A neighbour is a neighbour? Consumer diversity, trophic function, and spatial variability in benthic food webs

The short ecological history and the environmental conditions characterising the Baltic Sea make it inhabitable for only few species, resulting in a functional poverty of the system (number of functional groups and number of species within each group). Thus, it is a well-suited system for studies on biodiversity and ecosystem functions, particularly when moving from observation of patterns to determination of mechanisms behind observed relationships, such as the importance of species richness for trophic functioning. Predators in the benthic community are predominately generalists, with a potential for overlap in trophic function. Through analysis of stable isotope ratios (13C:12C and 15N:14N), we determined the trophic position of epibenthic consumers and estimated spatial variability in isotope ratios of food web components at nine shallow sandy sites. There were significant differences in isotope ratios both within species and for species assemblages between bays. The secondary consumer signatures in bivariate δ-space showed a partly overlapping transition from benthivores (Crangon crangon, Pomatoschistus microps) to more pelagically feeding predators (Gasterosteus aculeatus). At two sites (one sheltered, one moderately exposed), the gut contents of generalist predators revealed feeding on abundant prey and confirmed the potential for overlap in diets. Focusing on members of the benthic trophic pathway, we assessed the influence of secondary consumer diversity on trophic transfer, with temporal separation of resource use as a mediator of biodiversity effects. In a laboratory experiment, we were able to show temporally separated foraging for two epibenthic carnivores and that mechanisms behind the biodiversity effect differed in time. Many food web properties, such as function, resilience and resistance are linked to the diversity and characteristics of nodes and links in the food web (e.g. degree of redundancy, connectedness and trophic level) and our study provides information on some dimensions of the trophic niches of abundant species in shallow soft-bottom food webs.