Structure of small tropical islands freshwater fish and crustacean communities: a niche- or dispersal-based process?

Determining the relative importance of niche- and dispersal-based
processes in the structuring of animal communities is central in ecology.
Freshwater fish and crustacean communities of small tropical islands can
bring new insights for understanding these processes as all their species
present a pelagic larval stage which gives them important dispersal
capacities. Consequently, we hypothesized that dispersal-based process
maybe preponderant for structuring these communities from the regional- to
the local, i.e. survey site, scales. Gradient forest analyses allowed us to
model the effect of 12 environmental variables on freshwater taxa
abundances in two southwestern Indian Ocean islands: Mayotte (26 taxa) and
Reunion (21). A total of 153 surveys in Mayotte and 266 in Reunion were
used for building the models. Despite the strong heterogeneity of
environmental conditions between the two islands, the main factors
structuring freshwater fish and crustacean communities in both islands were
the elevation and the slope of the sites. The observed structure appeared
more pronounced for predatory species than for primary consumers and
omnivores. As predators generally have limited locomotor capacities, it is
concluded that dispersal-based process structures these communities not
only at large geographical scales but also at the intra-watershed scale, by
limiting the inland penetration (or dispersion) of species in relation to
their locomotor capacities. However, more knowledge concerning ecological
traits and taxonomic status of many species is needed to confirm this
assumption.