16. Community Interactions


16.7. The Impact of Human Actions on Communities


There is very little of the Earth’s surface that has not been altered by human activity. Often, the changes we cause have far greater impact than we might think (How Science Works 16.1). All organisms are interlinked in a complex network of relationships. Therefore, before changing a community, it is wise to analyze how its organisms are interrelated. This is not always easy, because there is much we still do not know about how organisms interact and how they use molecules from their environment. Several lessons can be learned from studying the effects of human activity on communities.



Whole Ecosystem Experiments

Many environmental issues are difficult to resolve because, although there are hypotheses about what is causing a problem, the validity of the hypotheses has not been tested by experiments. Therefore, when governments seek to set policy, there are always those who argue that there is little hard evidence that the problem is real or that the cause of the problem has not been identified. Several examples include: What causes eutrophication of lakes? What causes acidification of lakes and rivers? What are the causes of global climate change? What is the likelihood that emissions from coal-fired power plants are causing increased in mercury in fish? The most powerful tests of hypotheses related to these problems are experiments that take place on a large scale in natural settings. Several such experiments have been crucial in identifying causes of environmental problems and led to policy changes that have alleviated environmental problems.

Beginning in 1966, the Canadian government established the Experimental Lakes Area in western Ontario. Many lakes were designated for experiments that would help answer questions about environmental issues. One experiment tested the hypothesis that phosphorus was responsible for eutrophication (excessive growth of algae and plants) of lakes. Laboratory studies had suggested that carbon, nitrogen, or phosphorus could be responsible. To help answer which of these three nutrients was the cause of eutrophication, a dumb-bell-shaped lake was divided in two at its narrow "waist" by placing a plastic curtain across the lake. One portion had carbon, nitrogen, and phosphorus added to it and the other portion had only carbon and nitrogen added. The results were clear. The portion of the lake with the added phosphorus had an abundant growth of algae and turned green. The other portion of the lake with carbon and nitrogen but no phosphorus did not (see photo). As a result of this experiment, governments were justified in requiring detergent manufacturers to remove phosphorous compounds from their products and requiring sewage treatment plants to eliminate phosphorus from their effluent.

Other experiments on whole lakes have investigated:

• The effects of acid deposition on food webs in lakes— predator fish starve as their prey disappear.

• The effects of flooding of land by dams—there is an increase in the mercury content of fish and carbon dioxide and methane are released into the atmosphere.

• The effects of removal of aquatic vegetation—northern pike populations declined.

After each experiment, the Canadian government requires that the lake be returned to its pristine condition.




Introduced Species

One of the most far-reaching effects humans have had on natural communities involves the introduction of foreign species. Many of these introductions have been conscious decisions. Nearly all domesticated plants and animals in North America are introductions from elsewhere. Corn, beans, sunflowers, squash, and turkeys are exceptions. Cattle, horses, pigs, goats, and many introduced grasses have significantly altered the original communities present in the Americas. Cattle have replaced the original grazers on grasslands. Pigs have become a major problem in Hawaii and many other places in the world, where they destroy the natural community by digging up roots and preventing the reproduction of native plants. The introduction of grasses as food for cattle has resulted in the decline of many native species of grasses and other plants that were originally part of grassland communities. In Australia, the introduction of domesticated plants and animals—as well as wild animals, such as rabbits and foxes—has severely reduced the populations of many native marsupial mammals.

Accidental introductions have also significantly altered communities. Chestnut blight has essentially eliminated the American chestnut from the forests of eastern North America. Similarly, a fungal disease (Dutch elm disease) has severely reduced the number of elms. The accidental introduction of zebra mussels has significantly altered freshwater communities in eastern North America and has severely reduced the populations of native clams. Figure 16.34 shows two introduced species that have significantly altered communities in North America.



FIGURE 16.34. Introduced Species

Many introduced species become pests. Dandelions and mute swans are two examples.


Predator Control

Many kinds of large predators were actively destroyed to protect livestock. Predator control was also thought to be important in the management of game species. During the formative years of wildlife management, it was thought that populations of game species could be increased if the populations of their predators were reduced. For these reasons, many states passed laws to encourage the killing of foxes, eagles, hawks, owls, coyotes, cougars, and other predators that could kill livestock or use game animals as a source of food. Often, bounties were paid to people who killed these predators (figure 16.35).



FIGURE 16.35. Predator Control

At one time, people were paid to kill various kinds of predators. (a) These men receive $35 for each wolf killed. (b) This coyote was killed and hung on a fence because it was considered a threat to livestock.


