Omnivory

Technically, an omnivore is an animal that takes prey from more than one trophic level. Compilations of early descriptions of food webs indicated that omnivores are usually uncommon; this was taken to support expectations from simple model communities, where omnivory is destabilizing (Pimm, 1982). It was argued that in cases of omnivory, intermediate species both compete with and are preyed upon by top species and in consequence are unlikely to persist long. A more complex and realistic model incorporated 'life history omnivory', in which different life history stages of a species feed on different trophic levels, as when tadpoles are herbivores and adult frogs and toads are carnivores (Pimm & Rice, 1987). Life history omnivory also reduces stability, but much less than single life stage omnivory does. Intriguingly, omnivory is not destabilizing in donor-control models, and omnivores are common in decomposer food webs (Walter, 1987; Usio & Townsend, 2001; Woodward & Hildrew, 2002), to which donor-control dynamics can be applied.

In fact, an increasing number of studies indicate that omnivory is not uncommon at all, and that earlier indications of its rarity were an artefact of the webs being only poorly described (Polis & Strong, 1996; Winemiller, 1996). For example, Sprules and Bowerman (1988) found omnivory to be common in plankton food webs in North American glacial lakes, having identified all their zooplankton to species level and produced webs that were much more reliable as a result (Figure 20.17). Polis (1991) found similar results in his detailed study of a desert sand community. What is more, later modeling studies have undermined the whole suggestion that omnivory is inherently destabilizing. Dunne et al.'s (2002) simulation study detected no relationship between the level of omnivory and the stability of webs to species removal, while other models indicate that omnivory may in fact stabilize food webs (McCann & Hastings, 1997). It is sobering to note that

Figure 20.17 The prevalence of omnivory in glacial lakes in northeast North America (Sprules & Bowerman, 1988) is much greater than that observed in Briand's set of food webs (see Figure 20.9a). The degree of omnivory in a web is quantified as the number of closed omnivorous links divided by the number of top predators. A closed omnivorous link exists when a feeding path can be traced to a prey more than one trophic level away, and from that prey back to the predator through at least one other prey occupying an intermediate trophic level.

Figure 20.17 The prevalence of omnivory in glacial lakes in northeast North America (Sprules & Bowerman, 1988) is much greater than that observed in Briand's set of food webs (see Figure 20.9a). The degree of omnivory in a web is quantified as the number of closed omnivorous links divided by the number of top predators. A closed omnivorous link exists when a feeding path can be traced to a prey more than one trophic level away, and from that prey back to the predator through at least one other prey occupying an intermediate trophic level.

theoretical and empirical studies have managed to march in step twice in quick succession, but to quite different tunes. It reminds us that both sorts of study can only ever be as good as the assumptions on which they are inevitably based.

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