16. Community Interactions




Alien Invasion

Scientists concerned about survival of native plants.

Yard and garden centers often sell plant species that are not native to the area in which you live. Furthermore, homeowners often want unusual plants that are particularly colorful or have other striking characteristics. Some of these exotic plants are invasive. They have characteristics such as fruits or seeds that are easily spread from place to place. When this occurs, the exotic plant may become a pest because it competes with local native plants and replaces them, causing local extinctions of native species.

In the United States, there are many examples of exotic invasive species. Glossy buckthorn and autumn olive have replaced understory species in forests of the Northeast. Tamarisk (salt cedar) has become a dominant species along rivers in the Southwest. Brazilian pepper and Melaleuca have become major problems in south Florida. Kudzu (a vine) and water hyacinth have become significant problems in areas of the South. Purple loosestrife (see photo) has taken over wetlands in many areas of the northern parts of the United States and southern parts of Canada.

• What are the invasive exotic species found in your area?

• Why do some exotic species spread so rapidly?

• Should the kinds of plants you select to plant in your yard be regulated by state laws and/or local ordinances?


ü  Background Check

Concepts you should already know to get the most out of this chapter:

• The nature of food chains (chapter 15)

• The role of natural selection in shaping the evolution of organisms (chapter 13)


16.1. The Nature of Communities


Scientists approach the study of ecological interactions in different ways. For example, in chapter 15, we looked at ecological relationships from the point of view of ecosystems and the way energy and matter flow through them. But we can also study relationships at the community level and focus on the kinds of interactions that take place among organisms. Recall that a community consists of all the populations of different kinds of organisms that interact in a particular location.


Defining Community Boundaries

One of the first things a community ecologist must do is determine the boundaries of the community to be studied. A small pond is an example of a community with easily determined natural boundaries (figure 16.1). The water’s edge naturally defines the limits of this community. We would expect to find certain animals and plants living in the pond, such as fish, frogs, snails, insects, algae, pondweeds, bacteria, and fungi. But you might ask at this point, What about the plants and animals that live at the water’s edge? Are they part of the pond community? Or what about great blue herons that catch fish and frogs in the pond but build nests atop some tall trees away from the pond? Or should we include in this community the ducks that spend the night but fly off to feed elsewhere during the day? Should the deer that comes to the pond to drink at dusk be included? What originally seemed to be a clear example of a community has become less clear-cut.

The point of this discussion is that all community boundaries are artificial. However, defining boundaries—even if they are artificial—is important, because it allows us to focus on the changes that occur in a particular area, recognize patterns and trends, and make predictions.




FIGURE 16.1. A Pond Community

Although a pond seems an easy community to characterize, it interacts extensively with the surrounding land-based communities. Some of the organisms associated with a pond community are always present in the water (e.g., fish, pondweeds, clams); others occasionally venture from the water to the surrounding land (e.g., frogs, dragonflies, turtles, muskrats); still others are occasional or rare visitors (e.g., minks, heron, ducks).


Complexity and Stability

Each community has a particular combination of producers, consumers, and decomposers, which interact in many ways. Within the community, each species is a specialist in certain aspects of community function. One of the ways that organisms interact is by feeding on one another. Most organisms in a community participate in several food chains. When we recognize that many food chains in an area overlap, we see a pattern of interactions we can call a food web (figure 16.2).



FIGURE 16.2. A Food Web

As organisms feed on one another they establish a web of relationships known as a food web. This illustration shows the interactions between grazing and detrital food webs. In grazing food webs, photosynthesis by plants provides the energy for grazing animals that eat plants, which in turn provide energy for carnivores. Since all organisms die, they ultimately become part of a detrital food web in which dead organic matter and waste products supply the energy for a series of bacteria, fungi, and animals.


One of the common features of such a complex set of interrelationships is that natural communities are relatively stable. This stability allows us to identify and name various kinds of communities. In fact, there is a relationship between the degree of complexity and the stability of a community. Complex communities with many kinds of organisms interacting in numerous ways tend to be more stable than those that have few organisms and interactions. In other words, over hundreds of years, you would see the same kinds of organisms living in a woodlot, prairie, or ponderosa pine forest. On the other hand, many human-influenced communities are very unstable. Agricultural ecosystems tend to have very few kinds of organisms and cannot maintain themselves without human involvement.


Communities Are Dynamic

Although many kinds of communities are stable, that does not mean that there is no activity going on within the community. For example, while the human body is relatively stable, its stability results from the constant activity of various organs. Although at the broad level communities may appear to be unchanging, there is much change occurring among the organisms present. Producers are capturing sunlight, herbivores are eating plants, carnivores are eating animals, and decomposers are recycling materials. Populations of many organisms may rise and fall as well.

However, if the numbers of a particular kind of organism in a community increase or decrease significantly, some adjustment usually occurs in the populations of other organisms within the community. For example, the populations of many kinds of small mammals (mice, voles, lemmings, and so on) vary from year to year. If there are few small mammals to eat, the populations of their predators decrease. In addition, predators switch to different prey species, affecting other parts of the community.



1. Why is a food web better than a food chain as a way to describe a community?

2. As the number of insects declines in the autumn as the leaves fall from trees and the grass stops growing, what would you expect to happen to bird populations?