An ecological community consists of all the species—plants, animals, fungi, and microbes—that exist and interact in a given location. The interactions among the community members, such as those between a predator and its prey, shape ecological patterns and influence the evolution of populations (Figure 3.16). The web of interactions can be baffling. For example, hummingbirds dip their bills into deep flowers to drink nectar; at the same time, they inadvertently pollinate these plants in a mutually beneficial relationship, called a mutualism. Meanwhile, small insects, such as bees and flies, might visit these same flowers and compete with hummingbirds for nectar. If these bees are poor pollinators, then they impose a cost on both the flowers and the hummingbirds. Finally, it’s a little-known fact that hummingbirds sometimes eat insects for protein, so they may well snatch one of these flying insects in their flexible bills!

Nature is filled with elaborate mutualisms that go beyond pollination. For instance, Acacia ants live inside the thorns of some Acacia trees and protect them from herbivores; in exchange, they feed on a nutrient-rich liquid the plants secrete. Mycorrhizal fungi growing around plant roots help the plants acquire soil nutrients in exchange for energy-rich carbohydrates.

The ecological niche is a description of the physical and biological requirements of a species. A species’ niche includes where an organism lives—its habitat—and its trophic position in the ecosystem, for example, whether it’s a primary producer, a predator, or an herbivore (see Chapter 2 for more on trophic relations). However, the niche may also include such factors as the temperature range the species lives in, its water requirements, as well as when and where it feeds.

For instance, mountain gorilla habitat is mountain tropical rain forest, whereas that of the Bay checkerspot butterfly is temperate grassland. Both are herbivores; however, their approaches to herbivory are quite different. Mountain gorillas feed on more than 140 species of plants, which makes them a “generalist herbivore.” In contrast, Bay checkerspot caterpillars are “specialist herbivores” that feed mainly on just two plant species (Figure 3.17).

Competition involves interactions between individuals that depend on the same resources, such as food or space, for their survival and reproduction. Where the resources involved are in limited supply, competition for those resources may impact one or both of the competitors by reducing growth rates, reproductive rates, or probability of survival. Competition is most likely to occur between organisms with similar niches, since by definition they have similar resource needs. Because individuals of the same species share the same niche, competition is often strongest among them. This competition is called intraspecific competition, one of the density-dependent factors that regulate populations. Competition among individuals of different species, interspecific competition, is most likely to occur where the organisms have similar niches (Figure 3.18).

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Interspecific competition may influence the number and kinds of species that live together in a community. For instance, one species may exclude another from an ecological community and so reduce the total number of species living together. According to the competitive exclusion principle, if two species with identical niches compete for a limited resource (e.g., nectar), one or the other will be a better competitor and will eventually eliminate the other species. It follows from the competitive exclusion principle that when species live together, they generally have slightly different niches.

Studies of animals as diverse as insects, birds, fish, rodents, and lizards have shown that coexisting species use different resources, such as food, nesting sites, and feeding areas. Called resource partitioning, one of the best-known examples of the phenomenon occurs among coexisting species of woodland warblers. Up to five species of woodland warbler may live together in the forests of northeastern North America despite the fact that all species are about the same size and all eat insects. Since each species forages for insects in a different part of the trees, however, the spatial aspects of their feeding niches are different (Figure 3.19). Similarly, many hummingbirds coexist in the tropical rain forest, but they have bills of different lengths, which allow them to access different flowers.

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