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Chapter 43: Communities and Ecosystems

Learning Objectives

1. To become familiar with community interactions

2. To become familiar with successional changes over time

3. To elucidate the energy relationships characteristic of ecosystems

4. To become familiar with biogeochemical cycles

5. To briefly evaluate various types of ecosystems

Key Concepts

1. A species' habitat is where it lives; its niche includes the resources it requires and all other factors that limit its distribution and abundance. Interspecific competition may lead to specialized adaptations that allow a species to exploit a subset of the resources in its habitat. Other interactions among species include symbiosis and predation.

2. In primary succession, a new community forms; in secondary succession, the types of resident species change after a disturbance to the area. In both types of succession, pioneer species pave the way for changes in the community and the physical environment.

3. Ecosystems range greatly in size, and are constantly changing and interacting. Food chains and webs are built on primary producers that pass energy to successive levels of consumers. Pyramid diagrams represent energy flow or numbers or biomass of organisms in an ecosystem.

4. Biogeochemical cycles describe how elements move between organisms and the physical environment. Elements from the environment ascend food webs, and return to the physical environment when an organism decomposes. Some chemicals are concentrated as they ascend food webs, with toxic effects.

5. Earth has many different ecosystems. Here is a look at a few of them.

Chapter Concept 43.1: A Community Includes All Life in an Area

A species' habitat is where it lives; its niche includes the resources it requires and all other factors that limit its distribution and abundance. Interspecific competition may lead to specialized adaptations that allow a species to exploit a subset of the resources in its habitat. Other interactions among species include symbiosis and predation.

Textbook Reading Assignment: Pages 844 - 848

Ecology is the study of the interactions of organisms with one another as well as with their physical environment. Recall from Chapter 1 that an individual organism is a member of several different hierarchical systems:

For example, an individual is a member of a population (a group of organisms of the same species inhabiting a common area). That same individual is also a member of a community (a group of interacting populations) as well as a member of an ecosystem (a community of organisms along with the nonliving components of the environment).

Ecosystems consist of both nonliving (abiotic) and living (biotic) components. The abiotic components of ecosystems include factors such as wind, rain, light intensity, temperature, salinity, and chemical substances in the soil, air, and water. The biotic portion of the ecosystem consists of three types of organisms:

  • producers (autotrophs) – harvest energy from solar radiation or inorganic molecules
  • consumers (heterotrophs) -- obtain energy from consuming producers or other organisms
  • scavengers and saprophytes – survive on the energy provided in the molecules of deceased organisms

Biologists use the following terms to describe the way in which nutrition is obtained within an ecosystem:

  • herbivores – consume only producers
  • carnivores – consume other consumers
  • omnivores – consume producers, herbivores, and other carnivores
  • decomposers – break down dead organic material

Ecosystems have many properties that can be associated with individual organisms. For example, ecosystems obtain energy from the environment, transform chemicals, change over time, respond to environmental changes, and use energy to maintain a stable state. All the ecosystems taken together compose the biosphere. The biosphere extends over the entire surface of the Earth. It begins 3 kilometers under the surface and extends high into the atmosphere.

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Discussion Question 1

Which of the following contains the greatest number of organisms: a community, a population, or an ecosystem?

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Web Activity

The following link provides a government report concerning the state of our nation’s ecosystems:


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The place in which an organism lives within an ecosystem is termed its habitat and the way in which an organism uses its environment is termed its niche.

The distinction between the fundamental niche and the niche which the species actually occupies (realized niche) is an important one. The fundamental niche is defined as the potential ability of a species to realize resources, while the realized niche consists of the resources that the species actually uses within a community.

In many cases, competition can lead to the depletion of a resource over time. As a result, competition between species has the potential to eliminate one of the competing species. The competitive exclusion principle suggests that when two species directly compete for the same limiting resource, one species can eliminate the other. In reality, the competitive exclusion principle suggests that no two organisms can exploit the same resources at the same time and place.

Figure 43.2
As evidenced by Figure 43.2 in the text, in order to allow stable coexistence between species within a community, the fundamental niche is usually reduced to a realized niche. In some instances, species may evolve differences reducing the amount of competition (interspecific competition) to allow stable coexistence within a community.

In this example, Chthamalus is capable of surviving in both the upper and middle intertidal zones (fundamental niche), however, in the presence of Balanus, it inhabits only the upper intertidal zone (realized niche) in an attempt to reduce interspecific competition.

