Effects of food web complexity on top-down control in tropical lakes

Top-down control in ecosystems is dependent on food web structure. In this study, we developed 126 models describing different trophic link combinations in order to assess the effects of food web structure on the top-down response of shallow tropical lakes. We evaluated the effects of the presence of invertebrate predators, large-bodied herbivorous zooplankton and the degree of omnivory. The results showed that the presence of invertebrate predators and large-bodied herbivorous zooplankton can invert the relation between planktivorous/omnivorous fish and producers (algae). The fact that large herbivores are absent in tropical lakes and invertebrate predators are present in large quantities results in a positive correlation between piscivorous fish and algae biomass, contradicting the classical top-down response described for temperate lakes. We show that omnivory should not be analyzed as a feeding strategy in itself. Omnivory affects many food web processes and its effects are dependent on the trophic level. In our models, omnivory in intermediate trophic levels dampened the top-down control by fish, but omnivory in top trophic levels has an opposite effect increasing the fish carrying capacity and also the strength of the top-down trophic cascade, while simultaneously decreasing the shortest chain length between fish and algae, thus reversing the relation between these two trophic levels.