Chapter 16

Kangaroo rat. Experiments on seed-eating kangaroo rats and ants in Arizona have revealed that the two groups of herbivores compete with each other in complex ways. Pictured here is a bannertail kangaroo rat (Dipodomys spectabilis) with its cheek pouches stuffed with seeds.

Photo by Mary McDonald/naturepl.com.

In the deserts of the American Southwest, many species of herbivores make their living in unique ways. These include large mammals such as deer and antelope, several species of rodents, and numerous species of ants. For a long time, no one considered the possibility of strong competition among animal groups of very different sizes because ecologists assumed that when competition existed, only animals of similar size and type would compete with each other. In the 1970s, ecologist James Brown and his colleagues questioned this assumption. Their research revealed that herbivores of different sizes compete with each other, and that this competition can create a complex cascade of effects.

Brown and his colleagues conducted field experiments in which they manipulated the presence of seed-eating rodents and seed-eating ants. They built large fences around several plots of land that enclosed rodents alone, ants alone, rodents and ants together, or neither ants nor rodents. This allowed researchers to determine whether rodents and ants competed for their primary food resource, seeds.

The enclosure experiment provided clear evidence that rodents and ants compete for seeds. For example, when ants were excluded, the number of rodents increased by 18 percent and individual rodent growth increased by 24 percent. When the rodents were excluded, the number of ant colonies increased by 71 percent. The researchers also found that when either group was excluded, seed abundance in the enclosures increased substantially but when both groups were excluded, the number of seeds increased even more. These results provided further evidence that the two groups compete for seeds. The researchers also quantified the seed preferences of each group; although rodents preferred larger seeds and ants preferred smaller seeds, the size of seeds that both consumed overlapped a good deal.

The interactions between rodents and ants also shaped competition among plants in the experimental plots. When either rodents or ants were excluded, more seeds were left to germinate. This increased the plant density and led to more competition among plants. Although more competition did not reduce plant survival, it did reduce plant growth and reproduction.

For 4 decades researchers continued to observe the original plots as well as additional plots they created later. Over this period they saw a number of surprising effects. For example, they found that over the long term rodents actually helped provide more seeds for the ants. During the first year, eliminating rodents from the enclosures caused an increase in the number of ants, but several years later the number of ants in these enclosures began to decline. To determine the reason for the decline in ants, the researchers looked carefully at the seed diets of the two competing groups and found that the combination of rodents and ants collectively consumed the entire range of seed sizes. Competition for seeds kept rodent and ant densities sufficiently low so that neither species could eliminate its seed sources. It also promoted a diversity of plants that continually produced a full range of seed sizes. However, when rodents were excluded from the enclosures for several years, ants continued to consume the smallest seeds. With few small seeds left to germinate, the plant species that make small seeds dwindled. In addition, small-seed plant species were competitively inferior to the large-seed plant species. Because of these factors, plants with small seeds declined so much that ants no longer had enough food, and the ant population declined. In short, while rodents and ants compete over the short term, the ants rely on the rodents over the long term to maintain an adequate supply of plants that have the smaller seeds they can eat.

The interactions between ants and rodents in the desert highlights the fact that limited resources can control the abundance of competing populations. Such competition is pervasive and the interactions between competing species can be complex. In this chapter, we will see that competition can occur in a wide variety of ways that shape the composition of communities.

In Chapter 1, we defined competition as a negative interaction between two species that depend on the same limiting resource to survive, grow, and reproduce. Competition can help determine where a species can live in nature and how abundant a population can become. It can occur among many groups of organisms including predators, herbivores, and parasites.

In this chapter, we will explore different types of competition and the resources for which species compete. We will then examine models of competition, which extend the Lotka-Volterra models introduced in Chapter 14. Competition models allow us to predict the conditions that determine when a species will win a competitive interaction. Knowing how species compete for a resource is important, but we must also consider how other interactions might alter or even reverse the expected outcomes of competition, including abiotic effects, disturbances, and interactions with other species. At the end of the chapter we will examine a variety of cases that appear to describe competition but actually reflect other processes, including predation and herbivory.