The effects of small fish presence on a species-poor community dominated by omnivores: Example of a size-based trophic cascade

Abstract

Omnivory is important in determining species interactions and weakening possible trophic cascade effects. The present study is focused on determining if an indirect effect of fish presence (trophic cascade effect) can be observed regardless of a high abundance of omnivores. With this aim a field experiment was carried out in a Mediterranean salt marsh. The natural top predator of these systems is a small and endangered fish, the Iberian toothcarp (Aphanius iberus). We wanted to assess if the presence of this fish could trigger a trophic cascade in the aquatic community. Six mesocosms were installed in a salt marsh lagoon without A. iberus presence, for two months. Twenty adult females of A. iberus were added to three mesocosms (fish treatment), while the other three were used as control (without any A. iberus addition). Physical and chemical water characteristics, as well as aquatic invertebrate samples (from bacteria to amphipoda, the largest organisms detected beside fish) were collected at the begining and the end of the experiment. To assess the trophic position of organisms, stable isotope analyses were used (13C and 15N). Our results show that the presence of A. iberus produces a trophic cascade even though only two trophic levels were differentiated by the stable isotope analysis. Body size appears to be determinant, since trophic interactions observed within the food web were mainly body-size dependent. Moreover, an unexpected positive but weak effect of A. iberus on macrophyte biomass was observed, caused probably by the decrease of small phytoplankton abundance. This effect may suppose an advantage for A. iberus, since macrophytes provide refuge and a food supply for this small fish.