Seagrass beds and intertidal invertebrates: an experimental test of the role of habitat structure

The majority of field experiments have been carried out on relatively small spatial and short temporal scales, but some of the most interesting ecological processes operate at much larger scales. However, large-scale experiments appropriate to the landscape, often have to be carried out with minimal plot replication and hence reduced statistical power. Here, we report the results of such a large scale, un-replicated field experiment on the Mondego estuary, Portugal, which nevertheless provides compelling evidence of the importance of habitat structure for invertebrate community composition and dynamics. In this estuary, seagrass beds have suffered a dramatic decline over the last 20 years, associated with changes in invertebrate assemblages. In addition, the most abundant species in the system, Hydrobia ulvae, displays distinctly different population structures in those sites. The aim of the field experiment was to test the hypothesis that these differences are related to enhanced survival of snails due to protection from avian or fish predators so that they can grow to larger body sizes in the more complex habitat provided by seagrass. We tested this hypothesis through a large-scale experiment using artificial seagrass beds over a 12-month period. Adult snail densities were higher in the artificial bed plots compared to controls. However, these differences emerged only slowly, related to snail growth rate. This suggests that protection from epibenthic predators can have a significant effect on population structure and hence biomass and productivity of key species in this system. However, the invertebrate assemblage in artificial seagrass plots and the natural seagrass bed, remained statistically separate by the end of the experiment.