Predictability of the structure and dynamics of coral reef fish communities : effects of habitat, patch size and connectivity

Abstract

Despite widespread acceptance of non-equilibrial dynamics and recruitment limitation in coral reef fish communities, the empirical evidence for these processes is limited to assemblages of site-attached fish species on small, isolated patch reefs. Studies have indicated that the structure of these assemblages is unpredictable in space and time, but the extent to which this result can be extrapolated to vagile fish species and fish communities on connected reef habitats remains unclear. Few quantitative studies have examined spatial and temporal variation in populations of vagile fish species or whole communities on contiguous coral reef. The general aim of this study was to expand the database to the wider fish community and investigate relationships between the structure of fish communities and habitat characteristics on contiguous and patchy reef habitats at Heron and Wistari Reefs, Great Barrier Reef.To obtain baseline data on spatial variability in fish community structure, I examined the distribution and abundance of fish species varying widely in vagility on a number of sites on contiguous and patchy coral reef. Ordination and regression analyses indicated that spatial variation in the structure of fish assemblages on contiguous reef was significantly related to intrinsic habitat characteristics, including the composition and topographical complexity of the substratum, and depth. The structure of fish assemblages on patch reefs was unrelated to intrinsic habitat characteristics but was related to the connectivity of the patch to neighbouring patches. Separate multiple regression models explained significant proportions of spatial variation in the population densities of four site-attached and six vagile species on contiguous reef sites. For patch reef sites, models were significant for six species, five of which were not site- ttached. For site-attached species, models based on patch reef sites explained considerably less variability than models based on contiguous reef sites. For vagile species, there was little difference between models based on contiguous and patch reef sites. These result lend support to the importance of post-settlement migration in facilitating relationships between population density and habitat structure.In addition to examining spatial variation in community structure, I examined the degree of temporal variability in the structure of fish communities resident on contiguous and patch reef sites over a two year period. Ordination analyses indicated that most of the fish assemblages fluctuated randomly through time, although at some sites, communities tended to fluctuate around a stable community configuration. Community variation was highest during periods of high recruitment but recruitment events did not cause persistent shifts in community structure. For site-attached and moderately vagile species, but not highly vagile species, temporal variability in assemblage structure increased as sites became smaller and more isolated.To investigate further the mechanisms underlying spatial variation in the distribution and abundance of fish species, I reanalysed the data obtained from the surveys of contiguous and patch reef sites to examine spatial variation in species richness. Observed pattems were compared to those expected on the basis of two null models which randomly redistributed fish among sites to simulate stochastic non-equilibrial recmitment processes. Observed data differed significantly from the null models, suggesting that species richness was influenced by equilibrial interactions within fish communities (e.g., competition and predation). To compare the relationship between species richness and habitat structure on contiguous reef and patch reefs of different sizes, I used rarefaction and partial regression to remove the effect of sample size variation (number of fish at each site). Only a small amount of spatial variation in species richness was explained by habitat characteristics on contiguous reef sites (30%) and patch reef sites (25%). Much of the unexplained variability was likely due to species interactions within fish communities.  ........