The absence of predators can lead to many kinds of problems. In many metropolitan areas, deer have become pests. This is due to several reasons, including the fact that there are no predators, and hunting (predation by humans) either is not allowed or is impractical because of the highly urbanized nature of the area. Some municipalities have instituted programs of chemical birth control for their deer populations.

Only a few years ago, the alligator was on the endangered species list and all hunting was suspended. However, today increased numbers of alligators present a danger, particularly to pets and children. Hunting is now allowed in an effort to control the numbers of alligators, because humans are the only effective predators of large alligators.

Similarly, in Yellowstone National Park, elk, bison, and moose populations had become very large, because hunting is not allowed and predator populations had been small. In 1995, wolves were reintroduced to the park in the hope that they would help bring the elk and moose populations under control (figure 16.36). This was a controversial decision, however, because ranchers in the vicinity did not want a return of large predators that might prey on their livestock. They are also opposed to having bison, many of which carry a disease that can affect cattle, stray onto their land. The wolf population has increased significantly in Yellowstone and is having an effect on the populations of bison, elk, and moose. Regardless of the politics involved in the decision, Yellowstone is in a more natural condition today, with wolves present, than it was prior to 1995.



FIGURE 16.36. The Reintroduction of Wolves to Yellowstone

In 1995, wolves were captured in Canada and moved to Yellowstone National Park to reestablish a wolf population. Now, there is a large, healthy wolf population in Yellowstone.


By contrast, the state of Alaska allows the hunting of wolves because many Alaskans believe the wolves are reducing caribou populations below optimal levels. Caribou hunting is an important source of food for Alaskan natives, and hunters who visit the state provide a significant source of income. Many groups oppose the killing of wolves in Alaska, however. They consider the policy misguided and believe it will not have a positive effect on the caribou population.

As wolf populations have increased in parts of the northern Rocky Mountains and the Great Lakes region, they have been removed from the federal endangered species list. Several states have re-established hunting seasons for wolves. The primary justification is to minimize the impact of wolf predation on livestock.


Habitat Destruction

Some communities are fragile and easily destroyed by human activity, whereas others seem able to resist human interference. Communities with a wide variety of organisms that show a high level of interaction are more resistant than are those with few organisms and little interaction. In general, the more complex a community, the more likely it is to recover after being disturbed. The tundra biome is an example of a community with relatively few organisms and interactions. It is not very resistant to change and, because of its slow rate of repair, damage caused by human activity may persist for hundreds of years.

Some species are more resistant to human activity than are others. Rabbits, starlings, skunks, and many kinds of insects and plants are able to maintain high populations despite human activity. Indeed, some may even be encouraged by human activity. By contrast, whales, condors, eagles, and many plant and insect species are not able to resist human interference very well. For most of these endangered species, it is not humans’ acting directly with the organisms that cause their endangerment. Very few organisms have been driven to extinction by hunting or direct exploitation. Usually, the cause of extinction or endangerment is an indirect effect of habitat destruction as humans exploit natural communities. As humans convert land to farming, grazing, commercial forestry, development, and special wildlife management areas, natural communities are disrupted, and plants and animals with narrow niches tend to be eliminated, because they lose critical resources in their environment. Figure 16.37 shows various kinds of habitat destruction, and table 16.3 lists eight endangered and threatened species and the probable causes of their difficulties.



FIGURE 16.37. Habitat Destruction

Habitat destruction by the building of cities, conversion of land to agriculture, and commercial forest practices has a major impact on the kinds of organisms that can survive in our world.


TABLE 16.3. Endangered and Threatened Species



Reason for Endangerment

Hawaiian crow (Corvis hawaiinsis)

Predation by introduced cat and mongoose, disease, habitat destruction

Sonora chub (Gila ditaenia)

Competition with introduced species in streams in Arizona and Mexico

Black-footed ferret (Mustela nigripes)

The poisoning of prairie dogs (their primary food)

Snail kite (Rostrhamus sociabilis)

Specialized eating habits (they eat only apple snails), the draining of marshes in Florida

Grizzly bear (Ursus arctos)

The loss of wilderness areas

California condor (Gymnogyps californianus)

Slow breeding, lead poisoning

Ringed sawback turtle (Graptemys oculifera)

The modification of habitat by the construction of a reservoir in Mississippi that reduced their primary food source

Scrub mint (Dicerandra frutescens)

The conversion of their habitat to citrus groves and housing in Florida


Pesticide Use

Humans have developed a variety of chemicals to control specific pests. These chemicals have a variety of names. Herbicides are used to kill plants. Insecticides are used to kill insects. Fungicides are used to kill fungi. Often, all these kinds of chemicals are lumped together into one large category—pesticides—because they are used to control various kinds of pests. Although various kinds of pesticides are valuable in controlling disease in human and domesticated animal populations and in controlling pests in agriculture, they have some negative community effects as well.

