such as food and space increases. Populations that exceed their carrying capacity experience increased death rates, reduced birth rates, and sometimes sudden, catastrophic collapses.

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Apply Your Knowledge Ideas and concepts from population biology are often used to develop management plans for different animal species. In particular, the maximum sustainable yield (msy) concept is fre- quently the basis for management of commercially valuable species, such as fish stocks. msy is the largest harvest or catch that can be taken from a given population without reducing the size of that population.


Organisms that are r-selected tend to be small, short-lived, and opportunistic, and to grow through irregular boom-and-bust population cycles. They include many insects, annual plants, bacteria, and larger species such as frogs and rats. species considered pests typically are r-selected organisms that are capable of rapid growth when environmental conditions are favorable. In contrast, K-selected species are typically larger, grow more slowly, have fewer offspring and spend more time parenting them. Examples include large mammals, birds, and long-lived plants such as redwood trees. K-selected species are more prone to extinction than r-selected species because they mature later in life and have fewer offspring with longer ges- tation times.

many organisms fall between these two extremes and have some characteristics of both types. As we will see below, ecosys- tems tend to be dominated by r-selected species in their early stages with the bal- ance gradually shifting toward K-selected species.

In a growing population, survival and reproduction rates will not stay constant over time. Eventually resource limitations

will reduce one or both of these variables. Populations grow fastest when they are near zero and the species is uncrowded. A simple mathematical model of population growth implies that the maximum population growth rate occurs when the population size (N) is at one-half of the environment’s carrying capacity, K (i.e., at N 5 K/2).

In theory, if a population is harvested at exactly its natural rate of growth, the population will not change in size, and the harvest (yield) can be sustained at that level. In practice, how- ever, it can be very hard to estimate population sizes and growth rates in the wild accurately enough to achieve this maximum sustainable yield. [. . .]

Excerpted from Weeks, S., Moorcoft, P.R. (2007). Unit 4: Ecosystems. The habitable Planet: a systems approach to environmental science. Retrieved from Used with permission of Annenberg Learner.

Consider This Why are r-selected species most commonly found in unstable environments while K-selected species are generally found in stable environments?

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Apply Your Knowledge (continued) Assume you were tasked with managing a fish stock of 1,000 individuals and that the rate of growth (reproduction) of that fish stock was 20 percent or 0.2 annually.

What would the msy be in that time period?

What would happen to the population of fish if you limited total catch to a lower number than the msy?

What would happen to the population of fish if you allowed for a total catch greater than the msy?

Using the scientific method, how might you design an experiment to determine what the msy might be for a particular fish population?

What kinds of factors might make you cautious or nervous about making use of the msy concept as the basis for the management of wildlife populations?

1.5 Ecosystem Functions An organism’s ability to survive and thrive depends on the availability of resources, including light, water, and nutrients. Survival also depends on how that organism interacts with other organisms and what sort of niche it occupies within that ecosystem. Different species com- pete for limited resources, but generally over time a species will evolve to occupy a particular niche in an ecosystem. The excerpted selection below from The habitable Planet: a systems approach to environmental science by Annenberg Learner explains how disturbances to eco- systems, often from human activity, can disrupt the availability of resources and alter relation- ships between species, often with disastrous consequences.

A key concept in understanding how ecosystems work is that of the limiting factor. Consider that a plant needs sunlight, water, carbon dioxide, and certain essential nutrients to sustain it. Because a plant needs all of these factors in some combination, increasing one factor, such as carbon dioxide, may not result in increased plant growth. A more familiar analogy might be making pancakes, which requires a certain amount of flour, milk, and eggs. Doubling the amount of flour available will not result in any more pancakes unless you also increase the amount of milk and eggs.

An impact at one point in an ecosystem can have a ripple effect throughout an entire ecosystem. For example, phosphorous is often a limiting factor in many aquatic ecosystems. When excess phosphorous enters an aquatic ecosystem, such as through fertilizer runoff, it can cause an explosive growth in algae and eventually result in a sharp drop in oxygen levels in that system. This drop in oxygen can kill or drive off fish species, and this can have ripple effects throughout the entire food chain. Likewise, ripple effects can begin at higher trophic levels through the removal of a top predator in a food chain. The removal of that predator can lead to a popula- tion explosion at lower trophic levels, known as a trophic cascade. For this reason top predators are often considered keystone species because their removal can trigger impacts throughout an ecosystem.

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Different species in an ecosystem occupy ecological niches or positions within that ecosystem. Put another way, various species pursue specific survival strategies, make use of particular resources, occupy different regions, and engage with other species in prescribed ways in order to meet their survival needs. Because many species will end up competing for the same resources, their realized niche (positions they actually occupy) will be smaller than their fundamental niche (the full range of positions they could occupy in the absence of competition). Likewise, spe- cies that are specialists that depend on a very narrow range of food sources and conditions will generally be smaller in number than those species that are generalists that can take advantage of a wider range of food sources. Specialist species are therefore more prone to ecosystem dis- turbances, and most endangered species tend to be specialists. An understanding of concepts like niches, limiting factors, and keystone species helps environmental scientists better understand how an ecosystem works and predict the impact of a disturbance to that system.

By Annenberg Learner A key question for ecologists studying growth and productivity in ecosystems is which factors limit ecosystem activity. Availability of resources, such as light, water, and nutrients, is a key control on growth and reproduction. some nutrients are used in specific ratios. For example, the ratio of nitrogen to phosphorus in the organic tissues of algae is about 16 to 1, so if the available nitrogen concentration is greater than 16 times the phosphorus concentration, then phosphorus will be the factor that limits growth; if it is less, then nitrogen will be limiting. To understand how a specific ecosystem functions, it thus is important to identify what factors limit ecosystem activity.

Resources influence ecosystem activity differently depending on whether they are essential, substitutable, or complementary. Essential resources limit growth independently of other levels: if the minimum quantity needed for growth is not available, then growth does not occur. In contrast, if two resources are substitutable, then population growth is limited by an appropriately weighted sum of the two resources in the environment. For example, glucose and fructose are substitutable food sources for many types of bacteria. Resources may also be complementary, which means that a small amount of one resource can substitute for a rela- tively large amount of another, or can be complementary over a specific range of conditions.

