LAUSR

Aim To identify key environmental factors associated with local fish species richness across a large tropical marine region. Location Wider Caribbean region. Methods Species richness estimates were based on a sightings database covering the wider Caribbean region. Environmental variables considered were distance to key habitats, habitat area, temperature, depth, salinity, nutrient concentration, as well as natural and anthropogenic disturbance. We test the significance of associations between these factors and species richness, establish the shape of these relationships and use spatial cross-validation to test the generality of these results. Species-specific environmental associations within these relationships were then tested. Results Five environmental variables showed significant associations with species richness, but only two, temperature and depth, proved robust to spatial cross-validation. Temperature was the best performing environmental predictor, showing a unimodal relationship with species richness and optimum temperatures consistent across analytical choices and data sets. Relationships between species richness and other environmental factors were also typically unimodal. Variation in species-specific temperature associations was significant within species richness gradients; with a higher number of species associated with optimal temperatures. Main conclusions Temperature represents the dominant environmental predictor of fish species richness identified by our study, showing a hump-shaped curve, peaking at around 27.4°C. This meant that the warmest sites within our study were not necessarily the most species rich. Variation in species thermal niches may play a key role in driving species richness gradients, with significant general positive trend for fish species to occur in locations exhibiting optimum sea surface temperatures.