Top predators, are expected to have a stabilizing role in food webs due to their high mobility and ability to couple spatially separate habitats. However, the strength of interactions across habitats depends on the degree to which the predator generalize among habitat specific food resources. Resource generalization/specialization, in turn, depends on the constraints of the predator to flexibly change in behaviour and other foraging traits as morphology. We investigated the relationship between individual specialization and habitat coupling performance in Eurasian perch (Perca fluvialis), a common top predator in lakes across Europe. We sampled perch from two spatially separate habitats, pelagic and littoral sites in Lake Erken, Sweden. We measured individual diet and morphological specialization by using gut content and stable isotope analyses. We also assessed whether there was a genetic differentiation in perch between the different habitats using microsatellite analyses.
We show that habitat diversity and resource availability influence perch individual specialization and morphological variation. The morphological difference between littoral and pelagic habitats was consistent with previous observations of perch morphology: deeper body shape in the littoral habitat and more fusiform body shape in the pelagic habitat. We observed a stronger individual specialization of perch in the littoral habitat than perch in the pelagic habitat. However, littoral perch also used resources from the pelagic habitat, indicating a stronger ability to couple habitats. In contrast, fish from the pelagic habitat showed weaker individual specialization and a near complete reliance on pelagic resources, indicating a strongly reduced ability to couple habitats. We argue that differences in individual specialization can lead to asymmetry in habitat coupling in aquatic food webs. Although, perch showed a strong individual specialization and morphological adaptation to habitat there was no genetic differentiation of perch between habitats indicating strong trait plasticity in perch.