Resource availability serves as a so-called “bottom-up” control on an ecosystem: the supply of energy and nutrients influences ecosystem activities at higher trophic levels by affecting the amount of energy that moves up the food chain. In some cases, ecosystems may be more strongly influenced by so-called “top-down” controls—namely, the abundance of organisms at high trophic levels in the ecosystem. Both types of effects can be at work in an ecosystem at the same time, but how far bottom-up effects extend in the food web, and the extent to which the effects of trophic interactions at the top of the food web are felt through lower levels, vary over space and time and with the structure of the ecosystem.

Trophic Cascades and Keystone Species many ecological studies seek to measure whether bottom-up or top-down controls are more important in specific ecosystems because the answers can influence conservation and envi- ronmental protection strategies. For example, a study by Benjamin s. halpern [a marine ecol- ogist at the Center for Ocean solutions, University of California—santa Barbara] and oth- ers of food web controls in kelp forest ecosystems off the coast of southern California found that variations in predator abundance explained a significant proportion of variations in the

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abundance of algae and the organisms at higher trophic levels that fed on algae and plankton. In contrast, they found no significant relationship between primary production by algae and species abundance at higher trophic levels. The most influential predators included spiny lob- ster, Kellet’s whelk, rockfish, and sea perch. Based on these findings, the authors concluded that “[e]fforts to control activities that affect higher trophic levels (such as fishing) will have far larger impacts on community dynamics than efforts to control, for example, nutrient input, except when these inputs are so great as to create anoxic (dead) zones.”

Drastic changes at the top of the food web can trigger trophic cascades, or domino effects that are felt through many lower trophic levels. The likeli- hood of a trophic cascade depends on the number of trophic levels in the eco- system and the extent to which preda- tors reduce the abundance of a trophic level to below their resource-limited carrying capacity. some species are so important to an entire ecosystem that they are referred to as keystone spe- cies, connoting that they occupy an ecological niche that influences many other species. Removing or seriously impacting a keystone species pro- duces major impacts throughout the ecosystem.

many scientists believe that the reintroduction of wolves into yellowstone National Park in 1995, after they had been eradicated from the park for decades through hunting, has caused a trophic cascade with results that are generally positive for the ecosystem. Wolves have sharply reduced the population of elk, allowing willows to grow back in many riparian areas [the banks of rivers or streams] where the elk had grazed the willows heavily. healthier wil- lows are attracting birds and small mammals in large numbers.

“species, like riparian songbirds, insects, and in particular, rodents, have come back into these preferred habitat types, and other species are starting to respond,” says biologist Robert Crabtree of the Yel- lowstone Ecological Research Center. “For example, fox and coyotes are moving into these areas because there’s more prey for them. There’s been an erupting trophic cascade in some of these lush riparian habitat sites.”

Ecological Niches Within ecosystems, different species interact in different ways. These interactions can have positive, negative, or neutral impacts on the species involved.

Consider This how are the concepts of trophic cascade and keystone species related? how can these be used to argue for the protection and/or reintroduction of top predator spe- cies like wolves into areas like Yellowstone National Park?

. Nathan Hobbs/iStock/Thinkstock

Reintroducing wolves into Yellowstone National Park has had a positive effect on the ecosystem.

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Apply Your Knowledge a 2006 article in the journal Biological Conservation (Ripple and Beschta, 2006) described in detail a catastrophic trophic cascade event in Zion National Park in Utah. high visitor numbers helped drive down the population of cougars in the park, which led to higher populations of mule deer, which resulted in increased browsing of cottonwood tree seedlings along river and stream banks. This resulted in higher stream bank erosion and reduced populations of other terrestrial and aquatic organisms. In this situation:

Which animal was the keystone species?

how did lower cougar numbers help change the mule deer realized niche?

how did higher mule deer numbers help change the realized niche for cottonwood trees?

What kinds of strategies would you recommend the park consider to help address this problem? What might be some of the limitations or arguments against the strategies you would recommend?

Each species in an ecosystem occupies a niche, which comprises the sum total of its relation- ships with the biotic [living] and abiotic [non-living] elements of its environment—more sim- ply, what it needs to survive. In a 1957 address, zoologist george evelyn hutchinson framed the view that most ecologists use today when he defined the niche as the intersection of all of the ranges of tolerance under which an organism can live. This approach makes ecological niches easier to quantify and analyze because they can be described as specific ranges of vari- ables like temperature, latitude, and altitude. For example, the African Fish Eagle occupies a very similar ecological niche to the American Bald Eagle. In practice it is hard to measure all of the variables that a species needs to survive, so descriptions of an organism’s niche tend to focus on the most important limiting factors.

The full range of habitat types in which a species can exist and reproduce without any com- petition from other species is called its fundamental niche. The presence of other species means that few species live in such conditions. A species’ realized niche can be thought of as its niche in practice—the range of habitat types from which it is not excluded by compet- ing species. Realized niches are usually smaller than fundamental niches, since competitive interactions exclude species from at least some conditions under which they would otherwise grow. species may occupy different realized niches in various locations if some constraint, such as a certain predator, is present in one area but not in another.

In a classic set of laboratory experiments, Russian biologist G. F. Gause showed the difference between fundamental and realized niches. Gause compared how two strains of Paramecium grew when they were cultured separately in the same type of medium to their growth rates when cultured together. When cultured separately both strains reproduced rapidly, which indicated that they were adapted to living and reproducing under the same conditions. But when they were cultured together, one strain out-competed and eventually eliminated the other. From this work Gause developed a fundamental concept in community ecology: the competitive exclusion principle, which states that if two competitors try to occupy the same realized niche, one species will eliminate the other.

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Specialists and Generalists many key questions about how species function in ecosystems can be answered by looking at their niches. species with narrow niches tend to be specialists, relying on comparatively few food sources. As a result, they are highly sensitive to changes in key environmental con- ditions, such as water temperature in aquatic ecosystems. For example, pandas, which only eat bamboo, have a highly specialized diet. many endangered species are threatened because they live or forage in particular habitats that have been lost or converted to other uses. One well-known case, the northern spotted owl lives in cavities of trees in old-growth forests (for- ests with trees that are more than 200 years old and have not been cut, pruned, or managed), but these forests have been heavily logged, reducing the owl’s habitat.

In contrast, species with broad niches are generalists that can adapt to wider ranges of environmental conditions within their own lifetimes (i.e., not through evolution over genera- tions, but rather through changes in their behavior or physiologic functioning) and survive on diverse types of prey. Coyotes once were found only on the Great Plains and in the western United states, but have spread through the eastern states in part because of their flexible lifestyle. They can kill and eat large, medium, or small prey, from deer to house cats, as well as other foods such as invertebrates [an animal without a backbone] and fruit, and can live in a range of habitats, from forests to open landscapes, farmland, and suburban neighborhoods.