Addditional examples of such behavior might include two species of caterpillars that can both consume apples and roses reducing competition by specialization-one may specialize on apples while the other on roses. Any such change in morphology, life history, or behavior resulting from competition is termed character displacement.

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Discussion Question 2

Why must interspecific competition (competition between species within a community) be reduced in order for the stable existence of both species? In what ways do species reduce the amount of interspecific competition within communities?

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According to the competitive exclusion principle, each species has its own niche within a community. In some instances, species coexist within communities by reducing the size of their niche to prevent overlap.

Figure 43.4

For example, the study on resource partitioning suggests that American Warblers split up their niches within spruce trees. In other words, each species of bird hunts insects in a different part of the tree and nests at a separate time from other warblers. These data indicate that by reducing niche overlap, species are able to coexist within a community. Such a scenario results in a decrease in competition between species within a community.

Interestingly enough, predation can serve to maintain diversity by preventing competitive exclusion. For example, some predators prevent common species from overwhelming less common ones. When one prey species becomes common, predators begin to prey preferentially on that species. Predators switch from prey to prey species according to which one is the most common at the time. A predator that serves to promote diversity in this fashion is termed a keystone species.

Community Interactions

Symbiosis is defined as an intimate association between two organisms within a community. Symbiotic interactions can be harmful to both organisms involved, beneficial to both, beneficial to one and harmful to the other, and beneficial to one and harmless to the other.

Table 43.1 presents the various types of symbiotic interactions as well as how the organisms are influenced:

1.  Mutualism is a type of symbiotic interaction in which both individuals benefit. A lichen, which is composed of both an algae and a fungus, is an example of a mutualistic interaction. In this type of interaction, the algae provide food via photosynthesis (which benefits the fungus), and the fungus provides a place to live (which benefits the algae). Your textbook provides an example of mutualism between a cow and the microorganism living in the rumen allowing the cow to digest cellulose..

2.  Predation and parasitism are both symbiotic interactions in which one member benefits at the expense of the other. A predator is a consumer that kills and usually digests its prey. A parasite consumes nutrients from its host but does not necessarily kill the host. In fact, a successful parasite will not kill its host -- if it did, where would it live?

3.  Competition is a type of symbiotic interaction in which the interaction negatively affects both members. For example, if you and your neighbor are both competing for the same seed bank, every seed your neighbor consumes is essentially taken away from you, and vice versa.

4.  Commensalism occurs when one member benefits from the symbiotic interaction, while the other member remains unaffected. For example, certain fish live among the poisonous tentacles of sea anemones, thereby benefiting from the protection of their host. The sea anemone remains unaffected by the presence of the fish.

In many instances, interactions between two species influence the evolution of both. For example, many pollinating insects specialize on flowers that, in turn, are specially constructed to be pollinated by just a single type of insect species. Obviously, these two species have depended on one another for survival and reproduction for a long period of time. The interdependent evolution of two or more species whose adaptations appear to be selected by mutual ecological interactions is termed coevolution.

How Do Plants and Animals Defend Themselves?

Figure 43.5

Most organisms have several ways to protect themselves against potential predators.

Many organisms use camouflage to remain hidden from predators . For example, many organisms mimic twigs, leaves and branches to avoid detection by other consumers. Some organisms utilize chemical defenses designed to ensure the survival of the potential prey victim. Such defenses are characteristic of plants, bees, ants, wasps, beetles, poison dart frogs and skunks.

Other organisms display bright warning coloration which serves to convey a message of avoidance to potential predators. In general, predators tend to avoid animals with coloration that they associate with pain, illness or other unpleasant experiences.

Since predators tend to avoid organisms displaying "warning coloration", evolution has favored prey species displaying such coloration. In fact, many non-poisonous organisms have used this to their advantage by developing such coloration even though they are not poisonous. Such a phenomenon is termed mimicry and it is of great benefit to the mimic. Batesian mimicry involves a harmless animal imitating a dangerous one. In contrast, Mullerian mimicry involves two or more dangerous species evolving similar color patterns.

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Web Activity

The following site provides some examples of both Batesian and Mullerian mimicry:


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Chapter Concept 43.2: Communities Change Over Time

In primary succession, a new community forms; in secondary succession, the types of resident species change after a disturbance to the area. In both types of succession, pioneer species pave the way for changes in the community and the physical environment.

