Can omnivory explain variation in size-density relationships? The case of Leptonema in a neotropical stream

Abstract

Community size structure potentially depicts the constant amount of small abundant organisms sustaining larger, rarer ones. Strong size structuring is expected under a constant predator-prey mass ratio yet omnivory, as defined by organisms feeding at multiple trophic levels, can disrupt this pattern. Here, we investigated the size structure of macroinvertebrate communities in two tropical streams before and after a heavy rain event. We found that one taxon, Leptonema (a net-spinning caddisfly), was overabundant at all sites, strongly deviating from the allometric expectation given their body size. By analyzing their gut-content, we found evidence of true omnivory with inclusion of animal prey with increasing body size. As omnivory is a key factor in ecosystem stability, the presence of overabundant species could be an important element to consider when implementing conservation and restoration policies.