Multiple fish predators: effects of identity, density, and nutrients on lower trophic levels

The ongoing loss of predators is changing the composition of food webs, with largely un known consequences. In particular, the effects of multiple fish predators on food webs are difficult to predict due to the prevalence of omnivory and intra guild predation. At the same time, many ecosystems experience high nutrient loads that fuel food webs from the bottom up. To test the combined effects of both multiple predators and nutrient enrichment on food web composition, we created monocultures of perch, roach, and stickleback as well as mixed assemblages of all 3 species at 3 different density levels using an additive design, and induced nutrient en richment in half of the cages. The biomass of invertebrate herbivores and algae was measured. Stomach analyses of predators were used to detect prey switching. Herbivore biomass depended mainly on predator identity, while top-down effects on algae were mediated by predator density and nutrient en richment. Specifically, perch strongly reduced amphi pods and isopods, and roach mainly reduced gastropods but also isopods, while stickleback had weak overall effects on herbivores. These species-specific effects were attenuated in the mixed fish assemblages, probably due to prey switching. Algal growth strongly increased under high fish density and nutrient enrichment, but was not affected by predatorinduced changes in the herbivore composition. This study shows that identity effects from predators in isolation are attenuated in multiple predator assemblages, probably due to increasing interference among the predators. Algal biomass was enhanced by nutrient enrichment and high predator density, which affected algae probably through non-lethal effects, such as reduced activity of the herbivores. Diagram of food-web interactions (foreground) among fishes, herbivores and algae in the study system (background: brown algae and associated epiphytes and epifauna).