- Biodiversity loss in highly diverse systems such as coral reefs has been linked to significant declines in the ecosystem functions and services provided by marine species. Ecological functioning of coral reefs and the resistance of coral reef fish communities to disturbance depend on the functional traits of species that promotes ecological resistance, as well as traits spatial and temporal abundance. Trait-based approaches are increasingly being used in marine ecology to predict impacts on ecosystem function and services from environmental change. Functional diversity (FD) metrics quantify the trait diversity in biological assemblages and act as a proxy for the diversity of ecological functions performed in the community. Analyses of FD offers a potentially useful tool to identify functional changes in diverse, complex, and disturbed marine ecosystems such as coral reefs, yet this metric is rarely applied to evaluate community change. Few studies have applied trait-based diversity metrics to understand the ecological roles that fishes play in the trophic dynamics in coral reef systems, neither for monitoring spatiotemporal changes in the trophic organization of fish assemblages to evaluate management strategies and resistance of fish assemblages to disturbances.
To better understand the trophic structure in fish communities and identify any knowledge gaps, we conducted an extensive literature review of six trophic functional traits for juvenile and adult stages of 274 marine fish species known to occur in the U.S. Caribbean. Trait descriptions were more comprehensive for adult life stages than juveniles, and the trophic level was the most recorded trait, and social feeding behavior was the poorest recorded traits for species in adult and juvenile stages. From this review, we built a trait database that described fish trophic functions, but also gaps in trait knowledge in the Caribbean Basin.
In Chapter 3, we documented spatiotemporal variability in the trophic function of fish assemblages to identify changes in coral reef communities inside benthic habitats of the Buck Island Reef National Monument (BIRNM) in the U.S. Virgin Islands between 2002 and 2010. We combined six traits related to the trophic function of 95 fish species together with species biomass estimated from underwater surveys to calculate assemblage-level descriptors of functional richness, dispersion, and evenness. We found that coral reef and other hard bottom areas supported high levels of trophic functional richness and variation among habitat types, but with low functional redundancy evidenced by the unique trophic role that each fish species had within the functional trait space. Important temporal variations in functional trait composition during the last decade in the BIRNM, including change following a strong coral bleaching event in 2005, suggested that functional diversity descriptors are sensitive enough to track changes in the trophic organization of fish communities. In general, this chapter showed the utility of descriptors of functional diversity to evaluate changes in ecosystem functioning of heterogeneous habitats in the seascape.
In Chapter 4, we used descriptors of functional diversity and traits as tools to (1) evaluate the ecological performance of Marine Protected Areas (MPA); (2) study the association of fish functional diversity in ecosystem services such as production of fish standing biomass; and (3) identify the ecological resistance of fish communities to the coral bleaching event in 2005 in the U.S. Virgin Islands. We used four functional traits related to the trophic function of fish species and their biomass between 2002 and 2012 to calculate descriptors of functional richness, variation and evenness, community-weighted mean values of functional traits, and fish standing biomass inside and outside MPAs. We also used trait information to characterize trait values for tolerant and non-tolerant species to the coral bleaching event in 2005 that produced high mortality in coral reefs in the U.S. Virgin Islands. First, we found that functional diversity descriptors and traits are sensitive to capture spatial and temporal differences in the functional organization of communities, and are an excellent addition to monitoring the ecological performance of marine reserves. Reserve effects were found only for the Buck Island National Reef Monument in St. Croix, and we consider that the lack of reserve effects in other MPAs is a consequence of their short establishment duration, the legacy of overfishing, and possible functional homogenization in fish communities. Second, we found an association of functional diversity and fish biomass, which indicated that high fish functional richness and variation, and low evenness covaries positively with the total fish standing biomass at all levels of habitat protection. This finding highlights the importance of protecting the diversity of ecological roles in reef fish assemblages to maintain ecosystem services. Finally, we identified indicators of fish functional diversity erosion and a low ecological resistance of fish species to the coral bleaching event in 2005. These indicators included (1) the reduction in fish functional richness; (2) the lack of return of fish functional richness to its original levels after seven years of survey; (3) the dominance of broader diets in fish assemblages; and (4) the reductions in the trophic levels of disturbance-tolerant fish species. Based on the tolerance of each fish species to the coral bleaching event in 2005, we built a database of ecological resistance to disturbance. Managers can use this database to enhance the intrinsic ecological resistance of fish assemblages or to better understand further trends in the trophic organization of fish assemblages in each evaluated MPA.