II.7 Predator–Prey Interactions
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II.7 Predator–Prey Interactions Robert F. Denno and Danny Lewis OUTLINE 1. Evidence that predators reduce prey populations 2. Reciprocal density effects and predator–prey cycles 3. Mathematical models of predator–prey interactions 4. Factors stabilizing predator–prey interactions and promoting their persistence 5. Predation in complex food webs 6. Predation, biodiversity, and biological control 7. Evolutionary interactions between predators and prey 8. Epilogue In natural food webs, consumers fall victim to other consumers such as predators, parasitoids, parasites, or pathogens . Predators kill and consume all or parts of their prey and do so either before or after their catch has reproduced. A lynx stalking, attacking, and consuming a snowshoe hare is an example from the vertebrate world. Spiders snaring moths in their webs, assassin bugs lancing caterpillars with their beaks, and starfish ravaging mussel beds in rocky intertidal habitats are all instances of invertebrate predation . By contrast, parasitoids such as small wasps and flies usually attack only the immature stages of their arthropod hosts, thus killing them before they reproduce . Parasites live on (e.g., fleas and lice) or in (e.g., tape worms) host tissues, often reducing the fitness of their host but not killing it. Pathogens (e.g., viruses, bacteria, and fungi) induce disease and either weaken or ultimately kill their hosts. Although this chapter focuses on predators, there are many similarities among predator–prey, host– parasitoid, and host–pathogen interactions. GLOSSARY food web. Network of feeding relationships among organisms in a community functional response. The relationship between prey density and the number of prey consumed by an individual predator intraguild predation. A predation event in which one member of the feeding guild preys on another member of the same guild (predators consuming predators) keystone species. A species that has a disproportionate effect on its environment relative to its abundance mesopredator. A predator that is fed on by another predator, usually a top carnivore numerical response. The relationship between the number of predators in an area and prey density omnivory. Feeding at more than one trophic level such as occurs when a predator consumes herbivores as well as other predators top carnivore. A predator at the top of the food chain feeding on organisms at lower trophic levels (e.g., mesopredators and herbivores) trophic cascade. Reciprocal predator–prey effects that alter the abundance, biomass, or productivity of a community across more than one trophic link in the food web (e.g., removing predators enhances herbivore density, which in turn diminishes plant biomass) Unlike many consumers, predators are often generalized in their feeding habits, consuming a diversity of prey species that can even represent different trophic groups. For instance, coyotes (top carnivores) feed on other predators such as foxes (mesopredators), both of which consume rabbits (herbivores) and opossums (omnivores ). When predators consume other predator species, the act is called intraguild predation, whereas cannibalism occurs when predators consume members of their own species. Wolf spiders (Lycosa and Pardosa), for example, are notoriously cannibalistic, consuming smaller individuals in the population and even their own offspring. Although many predators are generalists, feeding on a diversity of prey species, there are some very specialized feeders. Desert horned lizards (Phrynosoma platyrhinos) are ant specialists, and ground beetles in the genus Scaphinotus feed selectively on mollusks and have a long head and mandibles adapted for reaching deep into snail shells. Predation can have widespread ecological, evolutionary , and economic effects on biological communities in both natural and managed habitats. Predation, for instance, can be a powerful evolutionary force with natural selection favoring more effective predators and less vulnerable prey. In an ecological sense, predators can dramatically affect the abundance and distribution of their prey populations. Moreover, the diverse feeding habits of predators form linkages that are responsible for the flow of energy through food webs, thus affecting food-web dynamics. Predators can act as keystone species, preventing superior competitors from dominating the community and promoting biodiversity at lower trophic levels. In contrast, the invasion of native ecosystems by exotic predators often has very negative effects on resident prey species. On a more positive note, invertebrate predators have been used as effective control agents of agricultural pests, increasing crop yields without the adverse consequences of pesticides . Thus, in both theoretical and applied contexts, it is imperative to understand the process of predation and its complex effects on species interactions, foodweb dynamics, and biodiversity. In the remainder of this article, we explore critical elements of predator– prey interactions, namely how predators and prey in- fluence...