Habitat Complexity and Human Presence Shape Pacific Coral Reef Size Structure

Human disturbances have wide ranging and diverse impacts on coral reef ecosystems. Reductions in biomass and biodiversity are well documented, particularly at local scales, and yet the impacts on community structure are less defined. Size-based analyses can be used to describe changes to community structure where, by quantifying the allometric relationship between body size and abundance, the size spectrum represents the distribution of body sizes in a community. Exploitation of large body sizes alters community structure and is reflected by a steepening of the size spectrum. Here, using an extensive dataset of fish abundance and body size at over 50 Pacific islands, covering several biogeographic regions, we explore the variation in community structure attributable to human impacts and environmental covariates.  Size spectra of populated reefs are steeper than their near-pristine, uninhabited counterparts, indicating a consistent and ocean-basin wide effect of fishing on the reef fish community. In contrast, habitat complexity and reef isolation drive size structure when humans are absent. Size based analyses complement the use of diversity and biomass metrics in teasing out the impacts of human disturbances on marine ecosystems.