Guild Structure in Coral Reef Fishes Does Not Conform to Competitive Exclusion-Based Assembly Rules

Coral reef fish communities are species-rich with seemingly high apparent niche overlap. Although competition among fish species is well documented at local scales and within specific species, the extent to which it affects community structure and composition remains unclear. Here, we examine whether competition-based models of community assembly can explain patterns of species occurrences within feeding guilds in a lagoonal reef fish metacommunity at One Tree Island on the southern Great Barrier Reef, and thus infer whether competitive exclusion is operating within guilds. Using presence-absence data from underwater fish surveys, we partitioned the fish community into feeding guilds and applied null-model analyses to test for evidence of competition using two classical assembly rules: Diamond's co-occurrence rule and Wilson's guild proportionality rule. Neither assembly-rule prediction was supported. Species within guilds co-occurred as frequently or more frequently than expected under null expectations, and guild composition varied more, rather than less, across sites. These results indicate that exclusion-based assembly rules do not provide a strong description of guild-level structure in this system and highlight the limits of extrapolating evidence of local competitive interactions to patterns of community assembly in species-rich communities.