Are macroinvertebrate communities influenced by seagrass structural complexity

A study was undertaken within a sub-tidal Zostera marina seagrass bed (Devon, U.K.), with the aim of elucidating the relationship between seagrass structural complexity and the size and composition of the associated macroinvertebrate community. Samples of macroinvertebrates were recovered from three designated areas of shoot density. Various physical characteristics were measured for individual plants, and an a priori complexity index was determined relevant to the associated target organisms. Resulting data were analysed using linear regression and multivariate techniques. Significant relationships were found between shoot density and number of leaves/shoot, leaf length, stem length and algal epiphyte biomass. Neither the number of species nor abundance of macroinvertebrates was significantly related with the derived complexity index. Multivariate analysis indicated that macroinvertebrate communities from the three areas of shoot density were significantly different, the pattern of macroinvertebrate community composition being best explained by sea-grass biomass. Linear regression of seagrass biomass with macroinvertebrate number of species and abundance revealed significant positive relationships. Regression also indicated that there was no significant increase in complexity with increasing seagrass biomass. The results suggest that within a seagrass bed the size and composition of the associated macroinvertebrate community is not determined by the structural complexity of the plants, but by the amount of plant available. This finding indicates a simple species-area relationship, and arguably one brought about as a result of a sampling artefact. Thus, the current paradigm that structural complexity of seagrass is responsible for increased species diversity, can only be justifiably applied to comparisons between seagrass and other habitats, and not within a seagrass bed itself.