Overlap between the niches of two species (more precisely, overlap between their resource use curves) causes the species to compete if resources are limited. One might expect to see species constantly dying off as a result, but in many cases competing species can coexist without either being eliminated. This happens through niche partitioning (also referred to as resource partitioning), in which two species divide a limiting resource such as light, food supply, or habitat.

Excerpted from Weeks, S., Moorcoft, P.R. (2007). Unit 4: Ecosystems. The habitable Planet: a systems approach to environmental science. Retrieved from Used with permission of Annenberg Learner.

1.6 Evolution and Natural Selection in Ecosystems Just as the species within an ecosystem change and evolve, ecosystems themselves go through natural changes in a process known as succession. Successional changes alter conditions for various species over time, favoring some at the expense of others. For example, a tract of forest blown over by a hurricane or burned to the ground in a wildfire will usually return to a forested state given enough time. Initially, r-selected species, those that reproduce quickly and colonize disturbed areas (see section 1.4), will dominate the site and take advantage of the lack of com- petition from other species. However, over time K-selected species will move in and the disturbed ecosystem will move through successional stages to a more diverse and complex system.

It might be tempting to look at an ecosystem, such as a forest, as relatively static and unchanging over time. However, ecosystems are constantly changing. The theory of evolution, first developed by Charles Darwin and other 19th-century scientists, suggested that competition between spe- cies for scarce resources will favor some species over others. Likewise, natural selection within

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species is a process where certain individuals in a given population will be better suited for sur- vival and will pass on their genes and traits to future generations.

Competition between species and natural selection within species results in an ongoing pro- cess of constant change in order to ensure the survival of a given species. For species that form a predator–prey relationship—such as bats and insects—ecologists have observed a process known as coevolution. Coevolution has been compared to an ecological arms race where the prey species might, over time, develop a defensive mechanism to thwart the predator that then is overcome by an evolved predator. The adapted section below from The habitable Planet: a systems approach to environmental science by Annenberg Learner explains how constant species interactions with each other and their environment in a given ecosystem results in a process of natural selection and evolution.

By Annenberg Learner As species interact, their relationships with competitors, predators, and prey contribute to natural selection and thus influence their evolution over many generations. To illustrate this

concept, consider how evolution has influenced the factors that affect the foraging efficiency of predators. This includes the predator’s search time (how long it takes to find prey), its handling time (how hard it has to work to catch and kill it), and its prey prof- itability (the ratio of energy gained to energy spent handling prey). Charac- teristics that help predators to find, catch, and kill prey will enhance their chances of surviving and reproducing. similarly, prey will profit from attri- butes that help avoid detection and make organisms harder to handle or less biologically profitable to eat.

These common goals drive natural selection for a wide range of traits and behaviors, including:

• Mimicry by either predators or prey. A predator such as a praying mantis that blends in with surrounding plants is better able to surprise its target. however, many prey species also engage in mimicry, developing markings similar to those of unpalatable species so that predators avoid them. For example, harmless viceroy butterflies have similar coloration to monarch butterflies, which store toxins in their tissues, so predators avoid viceroy butterflies.

• Optimal foraging strategies enable predators to obtain a maximum amount of net energy per unit of time spent foraging. Predators are more likely to survive and repro- duce if they restrict their diets to prey that provide the most energy per unit of han- dling time and focus on areas that are rich with prey or that are close together. [. . .]

. Yokpok/iStock/Thinkstock

The ability to blend in with, or mimic, its environment aids the praying mantis in capturing its prey.

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• avoidance/escape features help prey elude predators. These attributes may be behav- ioral patterns, such as animal herding or fish schooling to make individual organisms harder to pick out. markings can confuse and disorient predators: for example, the automeris moth has false eye spots on its hind wings that misdirect predators.

• Features that increase handling time help to discourage predators. spines serve this function for many plants and animals, and shells make crustaceans and mollusks harder to eat. Behaviors can also make prey harder to handle: squid and octopus emit clouds of ink that distract and confuse attackers, while hedgehogs and porcu- pines increase the effectiveness of their protective spines by rolling up in a ball to conceal their vulnerable underbellies.

some plants and animals emit noxious chemical substances to make themselves less profit- able as prey. These protective substances may be bad-tasting, antimicrobial, or toxic. many species that use noxious substances as protection have evolved bright coloration that signals their identity to would-be predators—for example, the black and yellow coloration of bees, wasps, and yellowjackets. The substances may be generalist defenses that protect against a range of threats, or specialist compounds developed to ward off one major predator. some- times specialized predators are able to overcome these noxious substances: for example, rag- wort contains toxins that can poison horses and cattle grazing on it, but it is the exclusive food of cinnabar moth caterpillars. Ragwort toxin is stored in the caterpillars’ bodies and eventu- ally protects them as moths from being eaten by birds.

Coevolution and Competition Natural selection based on features that make predators and prey more likely to survive can generate predator–prey “arms races,” with improvements in prey defenses triggering counter-improvements in predator attack tools and vice versa over many generations. many cases of predator–prey arms races have been identified. One widely known case is bats’ use of echolocation [the use of echoes to determine the location of something] to find insects. Tiger moths respond by emitting high-frequency clicks to “jam” bats’ signals, but some bat species have overcome these measures through new techniques such as flying erratically to confuse moths or sending echolocation chirps at frequencies that moths cannot detect. This type of pattern involving two species that interact in important ways and evolve in a series of recipro- cal genetic steps is called coevolution and represents an important factor in adaptation and the evolution of new biological species.

Other types of relationship, such as competition, also affect evolution and the characteristics of individual species. For example, if a species has an opportunity to move into a vacant niche, the shift may facilitate evolutionary changes over succeeding generations because the species plays a different ecological role in the new niche. By the early 20th century, large predators such as wolves and puma had been largely eliminated from the eastern United states. This has allowed coyotes, who compete with wolves where they are found together, to spread through- out urban, suburban, and rural habitats in the eastern states, including surprising locations such as Cape Cod in massachusetts and Central Park in New york City. research suggests that northeastern coyotes are slightly larger than their counterparts in western states, although it is not yet clear whether this is because the northeastern animals are hybridizing [cross- breeding] with wolves and domestic dogs or because they have adapted genetically to preying on larger species such as white-tailed deer.

