Are macroinvertebrate communities influenced by seagrass structural complexity?

A study was undertaken within a sub-tidal Zostera marina seagrass bed (Devo n, U.K.), with the aim of elucidating the relationship between seagrass str uctural complexity and the size and composition of the associated macroinve rtebrate community. Samples of macroinvertebrates were recovered from three designated areas of shoot density. Various physical characteristics were m easured For individual plants, and an a priori complexity index was determi ned relevant to the associated target organisms. Resulting data were analys ed using linens regression and multivariate techniques. Significant relatio nships were found between shoot density and number of leaves/shoot, leaf le ngth, stem length and algal epiphyte biomass. Neither the number of species nor abundance of macroinvertebrates was significantly related with the der ived complexity index. Multivariate analysis indicated that macroinvertebra te communities from the three areas of shoot density were significantly dif ferent. the pattern of macroinvertebrate community composition being best e xplained by seagrass biomass. Linear regression of seagrass biomass with ma croinvertebrate number of species and abundance revealed significant positi ve relationships. Regression also indicated that there was no significant i ncrease in complexity with increasing seagrass; biomass. The results sugges t that within a seagrass bed the size and composition of the associated mac roinvertebrate 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 resul t of a sampling artefact. Thus, the current paradigm that structural comple xity of seagrass is responsible for increased species diversity, can only b e justifiably applied to comparisons between seagrass and other habitats, a nd not within a seagrass bed itself.