Functional richness and turnover patterns reveal assembly rules structuring marine fish communities on the continental shelf of French Guiana

The presence and coexistence of species in a community depend on their abilities to survive and to maintain their fitness in a given environment. Determining how assembly processes structure communities across scales, also called 'assembly rules,' is still a subject of debate. These mechanisms are related to biotic and abiotic factors. At local scales, they are represented by 2 main assumptions: the limiting similarity hypothesis (interspecific competition) and the niche or environmental filtering hypothesis. At regional scales, dispersal limitation and environmental filtering are assumed to be the 2 main processes explaining the community assembly patterns. We tested these hypotheses using the trait-based approach and functional relationships between species in fish communities from the continental shelf of French Guiana. This study was based on 2 scientific surveys conducted in November 2016 and 2017 at 33 stations by bottom trawling. At the local scale, i.e. a single haul, co-existing species tended to be more functionally closely related than what would be expected randomly, indicating a dominant influence of environmental filters over interspecific competition. These observations revealed assemblages composed of species showing similar ecological functions and, thus, high functional redundancies. At the regional scale, i.e. the study area, comparisons between pairs of stations showed that species replacements (taxonomic turnover) between local assemblages corresponded to random variations while functional profile replacements (functional turnover) tended to be higher than random expectations. Results suggest that abiotic characteristics of habitats have a stronger effect on fish community assembly at local and regional scales than biotic interactions.