Complex plant-herbivore-predator interactions in a brackish water seaweed habitat

Seaweeds are inhabited by small mobile invertebrates that use their hosts as habitat, food and protection against predators. Patterns of distribution and abundance may therefore reflect complex trade-offs between suitable space, nutritional requirements and refuge. We tested differences in abundance and density of isopods (Idotea baltica) in four different seaweed habitats (Furcellaria, Fucus, Cladophora, Ulva; Island of Bornholm, Baltic Sea) and experimentally assessed different ecological interactions between isopods and seaweeds. We found marked differences in abundance of isopods among seaweed habitats in the field, with a prevalence for form-functionally simple seaweeds. Patterns of isopod densities on seaweeds in the field resembled patterns of grazing and growth rates, but were opposite to habitat selection in laboratory experiments. Habitat selection resembled patterns of standing seaweed biomass and cover in the field. Laboratory experiments also showed that all seaweed habitats dramatically reduced fish predation on isopods regardless of seaweed identity. The strong affinity of isopods to seaweeds likely reflects predator avoidance whereas their selectivity between seaweeds could be due to grazing interactions and habitat availability. We therefore conclude that in situ patterns of isopod distribution and abundance, at least in part, reflect complex interactions between bottom-up (food type and availability) and top-down (predator refugia) forces.