SCALE-DEPENDENT SPATIAL AND TEMPORAL VARIABILITY IN BIOGEOGRAPHY OF MANGROVE ROOT EPIBIONT COMMUNITIES

Studies across a range of spatial and temporal scales are needed to di scern multiple forces structuring communities. Subtidal prop roots of red mangroves host diverse assemblages of sessile marine epibionts tha t provide a model system for examining community development and maint enance at a variety of discrete spatial scales. During 1991-1992 we tw ice surveyed 11 sites at four cays in Belize, Central America, to quan tify spatial variability and temporal change in distribution and abund ance of root-fouling organisms at five sampling scales: (1) fronts and backs of roots (1-cm scale); (2) roots close to and extending away fr om peat bank (0.5-m scale); (3) along linear transects parallel to sho re (1-50 m scale); (4) on leeward and windward shores of cays (0.5-km scale); and (5) among cays (1-10 km scale). Although epibiont communit y structure differed widely among sites, all cays surveyed had similar seasonal values of water salinity, pH, and temperature. Within cays, windward sites had higher dissolved oxygen levels and water flow rates than leeward sites. At still smaller scales, outer roots and fronts o f roots received significantly more light and were subject to higher w ater flow rates than inner roots and backs of roots. Species richness, diversity, and mosaic diversity patterns indicated that epibiont asse mblages were distributed non-randomly in space: leeward sites were mor e speciose than windward sites, and fronts of roots were more speciose than backs. Jaccard's index of similarity, cluster analysis, and Kend all's coefficient of concordance showed hierarchical patterns of decre asing similarity with increasing sampling distance. Significant spatia l autocorrelation among Jaccard values occurred at 2-3 m intervals, po ssibly reflecting mean larval dispersal distances. Analysis of mosaic diversity among sites indicated the absence of a clear environmental g radient and supported the hypothesis that species distributions may re flect patterns of dispersal from initial source populations. While pre cise identity of species was unpredictable among roots, species groups based on taxonomy, morphology, and life history showed very consisten t distributions among sites that may reflect variability in local root environments: algae were most prevalent in well-lit areas and on wind ward sites, while sponges and ascidians predominated in leeward areas. Relative importance and dominance of both individual species and spec ies groups changed substantially between 1991 and 1992. Representative s of four species groups were transplanted across three spatial scales to assess whether post-settlement dynamics limit distributions of the se taxa. All transplants survived well for the first 6 wk of the exper iment. After 6 mo, all transplants exhibited similarly high levels of mortality regardless of treatment. Overall, the results indicate that larval supply may shape epibiont community composition on short time s cales and small and very large spatial scales, while variation in phys ical factors may influence distributions over the long term and at int ermediate spatial scales.