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Natural Ecosystem Change Just as relationships between individual species are dynamic, so too is the overall makeup of ecosystems. The process by which one natural community changes into another over a time scale of years to centuries is called succession. Common succession patterns include plant colonization of sand dunes and the regrowth of forests on abandoned farmland. While the general process is widely recognized, ecologists have offered differing views of what drives succession and how to define its end point. By analyzing the natural succession process, sci- entists seek to measure how stable ecosystems are at different stages in their trajectory of development, and how they respond to disturbances in their physical environment or changes in the frequency at which they are disturbed.

Figure 1.5: Succession

An example of succession is the change in the type of plants in an area over time. Certain organisms that initially colonize an area are replaced over time by others, which themselves are later replaced by other organisms.

In the early 20th century, plant biologist Frederic Clements described two types of succes- sion: primary (referring to colonization of a newly exposed landform, such as sand dunes or lava flows after a volcanic eruption) and secondary (describing the return of an area to its natural vegetation following a disturbance such as fire, treefall, or forest harvesting). British ecologist arthur Tansley distinguished between autogenic succession—change driven by the inhabitants of an ecosystem, such as forests regrowing on abandoned agricultural fields— and allogenic succession, or change driven by new external geophysical conditions such as rising average temperatures resulting from global climate change.

Tim e

Annual plants

Perennial plants and grasses

Shrubs Softwood trees– Pines

Hardwood trees

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As discussed above, ecologists often group species depending on whether they are better adapted for survival at low or high population densities (r-selected versus K-selected). suc- cession represents a natural transition from r- to K-selected species. Ecosystems that have recently experienced traumatic extinction events such as floods or fires are favorable envi- ronments for r-selected species because these organisms, which are generalists and grow rapidly, can increase their populations in the absence of competition immediately after the event. Over time, however, they will be out-competed by K-selected species, which often derive a competitive advantage from the habitat modification that takes place during early stages of primary succession.

For example, when an abandoned agricultural field transitions back to forest [. . .] sun-tolerant weeds and herbs appear first, followed by dense shrubs like hawthorn and blackberry. After about a decade, birches and other small fast-growing trees move in, sprouting wherever the wind blows their lightweight seeds. In 30 to 40 years, slower-spreading trees like ash, red maple, and oak take root, followed by shade-tolerant trees such as beech and hemlock.

A common observation is that as ecosystems mature through successional stages, they tend to become more diverse and complex. The number of organisms and species increases and niches become narrower as competition for resources increases. Primary production rates and nutrient cycling may slow as energy moves through a longer sequence of trophic levels.

many natural disturbances have inter- rupted the process of ecosystem succes- sion throughout Earth’s history, including natural climate fluctuations, the expansion and retreat of glaciers, and local factors such as fires and storms. An understand- ing of succession is central for conserving and restoring ecosystems because it iden- tifies conditions that managers must cre- ate to bring an ecosystem back into its nat- ural state. The Tallgrass Prairie National Preserve in Kansas, created in 1996 to protect 11,000 acres of prairie habitat, is an example of a conservation project that seeks to approximate natural ecosystem succession. A herd of grazing buffalo tram- ples on tree seedlings and digs up the ground, creating bare patches where new plants can grow, just as millions of buffalo maintained the grassland prairies that covered North america before European settlement.

Excerpted from Weeks, S., Moorcoft, P.R. (2007). Unit 4: Ecosystems. The habitable Planet: a systems approach to environmental science. Retrieved from Used with permission of Annenberg Learner.

Consider This For much of the 20th century park man- agers in the United states adopted a strict wildfire control and suppression policy, championed by the iconic smokey the Bear. From an ecological perspective, how might these efforts have been misplaced? how might certain disturbances, like fires, be beneficial for biodiversity in certain locations?

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Evolution in Action Public debates and discussions over evolution often center on whether or not such a thing exists. But for ecologists and other environmental scientists the evidence for evolution is all around them and constantly presenting itself in new ways. Perhaps part of the problem with the “debate” over evolution stems from the way scientists refer to evolution as a “theory.” For many of us a theory is simply a hunch, something we think might be true. For scientists, however, the term theory means something completely different. Recall from the discussion of the scientific method in the Introduction that even when an experiment matches up with a prediction and supports a hypothesis, scientists will continue to make additional predictions from the knowledge gained and to test these predictions. In this context a scientific theory is something that’s already well substantiated or supported and based on a large body of evi- dence developed through repeated observation and experimentation.

Ecologists and environmental scientists are also always seeing new evidence for evolution in the world around us. Take the case of how some insect populations have responded to the use of insecticide sprays in agricultural fields. Because insects produce such large numbers of off- spring (i.e., they are r-selected) there is a greater chance that some of them will have a genetic mutation that makes them less susceptible, or resistant, to the pesticide being sprayed. These individuals may survive pesticide applications and then reproduce, passing on the genetic mutation that made them resistant to the next generation (see Figure 1.6). This phenomenon has come to be known as pesticide resistance or the pesticide treadmill since farmers respond by spraying even more pesticide in a losing battle. The Pesticide action Network (http:// estimates that since 1945 between 500 and 1,000 insect and weed species have evolved to develop resistance to pesticides and herbicides.

Figure 1.6: Pesticide resistance process

Pests can develop resistance to pesticides over time.

Application of pesticides in response

to pest presence.

Some pests survive

pesticide application. Said to be resistant.

Resistant pests


Application of more

pesticide in response to

continued pest presence.

Pests survive,

no longer affected by pesticide.

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seCTION 1.7 Case hIsTOry—merCUry’s ImPaCT ON WIlDlIFe

1.7 Case History—Mercury’s Impact on Wildlife Mercury is a well-known contaminant that is facing increased scrutiny and regulation due to its impacts on human health. However, it’s less clear how mercury pollution is impacting wildlife populations around the world. In this article, “Mercury’s Silent Toll on the World’s Wildlife,” science and environmental journalist Rebecca Kessler describes how scientists are uncovering disturbing evidence of how even low levels of exposure to mercury can harm a wide variety of wildlife species. This raises the question of whether efforts to minimize human exposure to mer- cury will also benefit wildlife populations.