One problem associated with continual pesticide use is that pests become resistant to these chemicals. When a pesticide is used, most of the pests are killed. However, some may be able to resist its effects. When these survivors reproduce, they pass on their genes for resistance to their offspring, and the next generation is less susceptible to the pesticide. Ultimately, resistant populations develop and the pesticide is no longer useful.

Another problem associated with pesticide use is the effects they have on valuable nontarget organisms. Insecticides typically kill a wide variety of organisms other than the targeted pest species. Often, other species in the community have a role in controlling pests. Predators kill pests and parasites use them as hosts. Generally, predators and parasites reproduce more slowly than their prey or host species. Because of this, the use of a nonspecific insecticide may indirectly make controlling a pest more difficult. If such an insecticide is applied to an area, the pest is killed, but so are its predators and parasites. Because the herbivore pest reproduces faster than its predators and parasites, the pest population rebounds quickly, unchecked by natural predation and parasitism (figure 16.38).



FIGURE 16.38. Pesticide Use

Pesticides are used to control pests that injure or compete with crops and domesticated animals. Their use has several effects on the communities of plants and animals where they are used.


Today, a more enlightened approach to pest control is integrated pest management, which uses a variety of approaches to reduce pest populations. Integrated pest management includes the use of pesticides as part of a pest control program, but it also includes strategies such as encouraging the natural enemies of pests, changing farming practices to discourage pests, changing the mix of crops grown, and accepting low levels of crop damage as an alternative to costly pesticide applications.



The use of persistent chemicals has an effect on the food chain. Chemicals that do not break down are passed from one organism to the next, and organisms at higher trophic levels tend to accumulate larger amounts than the organisms they feed on. This situation is known as biomagnification.

The history of DDT use illustrates the problems associated with persistent organic molecules. DDT was a very effective insecticide because it was extremely toxic to insects but not very toxic to birds and mammals. It is also a very stable compound, which means that, once applied, it remained effective for a long time. However, when an aquatic area was sprayed with a small concentration of DDT, many kinds of organisms in the area can accumulate tiny quantities in their bodies. Because marshes and other wet places are good mosquito habitats, these areas were often sprayed to control these pests. Even algae and protozoa found in aquatic ecosystems accumulate persistent pesticides. They may accumulate concentrations in their cells that are 250 times more concentrated than the amount sprayed on the ecosystem. The algae and protozoa are eaten by insects, which in turn are eaten by frogs, fish, and other carnivores.

The concentration in frogs and fish may be 2,000 times the concentration sprayed. The birds that feed on the frogs and fish may accumulate concentrations that are as much as 80,000 times the original amount. Because DDT is relatively stable and is stored in the fat deposits of the organisms that take it in, what was originally a dilute concentration becomes more concentrated as it moves up the food chain (figure 16.39).



FIGURE 16.39. The Biomagnification of DDT

All the numbers shown are in parts per million (ppm). A concentration of one part per million means that, in a million equal parts of the organism, one of the parts would be DDT. Notice how the amount of DDT in the bodies of the organisms increases from producers to herbivores to carnivores. Because DDT is persistent, it builds up in the top trophic levels of the food chain.


Before DDT use was banned in the United States in 1972, many animals at higher trophic levels died as a result of lethal concentrations of the pesticide accumulated from the food they ate. Each step in the food chain accumulated some DDT; therefore, higher trophic levels had higher concentrations. Even if they were not killed directly by DDT, many birds at higher trophic levels, such as eagles, pelicans, and osprey, suffered reduced populations. This occurred because the DDT interfered with the female birds’ ability to produce eggshells. Thin eggshells are easily broken; thus, no live young hatched. Both the bald eagle and the brown pelican were placed on the endangered species list because their populations had dropped dramatically as a result of DDT poisoning. The ban on DDT use in the United States and Canada has resulted in an increase in the populations of both kinds of birds. Because the populations have recovered, the bald eagle was removed from the endangered species list in 2007 and the brown pelican in 2009.

