Effects of enrichment on three-level food chains with omnivory

Although omnivory (the consumption of resources from more than one trophic level) is widespread, this fundamental limitation to the applicability of f ood chain theory to real communities has received only limited treatment. W e investigated effects of enrichment (increasing carrying capacity, K, of t he resource) on a system consisting of a resource (R), an intermediate cons umer (N), and an omnivore (P) using a general mathematical model and tested the relevance of some of its predictions to a laboratory system of mixed b acteria (=R) and the ciliates Tetrahymena (=N) and Blepharisma (=P). The mo del produced six major predictions. First, N may facilitate or inhibit P. E nrichment may revert the net effect of N on P from facilitation to inhibiti on. Second, along a gradient of K, up to four regions of invasibility and s table coexistence of N and P may exist. At the lowest K, only R is present. At somewhat higher K, N can coexist with R. At intermediate it, either N a nd P coexist, or either consumer excludes the other clef ending on initial conditions. At the highest K, N may be excluded through apparent competitio n and only R and P can coexist. The pattern of persistence of Tetrahymena a nd Blepharisma along an enrichment gradient conformed fairly well to the sc enario allowing coexistence at intermediate K. Third, fur stable equilibria of the omnivory system, R always increases and N always decreases with R. The abundances of bacteria and Tetrahymena were suggestive of such a patter n but did not allow a strict test because coexistence occurred at only one level of enrichment. Fourth, an omnivore can invade an R-N system at a lowe r K than an otherwise identical specialist predator of N. Fifth, an omnivor e can always invade a food chain with such a specialist predator. Sixth, ov er ranges of K where both omnivory systems and otherwise identical three-le vel food chains are feasible, N is always less abundant in the omnivory sys tem, whereas the relative abundances of R and P in omnivory systems compare d to food chains may change with K. It is thus possible that total communit y biomass at a given It is lower in an omnivory system than in a food chain . Both the model and the experimental results caution that patterns of trop hic-level abundances in response to enrichment predicted by food chain theo ry are not to be expected in systems with significant omnivory.