Textbook Reading Assignment: Pages 848 - 850

Succession is the term used to describe the series of changes in number and kinds of organisms in a community over time. Ecologists distinguish two types of succession:

1. Primary succession - succession in an area where organisms have never been present.

2. Secondary succession - succession occurring in an area where an existing community has been severely destroyed by some disturbance.

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Discussion Question 3

According to the text, what type of succession occurred in Puerto Rico following Hurricane Hugo?

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A pioneer community is the first community in a succession and a climax community is the long-lived community that is the product of succession. Intermediate stages are occasionally termed successional communities. Pioneer and climax communities differ from one another in a number of ways. Pioneer communities consist of species that flourish in disturbed areas and reproduce rapidly. Climax communities consist of species that breed relatively slowly and gradually take over in undisturbed areas.

The reason for successional change over time is not clear. Some biologists argue that succession occurs because non-climax organisms degrade their environment while others argue that succession is due to varying growth rates. Whatever the reason, succession can be explained as a series of stereotypical changes over time.

Figure 43.7
In primary succession, pioneer organisms facilitate their own success by creating a thin layer of soil on bare rock or sand before other species can inhabit the area.

Figure 43.8

Secondary succession has a stereotypical sequence of events as well. During the process of secondary succession, the first plants to arrive are the annuals, which complete their life cycle in a single growing season. Within two years, annuals are replaced by perennials, which survive and produce seeds for two or more years. Once established, perennials outcompete annuals for light and water.

As succession continues, shrubs and sun-tolerant trees colonize the area and the resulting shade causes the demise of smaller sun-tolerant shrubs and herbs. These, in turn, are replaced by shade-tolerant ones. In time, shade-tolerant seedlings grow into shade-tolerant trees resulting in a climax forest. The characteristics of any climax community depend primarily upon climate, soil quality, terrain and the history of the area in question.

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Web Activity

The following government site contains an article on primary succession in the Hawaiian Islands:


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Chapter Concept 43.3: An Ecosystem Is a Community and Its Physical Environment

Ecosystems range greatly in size, and are constantly changing and interacting. Food chains and webs are built on primary producers that pass energy to successive levels of consumers. Pyramid diagrams represent energy flow or numbers or biomass of organisms in an ecosystem.

Textbook Reading Assignment: Pages 850 - 854

How Does Energy Flow Through Ecosystems?

While chemicals cycle through ecosystems, energy flow within ecosystems is unidirectional. Energy enters the ecosystem as solar radiation and is continually dissipated as it passes through the food chain.

Figure 43.11a
A food chain is a representation of who eats whom within an ecosystem. For example, the food chain described on the right includes a fox that eats a bird, which consumed a beetle, which munched on leaves. Each of the individual feeding levels within a food chain is termed a trophic level.

Organisms within a food chain are assigned to different trophic levels depending on where they obtain their energy. The first trophic level includes the primary producers, which obtain energy directly from the sun. The second trophic level includes the primary consumers, which obtain their energy directly from the primary producers. The third trophic level consists of the secondary consumers, which feed on the primary consumers and so on.

In reality, food chains become interconnected within ecosystems to form food webs, as depicted below:

Figure 43.10

Ecologists describe ecosystems in terms of biomass and productivity. Biomass is the total dry weight of all producers, consumers, and decomposers (scavengers and saprophytes) within a given ecosystem. Productivity represents a measure of the energy captured in the chemical bonds of new molecules per square meter of land each year.

Productivity values are relatively small with respect to the amount of solar radiation that actually contacts the Earth each year. The atmosphere reflects 30% of the solar radiation that reaches the Earth and absorbs 20% of it. Rocks, soil, and water absorb most remaining energy. As a result, biological organisms receive only a small percentage of the energy radiating from the sun.

Biologists can estimate how much of this solar energy is captured by producers. To do so, biologists must estimate the rates of photosynthesis and respiration in the producers. Since respiration consumes the glucose produced by photosynthesis, the difference between these values represents “net” stored energy, or primary productivity. Think of primary productivity as the amount of energy available within a trophic level for growth and reproduction.

Figure 43.11a
In addition to primary productivity, total biomass (the dry weight of all organisms within an ecosystem) is another important characteristic of an ecosystem. In general, the biomass within a trophic level is less than that of the level below it within the food chain (refer to the figure at the right).

For example, the biomass of primary producers is greater than that of the primary consumers. As a result, a graphic representation of food chain biomass resembles a pyramid in many cases (pyramid of biomass).

