Reef fish community structure across an ecological transition zone.

Transition zones exhibit a unique combination of abiotic characteristics derived from the merging of two distinct areas, hosting communities with different thermal tolerance and distribution ranges. Given these characteristics, these zones are key to unmasking the effects of climate change on biodiversity since rapid changes in the sea temperature can favor some populations more than others. This study aimed to investigate the community structure of reef fish in seven islands of the southwestern Atlantic in a transition zone. Our results revealed distinct patterns of thermal affinity separating reef fish communities from the north to south islands, indicating a higher tolerance for species experiencing colder and seasonal temperatures. We also found that the abundance distribution pattern corresponds to the presence of many rare and few dominant species. The northern islands exhibited greater species richness, density, and biomass compared to the southern islands, potentially due to differences in the oceanographic conditions of the region and protection status. Our findings underscore the importance of understanding how communities differ or shift in space, which is crucial for predicting the consequences of environmental changes on biodiversity and ecosystem functioning.