Benthic habitat structure explains broad-scale patterns in reef fish communities

Understanding how different ecological and anthropogenic drivers shape community structure is a central goal in ecology, particularly in spatially heterogeneous and rapidly changing systems. Fishes contribute to many key ecosystem functions and services on coral reefs, and a variety of physical, biological, and anthropogenic factors influence their distributions, habitat use, and ecological roles. Although habitat complexity is consistently shown to be important for reef fish ecology, it is rarely fully represented in large-scale analyses. When included, it is often measured using coarse or one-dimensional metrics, and seldom evaluated alongside other known ecological drivers. Here, we use three-dimensional habitat and overlapping fish census data collected at 89 sites throughout the main Hawaiian Archipelago to determine the role habitat structure plays in fish community structure compared to traditionally hypothesized environmental and anthropogenic drivers. We examined the impact of habitat structure (rugosity, fractal dimension, and coral cover), environmental conditions (depth, temperature, chlorophyll a, and wave exposure), and anthropogenic pressures (sedimentation, effluent pollution, coastal development, tourism, and fishing pressure) on four community descriptors: biomass, species richness, abundance, and community composition; the latter based on fish body size, diet, grouping behavior, and position in water column. Rugosity was the dominant driver of all community descriptors but was closely followed by environmental variables. The composition of behavioral traits in fish communities were strongly shaped by habitat structure, reflecting patterns in habitat use and predator-prey dynamics. Where structural complexity was not the primary effect, environmental conditions, such as temperature, were more strongly associated, particularly with body size distributions. Our results show that degraded reef conditions (i.e., reduced rugosity and coral cover) support communities with lower biomass and limited traits, which likely translates to a narrower range of ecosystem functions and services. These findings illustrate how different dimensions of habitat structure, in combination with environmental gradients, filter community traits and influence ecological organization. We offer a framework for predicting how management actions that alter habitat structure may cascade through fish communities to affect ecosystem functions and services. Maintaining structural features of reef habitats may therefore be essential to supporting the functional diversity and resilience of coral reef communities.