Shifts between biotic and physical driving forces of species organization under natural disturbance regimes

The high ecological values (i.e., the benefits that space, water, minerals, biota, and all other factors that make up natural ecosystems provide to support native life forms) and diversities found in tropical islands emphasize the importance of incorporating disturbance into ecological models. This is of major concern in appreciating how species will survive and adapt to changes and the consequences expected in terms of biodiversity. We predicted that in lotic systems, modification to natural disturbance regimes (fluvial action) would have strong consequences on community organization, with strong disturbance regimes reducing species competitive exclusion through changes in space occupation. We tested this prediction by relating microdistribution data from a crustacean species (Atya innocous, Decapoda, Atyidae) in small and large rivers in Guadeloupe to two, independently obtained sets of explanatory variables describing the physical environment, as well as the crustacean and fish competitors. Our results show that in rivers with high-energy flow, the driving forces for species coexistence were mostly environmental, whereas in rivers with low-energy flow, biotic interactions were prevalent. These differences linked to natural disturbance regimes revealed that disturbance was a stochastic factor, overlying the classical community-structuring factors and affecting global species relationships.