Several other chemical compounds are also of concern today because they are biomagnified in food chains. These include polychlorinated biphenyls (PCBs), dioxins, methyl-mercury, and many other compounds. Because these compounds can reach high concentrations in fish that are at the top of the food chain, many states and countries publish advisories that caution people not to eat large quantities of these fish. When the production of PCBs was halted in 1979, the level of contamination in fish declined.



20. Why do DDT and PCBs increase in concentration in the bodies of organisms at higher trophic levels?

21. What is the most common form of habitat destruction practiced by humans?

22. List three introduced species that have become pests, and explain why they became pests.

23. What happens to a community of organisms when predators are eliminated?

24. Describe two negative consequences of using pesticides.



Each organism in a community occupies a specific space, known as its habitat, and has a specific functional role to play, known as its niche. An organism’s habitat is usually described in terms of a conspicuous element of its surroundings. The niche is difficult to describe, because it involves so many interactions with the physical environment and other living things.

Interactions between organisms fit into several categories. Predation is one organism benefiting (predator) at the expense of the organism killed and eaten (prey). Parasitism is one organism benefiting (parasite) by living in or on another organism (host) and deriving nourishment from it. Organisms that carry parasites from one host to another are called vectors. Commensal relationships exist when one organism is helped but the other is not affected. Mutualistic relationships benefit both organisms. Symbiosis is any interaction in which two organisms live together in a close physical relationship. Competition causes harm to both of the organisms involved, although one may be harmed more than the other and may become extinct, evolve into a different niche, or be forced to migrate.

A community consists of the interacting populations of organisms in an area. The organisms are interrelated in many ways in food chains, which interlock to create food webs. Because of this interlocking, changes in one part of the community can have effects elsewhere.

Major land-based regional communities are known as biomes. The temperate deciduous forest, boreal coniferous forest, tropical rainforest, temperate grassland, desert, savanna, temperate rainforest, and tundra are biomes.

Aquatic ecosystems can be divided into marine and freshwater systems. Several kinds of marine ecosystems are: pelagic, benthic, coral reef, abyssal, and shoreline ecosystems. Freshwater ecosystems are generally separated into streams and rivers in which the water is running downhill and lakes and ponds in which it is not.

Communities go through a series of predictable changes that lead to a relatively stable collection of plants and animals. This process of change is called succession, and the resulting stable unit is called a climax community.

Organisms within a community are interrelated in sensitive ways; thus, changing one part of a community can lead to unexpected consequences. The introduction of foreign species, predator-control practices, habitat destruction, pesticide use, and biomagnification of persistent toxic chemicals all have caused unanticipated changes in communities.


Basic Review

1. The role an organism plays in its surroundings is its

a. niche.

b. habitat.

c. community.

d. food web.

2. The kind of interrelationship between two organisms in which both are harmed is _____.

3. A desert is always characterized by

a. high temperature.

b. low amounts of precipitation.

c. few kinds of plants and animals.

d. sand.

4. When a community is naturally changing with the addition of new species of organisms and the loss of others, _____ is occurring.

5. When two organisms cooperate and both derive benefit from the relationship, it is known as commensalism. (T/F)

6. Most of the plants and animals involved in agriculture are introduced from other parts of the world. (T/F)

7. A biome that has trees adapted to long winters is known as a

a. tundra.

b. temperate deciduous forest.

c. boreal coniferous forest.

d. temperate rainforest.

8. If a forest is destroyed by a fire, it will eventually return to being a forest. This process is known as _____ succession.

9. A collection of organisms that interact with one another in an area is known as a

a. community.

b. biome.

c. succession.

d. biomagnification.

10. The idea that no two organisms can occupy the same niche is known as the competitive exclusion principle. (T/F)

11. The tundra biome has permafrost. (T/F)

12. Plankton organisms are strong swimmers. (T/F)

13. Which of the following organisms are common in freshwater ecosystems and rare in marine ecosystems?

a. algae

b. fish

c. zooplankton

d. insects

14. Which of the following have resulted in reductions in species native to an ecosystem?

a. introduced species

b. habitat destruction

c. pesticide use

d. all of the above

15. In marine ecosystems, pelagic organisms are located on the bottom. (T/F)



1. a 2. competition 3. b 4. succession 5. F 6. T 7. c 8. secondary 9. a 10. T 11. T 12. F 13. d 14. d 15. F


Thinking Critically

Natural and Managed Ecosystems

Farmers are managers of ecosystems. Consider a cornfield in Iowa. Describe five ways in which the cornfield ecosystem differs from the original prairie it replaced. At what trophic level does the farmer exist?