10:45-12:00 Session IIIb Dynamics of Coastal Ecosystem
Processes regulating coral community diversity:
towards bridging the empirical-theoretical gap
R. van Woesik
Department of Marine Sciences, University of the Ryukyus, Nishihara,
Okinawa 903-0213, Japan
E-mail: b984138@sci.u-ryukyu.ac.jp
Some unresolved questions in tropical marine ecology are (1) what mechanisms and processes generate and maintain coral diversity? And (2) how is that diversity related to different features of the ecosystem? Moreover, the mechanisms organising coral communities are equivocal, (3) are they niche assembled, dispersal assembled or disturbance assembled?
In a local-regional comparison, a non-saturating relationship highlights mechanisms involving dispersion, increased niche overlap and increased specialization, while local saturation accentuates mechanisms involving competitive interactions and stochasticity. Several authors have shown a strong relationship between local and regional scleractinian coral-species richness, with no indication of local saturation. This indicates that interspecific interactions may have minimal influence on macro-scale distribution and abundance patterns and define only zonal variability. However, disturbance can obscure the effects of competition, even when disturbances are infrequent. Recent studies however show no relationship between disturbance history and local species richness in a tropical forest. Tropical forest studies suggest recruitment limitation is the principal mechanism determining local species richness, while my research on coral reefs indicates coral communities are not recruitment limited (i.e., dispersal assembled). I found inconsistencies between coral recruitment patterns and adult coral communities, suggesting that local environmental conditions inflict post-settlement mortality and that these processes are a vital structuring component of coral communities.
I introduce new data from Okinawa, Japan and briefly outline how the empirical data relates to diversity theory. A metapopulation and metacommunity model is discussed. The metapopulation model predicts a species' probability of site occupancy and the metacommunity model predicts the number of species supported at any one site. The models accentuate the primary mechanisms that generate and maintain local (10's m2 - 10's km2) coral diversity based on a regional (100's km2) perspective. Local diversity appears regulated by differential post-settlement mortality and degree of reef connectivity. High post-settlement mortality leads to local extinction of some species. My studies in Queensland, Australia demonstrated strong negative relationships between coral species richness and productivity (measured by chlorophyll a concentrations in the water column). Indeed, harsh local environments cause high levels of local extinction and these environments support few coral species. In habitats supporting high coral diversity I argue that there is little differentiation in best competitor. This scenario agrees with diversity theory, which suggests that the more species there are in a community the less connected they are, and that there is less probability that any one species is a keystone species. This scenario, with probability theory as its underlying tenet, is plausible on a local scale but breaks-down on a regional scale. In a group of small islands with abundant and variable environmental conditions, high site variability may alone maintain high regional species diversity.
