Sponge Community Dynamics on Caribbean Mangrove Roots: Significance of Species Idiosyncrasies

Descriptions of the rich sponge faunas inhabiting mangrove roots at various Caribbean sites are unanimous in pointing out the heterogeneity of species distribution and abundance patterns at all scales, from different portions of a single root to geographic subregions. Abiotic factors have often been implicated by correlation, but ecological interactions, and the life history and morphological characteristics of the sponge species, may also play key roles. Published studies vary widely in methods used, hampering direct comparisons of results, and raising the possibility that conclusions might be influenced by methods. I have been exploring the processes underlying distribution and abundance patterns by applying identical methods to studying community composition and dynamics at two sites in Belize (Twin Cays) and one site in Panama (Bocas del Toro). Established communities on roots have been fully censused, by volume and numbers of individuals, yearly for three years (i.e., four censuses). Community composition, when evaluated in terms of total volume of component species, is very similar at these three sites, although abiotic factors differ and geographic distances between sites range from 330 m to 1,200 km. The nine species found on censused roots at all three sites constituted a total of 89%, 84%, and 73%, respectively, of the total sponge volume at these sites. In general, species exhibited similar patterns of growth, size decrease, and mortality at all sites where they were found, suggesting that these are species-level characteristics. Numbers of individuals and volume provide very different assessments of the relative importance of different species in these communities. Community change over time appeared to be substantial, when measured in terms of shifts in total numbers of individuals or total sponge volume. However, taking into account dynamics of individual species provides a very different view, as most large changes in numbers or volume were not community wide but tended to reflect life history characteristics typical of early successional stage species or idiosyncratic responses of one or a few species to particular environmental circumstances.