Loops, or reciprocal predation, in which two species feed on each other or a third species feeds on one and is eaten by the other, should be rare or absent because the size range of prey is constrained by physical limits and because loops potentially reduce population recovery following disturbance (Pimm 1982, Pimm and Rice 1987). Intraguild predation involves predation among members of the predator guild on each other. Cannibalism is considered a "self-loop" (see Fox 1975a).
Polis (1991b) and Reagan et al. (1996) reported the occurrence of a substantial number of loops, especially involving arthropods. In most cases, each species in the loop preys on juveniles of the other species. For example, in a tropical forest in Puerto Rico, adult centipedes prey on young frogs, whereas adult frogs prey on young centipedes. Polis (1991b) reported that several species of desert ants regularly prey on each other. Other predators constituted 9% of the overall diet of the aquatic heteropteran, Notonecta hoffmanni, studied by Fox (1975b). Longer loops involving up to four species have been observed (Reagan et al. 1996). Reagan et al. (1996) found that 35% of 19,800 observed chains (corrected to exclude loops) include at least one species involved in at least one loop.
Furthermore, a number of studies have demonstrated significant reduction in predator abundances as a result of intraguild predation (e.g., Denno et al. 2004, Erbilgin et al. 2004, Perez-Lachaud et al. 2004, Rosenheim 2005). Rosenheim (2005) demonstrated through enclosure/exclosure experiments that abundance of the anthocorid bug, Orius tristicolor, was significantly reduced by intraguild predation by big-eyed bugs, Geocoris spp., and lacewing, Chrysoperla sp., larvae in cotton fields in California, United States, interfering with top-down control of spider mite, Tetranychus spp., prey. Reciprocal, or intraguild, predation may be pervasive within arthropod predator guilds, complicating measurement of food chain length and explaining irruptions of prey species when multiple predator species are present.
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