The total amount of energy within a food chain can be represented in a similar fashion. Because of their photosynthetic ability, the primary producers use solar energy to synthesize food that supplies the entire food chain.

Because every chemical reaction releases energy in the form of heat, the producers can only convert a fraction of the light energy into food. In addition, primary consumers use the chemical energy of the producers to form their own molecules. Once again, most of the available energy for work is lost as heat.

Figure 43.11a
As a result, the total amount of energy within a food chain decreases as you move from the primary producer to the top consumer. The resulting pyramid of energy represents the amount of energy at each trophic level within the food chain.

Figure 43.11b
Ecologists have estimated that approximately 10% of the energy available for growth and reproduction at one trophic level is passed on to the next. This is sometimes referred to as the ten percent law.

Figure 43.12a
Due to the fact that energy is lost as it is passed up the food chain, a pyramid of numbers is similar to the biomass and energy pyramids. In general, numbers of individuals decrease as you go from one trophic level to the next.

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Discussion Questions 4 and 5

The following question refers to the ten percent law: Assume that the total amount of energy the primary consumers take in is 2000 kcal/year. Only 10% of the 2000 (200 kcal/year) is made available to the secondary consumers. Likewise, only 10% of the 200 (20 kcal/year) is made available to the tertiary consumer. Using this information, can you explain why most food chains are limited to three or four trophic levels?

According to the text, why do parasitic organisms have an inverted pyramid of biomass?

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Web Activity

  The following article (http://www.cnn.com/2000/NATURE/07/14/food.chain.enn/index.html) concerns itself with the number of trophic levels found within ecosystems. Contrary to the traditional view, the author argues that the size of an ecosystem, not the amount of available food energy within it, determines the size of a food chain. What do you think about the author’s argument? How does it differ from the traditional view presented in the text?

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Chapter Concept 43.4: How Chemicals Cycle Within Ecosystems--Biogeochemical Cycles

Biogeochemical cycles describe how elements move between organisms and the physical environment. Elements from the environment ascend food webs, and return to the physical environment when an organism decomposes. Some chemicals are concentrated as they ascend food webs, with toxic effects.

Textbook Reading Assignment: Pages 855 - 860

Although the biosphere dissipates heat, it serves to recycle materials. Within ecosystems, materials like carbon, nitrogen, oxygen, and phosphorous continually recycle and are rarely lost. These cycles are termed biogeochemical cycles.

The Water Cycle

Figure 43.13
Without water, life on this Earth would cease to exist. Most of the Earth’s water (approximately 97%) is found in the oceans. The remainder is found in lakes, streams, seas, ponds, and rivers. The water in these sources, as well as in plants, is evaporated by solar radiation into the atmosphere. The resulting atmospheric water vapor condenses into rain, which falls back to the surface of the Earth. Most of this rain occurs over the continents, where it then returns to the ocean as runoff in streams and rivers. Sometimes groundwater is located within underground aquifers, which release water to wells or springs.

Figure 43.14

The Carbon Cycle

Most carbon in the biosphere is found as bicarbonate (HCO3-) in oceans and carbon dioxide (CO2) in the atmosphere. Each year, terrestrial producers convert approximately 12% of atmospheric carbon in carbon dioxide into complex organic compounds. This carbon is incorporated within the food chain, where it is eventually returned to the oceans and atmosphere, as carbon dioxide, through the process of aerobic respiration.

The Nitrogen Cycle

Molecular nitrogen (N2) is the most abundant element in the air we breathe. Yet, large quantities of this nitrogen are not available to organisms. Why is this the case?

Figure 43.15

One reason molecular nitrogen is in short supply is because it takes a large amount of energy to break the triple bond that holds the two nitrogen elements together.  Nitrogen fixation is accomplished by nitrifying bacteria in soil and water.  These organisms are capable of breaking this bond and converting the nitrogen into ammonia (NH3) and nitrate (NO3).  The resulting nitrogen is termed fixed nitrogen

Such a scenario allows nitrifying bacteria living in conjunction with certain plants to convert atmospheric nitrogen into a form that plants can use which is ultimately passed up the food chain. 

When organisms break down proteins during respiration, they release their nitrogen as ammonia (fish), urea (mammals), or uric acid (birds, reptiles and insects).  Decomposers then obtain nourishment from these products by converting them back into ammonia, which plants can then use again, completing the cycle.  Finally, denitrification, accomplished again by bacteria, converts nitrate back into nitrogen gas.

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Discussion Question 6

How would the absence of nitrifying bacteria affect life on Earth?