Mercury contamination provides an excellent case study for some of the concepts already explored in this chapter. Low levels of mercury can exist naturally in many ecosystems, but certain human activities can increase levels of mercury in the environment significantly. For example, when coal is burned in a power plant the emissions that enter the atmosphere often contain trace amounts of mercury. This mercury can travel thousands of miles and get washed out of the atmosphere, entering streams, lakes, the ocean, and other water bodies. Once in the water this mercury converts to methylmercury, which is both toxic and easily absorbed by algae at the base of the aquatic food chain. Small fish consume the algae and in turn are consumed by larger fish, allowing the mercury to bioaccumulate in their tissue. As mercury-laden fish are eaten by animals higher up the food chain we can see biomagnification and impacts of mercury contamination far from the water. Even low levels of exposure to mercury can impact the ability of some species to reproduce, altering species composition in an ecosystem and possibly trigger- ing trophic cascades.

Another interesting feature of this case history is the example it provides of the scientific method in action. It describes the results of a research project that fed mercury-contaminated food to captured ibises (a species of wading bird) and then tracked their breeding behavior. This research provided new insights into our understanding of mercury contamination, although the researchers acknowledged that there are possible differences in how mercury enters and moves through food chains in the wild. Either way, the scientific research discussed in this article makes clear that low levels of mercury exposure over prolonged periods could be resulting in unex- pected and serious impacts on wildlife. As such, regulatory efforts to control mercury emissions from human activities need to consider these impacts and not just those to human health.

By Rebecca Kessler This month [January 2013], delegates from over 140 countries gathered in geneva and final- ized the first international treaty to reduce emissions of mercury. The treaty—four years in the works and scheduled for signing in October—aims to protect human health from this very serious neurotoxin.

But barely considered during the long deliberations, according to those involved in the treaty process, was the harm that mercury inflicts on wildlife. While mercury doesn’t kill many ani- mals outright, it can put a deep dent in reproduction, says David Evers, chief scientist at the Biodiversity Research Institute (BRI), who serves on a scientific committee informing the process. “It is a bit of a silent threat, where you have to kind of add up what was lost through studies and demographic models.”

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harmful levels of mercury have turned up in all sorts of animals, from fish and birds living around the world to pythons invading the Florida Everglades and polar bears roaming far from any sources of pollution. In recent years, biologists have been tracking mercury’s foot- prints in unexpected habitats and species. Their research is illuminating the subtle effects of chronic exposure and is showing that ever-lower levels cause harm.

Coal burning, gold mining, and other human activities release mercury into water bodies or the atmosphere, where it can travel great distances before set- tling back to earth. mercury contami- nation is ubiquitous and hotspots are common around the world, with fish and human hair collected in 14 coun- tries regularly exceeding U.s. envi- ronmental Protection Agency (EPA) standards, according to a BRI report released just before the geneva nego- tiations. And while mercury emis- sions are declining in North America and Europe they are rising quickly in the developing world, according to the United Nations Environment Pro- gramme, the treaty coordinator.

The new global treaty bans the pro- duction, import, and export of cer- tain mercury-containing products, requires governments to create plans to reduce mercury in small gold mining operations, and puts some controls on industrial facilities—but some environmental groups warn that it is too weak. The U.s. is going further. On January 1, an export ban on elemental mercury took effect, and the EPA is finalizing new limits on coal plant emissions.

“In the end the treaty will reduce mercury that’s being released into the environment. And I think the question will be, as we move along, ‘Is it enough?’—especially for areas that are sensitive to mercury input. And then ‘Is it enough for wildlife conservation purposes?’ which really wasn’t addressed all that well,” Evers says.

Exposed animals have trouble ridding their bodies of mercury, and it accumulates in tis- sue with every link in the food chain. Long-lived predators tend to carry the heaviest loads. Research and public attention have largely focused on contaminated fish, the main route of human exposure. In water, mercury converts quickly to methylmercury, its most toxic and bioavailable form, so for many years wildlife biologists trained their sights on aquatic, fish- eating birds and mammals, says Bill hopkins, a Virginia Tech physiological ecologist.

lately, though, hopkins and others have uncovered mercury in reptiles, amphibians, insects, spiders, terrestrial songbirds, and a wider variety of mammals than expected. “All these dif- ferent groups can be exposed to mercury and pass it on to their babies,” says hopkins.

AP Photo/Esteban Felix

Mining operations—like this illegal one in Peru— release toxic mercury into the environment. In the Peruvian state of Madre de Dios alone, 35 metric tons of mercury is released annually by miners, slowly poisoning people, plants, animals, and fish.

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mercury is also turning up in strange places, he says. Invertebrate-eating songbirds living in the floodplain bordering a contaminated Virginia river had as much mercury in their blood as the river’s fish-eating birds, and sometimes more, showing that mercury pollution doesn’t stay put in aquatic habitats. scientists have found mercury-laden food chains in mountainous forests, and shown that methylmercury forms in the woods, as well as in water. BRI scien- tists and collaborators discovered high levels in many invertebrate-eating songbird and bat species living in varied habitats across the U.s. Northeast and mid-atlantic states, including remote uplands. The pollutant has also emerged as a serious problem in the arctic.

mercury plays havoc on vertebrates’ development and their neurological and hormonal systems, and doses too low to kill can cause problems that aren’t always obvious in the wild, experts say. “methylmercury is one of most toxic environmental pollutants we’ve ever come upon,” says gary heinz, a recently retired federal wildlife biologist who studied it over four decades.

In the earliest studies of these sublethal effects in the 1970s, heinz reported that captive mal- lards fed mercury-laced food laid fewer eggs than control ducks and laid them outside the nest. Also, their ducklings didn’t respond well to their calls. Numerous examples have accu- mulated since. Fish form loose, sloppy schools and are slow to respond to a simulated preda- tor. several bird species sing different songs. loons lay smaller eggs, and they incubate their nests, forage, and feed their chicks less. salamanders are sluggish and less responsive to prey, hopkins and colleagues found. egret chicks are similarly lethargic and unmotivated to hunt.

Changes like these could be grave for wild animals, says Peter Frederick, a University of Flor- ida ecologist who was part of the egret study. “Getting lunch or a mate depends on millisec- onds and millimeters. you have to perform that courtship dance just right. you have to make the calls just right. you have to stab your prey to within a millimeter. If you’re off by a micro- second, it’s gone,” he says.

