Fishing down nutrients on coral reefs: Selective harvest reduces nutrient capacity in coral reef ecosystems
Overfishing is widely recognized as one of the major drivers of global coral reef declines with much emphasis placed on altered ecosystem function due to biodiversity loss. Recent research has demonstrated that fish communities play critical roles in the supply and storage of nutrients (i.e., nutrient capacity) within these ecosystems. That is, biodiversity and community structure strongly regulate nitrogen (N) and phosphorus (P) supply and storage from fish communities and that healthy fish communities constitute one of the largest sources of nutrients to coral reefs. Yet the degree to which human impacts (e.g., fishing) may affect these processes remains unclear. Here we test the implications of fishing for ecosystem-level nutrient capacity across extensive spatial scales throughout the Caribbean. Using species-specific models, N and P storage and supply were estimated across 110 fish communities in 43 sites within 4 countries, including fished and unfished areas. We tested two specific hypotheses: (1) Fishing will reduce both storage and supply of nutrients in reef fish communities across large spatial gradients. (2) Reductions in nutrient storage and supply will be driven by reduced species richness and altered trophic structure of fish communities.
All measures of nutrient supply and storage of N and P were substantially reduced on fished versus unfished reefs, reducing nutrient supply and storage by 40-46%. Human population density closest to reef, used as a proxy for fishing pressure, was negatively correlated with nutrient capacity; reef habitat type and size of protected areas were also marginally important predictors. Despite substantially reducing ecosystem function, fishing pressure did not significantly reduce species richness across the study reefs, contrasting theory that suggests losses in ecosystem function should be proportional to losses in richness. Instead, altered trophic structure was the central driver of reduced nutrient processes. That is, substantial reductions in nutrient processes per functional group were found despite only minor reductions (and at times increases) in richness. Conservation efforts should target trophic structure and community biomass to maintain high levels of nutrient capacity in these diverse ecosystems. Incorporating consumer-mediated nutrient dynamics into conservation and management strategies may be critical to ensure more rapid recovery of these imperiled ecosystems.