THE ROLE OF OMNIVORY IN THE DYNAMICS OF FOOD CHAINS
Keywords:Omnivory, Adaptative forraging, Stability, Food chain.
Food chain theory is one of the most tested ecological theories and has been applied with a relative success in the restoration of eutrophic temperate lakes. However, its applicability to the restoration of eutrophic tropical lakes has been questioned as some of its assumptions ignore common features that are dynamically important in food chains of these systems, which includes the high degree of omnivory found among tropical fish. The aim of this work was to theoretically investigate the consequences of omnivory by predators for the population dynamics of autotrophs and herbivores in a food chain with three trophic levels. Two differential equations were used to describe the dynamics of the system along a gradient of predator density and prey preference by predators. The results from the graphical analysis of isoclines show that increasing predator density and/or prey preference for herbivores tends to stabilize the dynamics of interactions between autotrophs and herbivores. On the other hand, some degree of omnivory seems to be necessary to allow the persistence of herbivores in the system when predator density is high. This result is consistent with the idea that food chain with weak omnivorous interactions are more persistent and, therefore, more stable than food chains without omnivory. The mechanism responsible for the stabilizing effect of omnivory in our model is the prey switching by predator, which change the predator functional response from a type II to a type III as the degree of omnivory increases. Therefore, we conclude that an adaptative foraging of omnivorous predators enhance the persistence of herbivores in the system, at least in the absence of a numerical response of predators.