Although we have focused on a two-species system, almost all communities have more than two species. Prey-predator interactions play a major role in food web structure.
There is some evidence to support the hypothesis that a predator's choice of prey is affected by relative prey abundance. Because of spatial heterogeneity, a predator may not encounter all possible prey species in its feeding area. For example, if species 1 and 2 exist in different areas, the predator will consume each prey in its respective area. In each area, there is a simple relationship between one prey and one predator, whereas a two-prey, one-predator system exists in the total habitat. In a similar way, if a predator consumes prey 1 in the summer and prey 2 in the winter, a simple relationship exists in each season. Therefore, it is important to discriminate temporal and spatial scales for food web structures. Population dynamics depend on the long-term structure of food webs in the overall habitat. Evolutionary changes due to genetic traits work on the same scale as population dynamics. In contrast, optimal behavior likely depends on the local and temporal conditions in the feeding area. Food web structure obtained by a long-term field survey is usually much more complicated than food webs obtained by short-term or local experiments.
One of the biggest problems in the ecology of prey-predator systems is a lack of empirical evidence. Population densities are rarely assessed from fossil records. Many empirical studies of prey-predator systems avoid using long-lived, large-bodied predators. There is a gap between the species that are used in studies of optimal behavior and studies of prey-predator systems, despite the fact that adaptive strategies depend on changes in population size, and that prey-predator dynamics depend on behavioral changes in either prey or predators. However, geographic, chemical, and genetic techniques will help to bridge the gap between evolution and ecology in empirical studies of prey-predator systems.
See also: Animal Prey Defenses; Prey-Predator Models.
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