Frederick discovered a remarkable example in white ibises from the everglades. There, mer- cury levels are low but constant, and ibises seem to nest less and abandon their nests more often than elsewhere. To see if chronic mercury exposure was responsible, Frederick cap- tured 160 ibis nestlings and fed them food with mercury levels similar to their wild fish prey. he and his team observed the birds for three years to see if it affected their breeding behavior.

as expected, the dosed birds produced far fewer offspring than undosed controls. There were the usual reasons: eggs didn’t hatch and chicks died under lousy parenting. But Frederick was wholly surprised to see widespread homosexual pairing among the dosed males and to find this caused much of the reproductive deficit. Avian homosexuality usually occurs with stark sex imbalances—which wasn’t the case here, Frederick says.

No one had ever reported homosexuality as an effect of mercury, or any other contaminant for that matter, Frederick says. moreover, the effects appeared in ibises he’d fed as little as 0.05 ppm [parts per million] of mercury in their food—one-tenth of what heinz fed his mallards. Further work indicated that hormonal changes wrought by mercury’s effects on the ibises’ endocrine systems were at work. In a 2011 paper, Frederick and a colleague estimated that out in the Everglades, mercury could cut the number of ibis fledglings by half—easily enough to curtail the population.

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No one has checked wild ibises for poor parental behavior or homosexuality, which might lay the blame more squarely on mercury, he says. (Different species react to mercury differ- ently, and Frederick stresses that for many reasons his results in no way suggest that mercury might play a role in human homosexuality.) Nevertheless, the broader implications for chron- ically exposed wildlife are chilling. “We can be essentially neutering populations by cutting off reproduction through the endocrine system,” he says. “This could easily be going on in the wild with many kinds of contaminants. mercury is not the only endocrine disruptor.”

Like Frederick’s study, much of the research on mercury’s sublethal effects has been con- ducted on captive animals. In nature, it’s very difficult to get the large sample sizes and con- trol groups needed to identify subtle differences statistically, says Erick Greene, a conserva- tion biologist at the University of montana.

studying ospreys living near montana’s polluted Clark Fork river, greene and two colleagues found that about half the eggs laid by high-mercury birds fail to hatch. But they’ve been puz- zled as to whether the surviving chicks are affected. In humans, blood levels around .005 ppm can cause cognitive deficits, Greene says. But his osprey chicks commonly have levels 100— and even 1,000—times higher. The chicks seem to do fine in the nest, he says. “They may look all right, but I don’t know if I would recognize a mentally impaired osprey chick.”

Once they’re fledged they soon migrate south, out of sight. Greene suspects they may have trouble making the demanding migration to Central or south america (where mercury flows freely in small gold mining operations), or just figuring out how to survive on their own. his team has begun outfitting fledglings with satellite transmitters to determine how far mer- cury-loaded birds get compared to their normal peers, and how long they live.

It’s one thing to show that wild animals are exposed to harmful levels of mercury, but solid evi- dence that whole populations are harmed is harder to come by, experts say. A notable excep- tion is loons. evers and more than a dozen colleagues amassed an impressive 18-year data set of nearly 5,500 mercury measurements from loons on 700 lakes across 17 U.s. states and Canadian provinces. They showed that when mercury in loon blood hits 3 ppm, the number

of young fledged drops by 41 percent— and that enough loons are affected to set back some New hampshire and maine populations.

In a forthcoming paper, hopkins and another researcher go a step further with a population model they developed based on four years of field data on American toads. Toads readily move between small populations scattered throughout the landscape. mercury exposure can kill eggs and tadpoles, and survivors are often small and slow to mature. The model revealed that not only can mercury kill enough tad- poles to wipe out small populations, but that nearby uncontaminated populations

Consider This Understanding the impacts of mercury on wildlife is a challenging task, and the uncertainty surrounding the science can be used by some to argue against regula- tion of this contaminant. At what point do you think there is enough evidence to take action to address an environmental problem like this? how can the cautious approach of scientists “testing hypoth- eses” be reconciled with political demands for “scientific proof ” before taking action?

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can also drop or go extinct because there are too few toads around to replenish them if their numbers happen to dip for other reasons. hopkins says he thinks the paper will change biolo- gists’ understanding of contaminants. “Contaminant effects in one population can actually affect adjacent populations that aren’t being exposed to that contaminant,” he says.

Whatever its weaknesses, the new treaty represents a “great step forward,” says Evers, and the good news is that once local sources are controlled, mercury in nearby wildlife can drop quickly. The bad news is that mercury from coal burning can travel great distances—for instance, from China to North America—before settling.

Overall, Evers says the forecast for wildlife is cloudy. When it comes to mercury, “the more we look the more we find, and the more we find the lower that toxicity level is going,” he says. “right now at a global level, mercury is just being released more and more in the system. Those trend lines are going in the wrong directions.”

Kessler, R. (2013, January 31), “Mercury’s Silent Toll On the World’s Wildlife,” Yale Environment 360. Copyright © Rebecca Kessler. Reprinted by permission of the author.

Summary & Resources

Chapter summary In order to better understand how the natural world works, ecologists and other scientists study it on very different scales. For example, we started this chapter with the concept of biomes, which are broad-scale areas of the planet that are characterized by similar climates, plants, and animals. The biome concept allows ecologists to classify the world into a rela- tively small number of categories, and then to examine how basic factors such as tempera- ture, moisture, nutrient levels, light, and other physical conditions affect the abundance and diversity of life in locations with similar characteristics.

Ecosystems are studied on a smaller scale than biomes, and they can be defined as a system or unit (for example a forest or grassland) and the living (biotic) and non-living (abiotic) com- ponents of that system. Ecologists who study ecosystems investigate the way energy enters the system, usually in the form of sunlight, and how it moves through that system at various trophic levels. Ecologists also look at how water and nutrients cycle through ecosystems as well as the impact on the abundance and/or diversity of species based on the availability of energy (net primary productivity), nutrients, and water.

On an even smaller scale, ecologists study individual species and organisms in specific ecosys- tems. In order to do this, ecologists focus on concepts of population biology and an organism’s life history strategy—how it develops and reproduces. In addition, ecologists pay close atten- tion to how different species interact and compete with one another for limited resources in a given ecosystem. Competition results in species occupying niches within ecosystems and also helps to drive the process of evolution to ensure the ongoing survival of an individual species.

Throughout all of these stages, ecologists rely on the scientific method to perform their work. They first observe the natural world and form questions and hypotheses. Next, they acquire actual data on the system and the question they seek to answer. They examine and analyze

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Working Toward Solutions While the case history reading in section 1.7 focused on the dangers of mercury to wildlife, this contaminant can also have serious impacts on human health. mercury exposure is espe- cially problematic for children and pregnant women because it can interfere with neurological or brain development and function. As a result, many states issue advisories on how much fish should be eaten by these groups. Even though individuals are usually not directly related to mercury emissions, there are things you can do to reduce emissions indirectly as well as limit your own exposure to mercury pollution.

