Role of body size and habitat complexity in the diet of the invasive Micropterus salmoides (Lacépède). Optimal foraging theory matters

Although biological invasions are becoming more frequent, their underlying ecologi- cal mechanisms often remain unknown. One of the most poorly understood aspects is the relationship between ontogenesis and the trophic role of alien spe- cies in invaded ecosystems. The aim of this study was to investigate the role of littoral habitat complexity in the shift from benthivory to piscivory of Microp‐ terus salmoides, one of the most widely introduced and invasive fish species. Specifically, populations from two habitats differing in terms of aquatic veg- etation cover within the same ecosystem were stud- ied. Specimens of M. salmoides and its potential prey collected in both habitats were analyzed for C and N stable isotopes. The consumption of macroinverte- brates decreased with body size, but in the less com- plex habitat, M. salmoides shifted its diet to piscivory at an earlier stage of its life cycle. In this habitat, the size-based food web appears highly connected, as largemouth bass have diffuse weak interactions with multiple prey species occupying a range of trophic levels. This may lead bass to threaten the native fish not only by competition but also by predation. Large piscivorous individuals preferred conspecific fry as prey, on which they concentrated particularly in the more complex habitats, where diet specialization was marked. Since preying on conspecifics is energetically costly, according to Optimal Foraging Theory it only becomes advantageous when competition for other food items is intense. This evident trophic plasticity may favor the success of bass in invaded ecosystems and should be considered when deciding management policy, which should also include the preservation of habitat complexity.