Environmental drivers of functional diversity of near-pristine coral reef fish communities at marcoecological scales

Background/Question/Methods

Elucidating the role of environmental variation in driving spatial patterns in biodiversity is critical to predict the impacts of global environmental change on community structure and ecosystem processes.  Our ability to link environmental variation to biodiversity patterns, however, is often hampered by the confounding effects of human-driven species loss and our inability to study these relationships in systems that provide pristine reference points. We used coral reef fish communities as a model system to understand baseline relationships between key environmental variables and patterns in functional diversity for 23 near-pristine islands in the Pacific Ocean.  We derived environmental variables from field measurements collected at fish survey locations (depth, benthic complexity), data extracted from remote sensing products (sea surface temperature, chlorophyll a concentration, bathymetric slope, reef connectivity), and island-level characteristics (island type).  We used a mixed-effects modelling framework to evaluate the role of taxonomic diversity (i.e., species richness) and environmental variables in influencing four components of functional diversity: richness evenness, divergence, and redundancy.  To understand if observed patterns were scale-dependent, we performed our analyses at two levels: site and island. 

Results/Conclusions

We found effects of environmental variables on functional diversity, but relationships were dependent on the diversity component evaluated and the scale of the analysis. Functional richness was largely driven by variation in taxonomic diversity, with additional positive effects of habitat complexity. Functional evenness was negatively related to taxonomic diversity, but was not well-predicted by environmental variables. At the site level, functional divergence was highest at shallower, warmer locations.  Functional divergence was negatively related to estimates of water-column primary productivity at the island level. Functional redundancy was highest at shallower, warmer, and more complex reefs at the site level.  At the island level, functional redundancy was positively related to temperature and negatively related to primary productivity and taxonomic diversity. These results suggest that various components of functional diversity may be affected by different environmental drivers and as such may respond differently to global environmental change (e.g., rising ocean temperature or coastal eutrophication).  As individual components of functional diversity may have differential impacts on ecosystem processes, understanding these baseline relationships between environmental variables and functional diversity is key to predicting subsequent impacts on ecosystem functions like the amount and stability of secondary fish production.