For a general overview of mercury pollution and how it impacts human health, visit the fol- lowing pages: /factsheet.cfm /mercury


that data against the original hypotheses that drive the research, and finally, they reach a con- clusion based on the results. These conclusions are usually disseminated and shared with the broader scientific community so that the results can be questioned and challenged, and the original research can be repeated to see if similar results are obtained.

The end result of decades of this sort of work is a generally well-developed understanding of how our world works, although Earth’s systems are so complex that there are still many things that we do not understand. Likewise, questions that were once thought to be settled are sometimes revisited as new information is discovered.

The remainder of this book will focus primarily on how human actions affect the natural world. much of the information presented will be based on an understanding first of how an ecosystem functions and then how human actions—such as the introduction of a pollutant, removal of a species through over-hunting, or conversion of part of the ecosystem to another use—alter those systems. Chapter 2 will focus specifically on changes in human population over time. Indeed, understanding human population is important since, ultimately, it is the number of people, and the level of material consumption that they engage in, that determine the scale and scope of the human impact on the environment.

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Working Toward Solutions (continued)

For some discussion of what you can do to help reduce and prevent mercury pollution, visit: /p2-pollution-prevention/reducing-toxicity/preventing-mercury-pollution.html


1. Which biome would be expected to have the warmest and wettest conditions? a. Coniferous forest b. Desert c. Tropical forest d. Temperate grassland

2. The major sources of human emissions of the pollutant mercury are a. disposal of thermometers and hospital waste. b. car and truck exhaust. c. coal burning and gold mining. d. agriculture and cattle ranching.

3. Which of the following is NOT an example of an important biogeochemical cycle? a. The water cycle b. The phosphorous cycle c. The solar cycle d. The carbon cycle

4. The population biology concept that refers to the maximum number of organisms that a given environment can support is

a. survival rate. b. reproductive rate. c. K-selection. d. carrying capacity.

5. When a top predator is removed from an ecosystem it can have dramatic impacts on the entire food web. These impacts are referred to as

a. biomagnification. b. bioaccumulation. c. trophic cascades. d. photosynthesis.

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6. Which of the following is NOT an example of an avoidance/escape feature used to deter predators from attacking prey?

a. A panda feeding only on bamboo b. Fish swimming in a school c. Wildebeests moving in a herd d. A moth with false eye spots on its hind wings

7. Because mercury tends to accumulate in an animal’s tissue we would expect what kinds of organisms to carry the highest amounts of this toxin?

a. Long-lived predators b. Primary producers c. short-lived predators d. Detritivores

8. The latitudinal biodiversity gradient predicts that a. biodiversity will be highest at high latitudes. b. biodiversity will be evenly distributed across latitudes. c. biodiversity will be spread randomly around the planet. d. biodiversity will be highest in tropical areas at low latitudes.

9. The average rate of energy transfer between one trophic level and the next is a. 1 percent. b. 10 percent. c. 30 percent. d. 50 percent.

10. Which of these human activities is among the major causes of adding to and altering the nitrogen cycle?

a. Fossil fuel combustion b. Over-pumping of groundwater c. Deforestation d. Over-fishing

11. Population biologists indicate that every organism in an ecosystem divides its energy expenditures among three competing goals, which are

a. growing, surviving, and photosynthesizing. b. growing, hibernating, and bonding. c. migrating, surviving, and respiration. d. growing, surviving, and reproducing.

12. The main reason why species like the coyote are found in a wide range of environ- ments is because they are

a. invertebrates. b. generalists. c. keystone species. d. specialists.

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13. Immediately after a forest ecosystem has been wiped out by a flood or fire, we would expect what kind of species to dominate the area?

a. R-selected b. Keystone species c. specialists d. K-selected

14. Wildlife biologists are primarily concerned about the impact on wildlife of chronic, long-term, low-level exposure to mercury because this could be leading to

a. changes in vocal chords. b. changes in sleep patterns. c. changes in reproductive behavior. d. changes in eating habits.

Answers 1. c. Tropical forest. The answer can be found in section 1.1. 2. c. coal burning and gold mining. The answer can be found in section 1.2. 3. c. The solar cycle. The answer can be found in section 1.3. 4. d. carrying capacity. The answer can be found in section 1.4. 5. c. trophic cascades. The answer can be found in section 1.5. 6. a. a panda feeding only on bamboo. The answer can be found in section 1.6. 7. a. long-lived predators. The answer can be found in section 1.7. 8. d. biodiversity will be highest in tropical areas at low latitudes. The answer can be found in section 1.1. 9. b. 10 percent. The answer can be found in section 1.2. 10. a. Fossil fuel combustion. The answer can be found in section 1.3. 11. d. growing, surviving, and reproducing. The answer can be found in section 1.4. 12. b. generalists. The answer can be found in section 1.5. 13. a. r-selected. The answer can be found in section 1.6. 14. c. changes in reproductive behavior. The answer can be found in section 1.7.

Key Ideas

• The world’s diverse ecosystems are classified into a smaller number of similar types known as biomes. Biodiversity and species richness within different biomes is determined by a combination of factors, mainly temperature, precipitation, light, and nutrients.

• Nearly all life on Earth is powered by solar energy through photosynthesis. Primary producers like plants and algae form the basis of most food webs. As energy is trans- ferred from primary producers to organisms at higher trophic levels, much of the original energy is lost, resulting in pyramid-shaped food chains.

• Unlike energy, which flows through ecosystems, matter like nitrogen and phospho- rous cycles within ecosystems. Because the law of conservation of mass holds that matter can be neither created nor destroyed, human activities that introduce matter like nitrogen and phosphorous into an ecosystem can have undesirable impacts.

• Population biology or population ecology helps explain why different organisms adopt different life history strategies in order to survive. R-selected species repro- duce quickly and are generally found in unstable environments. K-selected species reproduce slowly and tend to be found in stable environments.

• Trophic cascades can occur when populations of keystone species near the top of a food web are altered. Not all trophic cascades are negative. Reintroducing a keystone

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species can set in motion a trophic cascade with positive effects. Ecologists utilize the niche concept to help predict when changes to a species population might trig- ger a trophic cascade.