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Web Activity  

Visit http://www.wri.org/wri/wr-98-99/nutrient.htm for an article describing how nutrient overload affects the nitrogen cycle within ecosystems.

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Figure 43.16
The Phosphorous Cycle

The weathering of rocks makes phosphate ions (PO43- and HPO42-) available to plants which is incorporated into ATP or DNA/RNA nucleotides. This phosphate is then passed on to the next trophic level when consumers eat the producers. Consumers assimilate this phosphate into teeth, bones, shells, etc. As these organisms die, their phosphates, once again, become available for plants to repeat the cycle.

Notice from the diagram on the right that some phosphates run into aquatic systems where it is incorporated by photosynthetic algae. These phosphates eventually become part of sedimentation and are re-exposed to weathering after upheaval.


Soil, water, and air pollution are all extensions of a growing human population. Some materials released into the environment may be of little concern, while in larger quantities they overburden natural food chains and create serious side effects.

Figure 43.18
A classic example is the pesticide DDT. Developed prior to World War II, DDT was the first major synthetic organic chemical pesticide. While DDT, and some of its derivatives, were applied in small amounts within ecosystems, once they found their way into food chains, they were passed up the chain, increasing in concentration at each new trophic level.

Contaminants that can persist and move through food chains, increasing their concentration at each succeeding trophic level, are said to exhibit biomagnification. To reduce the chance of biomagnification, chemicals are now designed to begin to break down once they are exposed to the environment.

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Web Activity

The following site provides an article discussing the population rebound of the bald eagle as a result of the cessation of DDT spraying within ecosystems: http://www.edf.org/pubs/NewsReleases/1997/Jun/e_ddt.html

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Chapter Concept 43.5: A Sampling of Ecosystems

Earth has many different ecosystems. Here is a look at a few of them.

Textbook Reading Assignment: Pages 860 - 864

Don't worry too much about this section of the chapter; it is meant to introduce you to the characteristics of a few ecosystems. Briefly read through it and be familiar with some of the differences between these ecosystem types. I will not ask you to recall any of the detail provided in this section of the text.


Self Test Questions

1.Which of the following is composed of communities?

a. ecosystem

b. population

c. community

d. tissue

2. Which of the following is a biotic component of an ecosystem?

a. rain

b. organisms

c. soil nutrients

d. salinity

3. Which trophic level within a food chain contains the autotrophs?

a. primary consumer

b. secondary consumer

c. secondary producer

d. primary producer

4. Most dead organisms are decomposed by _____________, which live off the energy stored in particles of dead organisms.

a. saprophytes

b. omnivores

c. sporophytes

d. gametophytes

5. Approximately what percentage of the energy at one trophic level is made available for growth and reproduction at the next?

a. 10%

b.  15%

c. 70%

d. 90%

6. Which of the following represents a measurement of the dry weight of all the organisms living within an ecosystem?

a. productivity

b. biomass

c.  food web

d. biogeochemical cycles

7. In the water cycle, atmospheric water is returned to Earth via

a. transpiration

b. evaporation

c. rain

d. percolation

8. A __________ species is the first to invade during primary succession..

a. pioneer

b. climax

c. succession

d. keystone

9. Examples of fixed nitrogen include

a. ammonium ions

b. nitrate ions

c. molecular nitrogen

d. A and b are correct.

10. The conversion of nitrate into molecular nitrogen is termed

a. nitrification

b. denitrification

c. ammonification

d. condensation

Answers to Self Test Questions

1. a

2. b

3. d

4. a

5. a

6. b

7. c

8. a

9. d

10. b

Answers/Insight to Discussion Questions

1. ecosystem

2. Interspecific competition must be reduced within organisms in order to allow species to coexist. If too much competition exists between species, individuals eventually perish since they are all competing for the same resources. Species reduce the amount of interspecific competition by specializing on different resources.

3. Secondary succession

4. Most food chains are limited to four trophic levels due to the fact that only 10% of the energy available at one trophic level is made available for growth and reproduction at the next. As a result, ecosystems only have enough energy to sustain three to four trophic levels (in other words, ecosystems do not have enough energy to sustain a fifth or sixth trophic level—the energy runs out of the food chain before these levels are reached).

5. Because a single large animal supports many individual parasites, an inverted pyramid of numbers results.

6. In the absence of nitrifying bacteria, N2 could not be broken down and, therefore, would not be made available for living organisms.

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