Interspecific-Competition Strongly Constrains Species-Richness and Species-Abundance Evenness in a Tropical Marine Molluscan Community Inhabiting Caulerpa Beds, as Compared to Coral-Reefs

Increasing species-richness at the local scale (within species communities) is accommodated, first, by the diversification of the niches respectively associated to species. Yet, in case of excessive supply in colonizing species issued from the regional pool, the corresponding increase in the number of solicited niches may lead to some “niche-overcrowding” resulting in significant niche-overlaps. Then, second, strong interspecific competition for shared resource can arise, triggered by the density in individuals among those species co-occurring at niche-overlaps. Accordingly, the accommodation of species-richness within a local community involves a balance between (i) the positive contribution of improved niche-diversification and (ii) the negative consequence of induced interspecific-competition at increasing niche-overlaps once the number of colonizing species becomes too large. This balance can strongly differ according to the local ecological conditions, since the latter are expected to strongly influence the range of “overcrowding-free” diversification of niches. So that, concretely, each community requires a specific analysis, in order to disentangle and quantify the respective contributions of the niche-diversification and the intensity of interspecific-competition to this balance. And, in particular, their respective roles upon both the species-richness and the degree of unevenness of species abundance within community. Beyond its speculative interest, this deeper understanding of the process involved in the hierarchic-like organization of species within community also answers more practical concerns, in particular the stability of species-richness, partly dependent on the intensity of interspecific-competition.

In this perspective, we quantify and compare how species-richness accommodation proceeds in two major taxonomic groups, Bivalves and Gastropods respectively, both belonging to a same molluscan community inhabiting Caulerpa beds, in the intertidal-zone of Siquijor Island (Philippines). Then, after having compared these two different taxonomic groups, the influence of environmental conditions on species-richness accommodation is addressed, showing that “Caulerpa-beds” habitat features far-less rewarding to Gastropods communities than can be the classical “coral-reef” habitat.