• Constant interactions between species in an ecosystem as well as changes to that eco- system force organisms to evolve through a process of natural selection. Predator–prey interactions often result in an ongoing series of adaptations known as coevolution.

• mercury emissions from human activities like burning coal and gold mining can increase levels of this contaminant in ecosystems far from any source. Once present, even low levels of mercury can bioaccumulate in individual organisms and biomag- nify up the food chain, wreaking havoc with wildlife populations.

Critical Thinking and Discussion Questions

1. Biodiversity in tropical regions can be attributed to an abundance of the key influ- ences on ecosystems: temperature, moisture, daylight, and nutrients. Predators and pathogens may also contribute to biodiversity because higher predation limits competition in the trophic levels, thus allowing more species to coexist. Explain how population stability might be enforced in this scenario and how you might approach proving a hypothesis linking predation to biodiversity.

2. Energy cycles through ecosystems at different trophic levels. At each trophic level, energy is reduced. Describe this process. What are some factors that affect energy transference and nutrient cycling? Consider photosynthesis, keystone species, decomposition, and bottom-up and top-down controls and discuss their relationship to energy reduction.

3. According to the principle of conservation of matter, the same amount of water exists as has always existed. If this is the case, how do you explain current water scarcity issues world populations face? What role does material cycling play in this scarcity? Explain.

4. The World Wildlife Fund (WWF) lists the following animals as the top ten most endangered species in the world ( /Ten-to-watch-in-2010): tiger, polar bear, Pacific walrus, magellanic penguin, leath- erback turtle, bluefin tuna, mountain gorilla, monarch butterfly, Javan rhinoceros, and giant panda. Are most of these r-selected or K-selected species? What is it about most of these species that will make their protection and long-term survival espe- cially difficult?

5. Trophic cascades can have both positive and negative effects. Why is it crucial for environmental agencies to predict whether or not a trophic cascade will produce the desired effect of their addition, elimination, or reduction of a species occupying an ecosystem niche? Can negative effects be desirable?

6. Imagine you are part of a conservation team tasked to restore a damaged ecosystem. Describe the succession stages that must occur based on the goals you have in mind for the project. you may speak in abstract terms discussing stages, trophic levels, and types of succession, or you might consider using an existing endangered ecosystem to explore this task in concrete terms.

7. Research on mercury contamination of wildlife reveals that this toxin is showing up in unexpected places and in unexpected species (such as polar bears in the arctic). how might mercury be ending up in such distant locations, and why are some of the highest levels of mercury contamination occurring in long-lived predators like polar bears?

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Key Terms

bioaccumulation The buildup of a sub- stance, such as a toxin, in an organism’s body.

biomagnification  An increase in the con- centration of a pollutant as you move higher up the food chain.

biome a major ecological community type characterized by a similar climate, soil, plants, and animals.

carrying capacity The maximum popula- tion of a particular species that an environ- ment can support over a given period of time.

coevolution The evolution of two or more species that occurs as a result of their interactions with each other over a period of time.

fundamental niche The full range of habi- tat types and resources a species can pos- sibly occupy and use without competition from other species.

generalists species with a broad ecological niche who can eat a variety of food and live in many types of environments.

gross primary productivity The total amount of new organic matter produced by photosynthesis.

keystone species A species upon which many other species depend and whose dis- appearance initiates significant changes in an ecosystem.

limiting factor A variable that limits the reproduction, growth, and/or survival of organisms.

lipophilic The tendency of many toxins to dissolve in fat.

mimicry The ability of certain species to blend in with their environment or to develop a resemblance to other species for the purpose of hunting prey or for protec- tion from a predator.

net primary productivity The amount of energy that remains in an ecosystem after cellular respiration has occurred.

niche The way of life or role of a species in an ecosystem.

photosynthesis The biological process that captures light energy and uses it to con- vert carbon dioxide and water into glucose (sugars).

prey An organism that is captured and serves as a source of food for another organ- ism (predator).

realized niche The range of habitat types and resources from which a species is not excluded by competition from other species.

specialists species with a narrow ecologi- cal niche who can only eat a few types of food or live in limited types of habitats.

succession The sequence of changes in an ecosystem over time.

trophic cascades The cascading effect that a change in the size of one population at the top of a food web has on the populations below it.

trophic levels sequential stages in a food chain where organisms that are the same number of steps away from the original energy source are grouped together.

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Additional Resources • For a detailed explanation of the characteristics of each of the world’s major biomes,

visit and also http://www.nature .com/scitable/knowledge/library/terrestrial-biomes-13236757 and then have a look at this clever music video describing the world’s major terrestrial biomes:

• For a detailed example and explanation of the concepts of trophic levels and energy transfer between producers and consumers, visit trophic/trophic2.html and also take a look at the easy-to-follow video at http:// This website gives an even more detailed review of photosynthesis and energy transfer through ecosystems ( -ecosystems-13254442) while this website gives a detailed review of the concept of primary production ( /terrestrial-primary-production-fuel-for-life-17567411).

• A couple of very simple animations that illustrate the nitrogen cycle can be found here _nitrogen/em05_pg20_nitrogen.html and here /asset/lsps07_int_nitrogen/. A much more complex representation of the nitrogen cycle, showing the impact of human activities on this process, can be found here A very detailed review of the law of conservation of mass and how this works in eco- systems can be found here /the-conservation-of-mass-17395478.

• A more in-depth explanation and discussion of population ecology concepts, especially as it applies to msy and managing game populations, can be found here http://www -game-populations-50937864. For a discussion of the concept of maximum sustained yield and some of the challenges of managing fish populations, visit http://assets

• This CNN piece explains the trophic cascade concept and the story of wolves in yellow- stone National Park ( while this video ( provides a very simple example of how trophic cascades work in a salt marsh ecosystem of Cape Cod. A more detailed discussion of the trophic cascade concept can be found here /knowledge/library/trophic-cascades-across-diverse-plant-ecosystems-80060347, and the article on trophic cascades and cougars in Zion National Park can be accessed here 042408/ripplebeschta.biolcons06.pdf.

• The Nature education Project has a series of readings on evolution and ecology. start with this introduction ( -13228138) and then move on to more detailed and challenging readings here http:// -traits-15164254, here -molecular-techniques-to-answer-ecological-questions-15643181, and here -species-26230527.

ben85927_01_c01.indd 61 1/20/14 1:03 PM

ben85927_01_c01.indd 62 1/20/14 1:03 PM

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