Ecosystem function is a central focus of ecological theory and practice in the Anthropocene. In ecological theory, this focus manifests as an interest in quantifying ecosystem function and decomposing ecosystem function into component parts. Ecological restoration similarly focuses on functional metrics, though from the more pragmatic perspective of assessing the efficacy of restoration efforts. Proposed metrics of ecosystem function have focused on measures that positively correlate with ecosystem function. At the same time, restoration ecology would benefit from the ability to assess small changes in ecosystem function, especially in low-function (heavily degraded) habitat. Ecosystem generalists benefit from reductions in species richness and ecosystem function and so make attractive candidates for inverse metrics of ecosystem function. We propose that in coral-reef ecosystems, gnathiid fish parasites are a candidate inverse metric of ecosystem function. We measured abundance of Gnathia marleyi—a gnathiid parasite common on Eastern Caribbean shallow coral reefs; substrate cover including live and dead stony coral, macroalgae, and sand; and counts of fish by functional group at seventeen 200 m2 plots on five Eastern Caribbean islands for a total of 938 data points.
Results/Conclusions
For the smallest (youngest) gnathiid juvenile stage, we found a negative correlation of gnathiid abundance with local cover of living stony coral. We also examined parasite hyperabundance—assessed by 90th percentile parasite counts—for each of the seventeen plots as a metric of the most stressful conditions for fish hosts on each plot. Of the seventeen plots studied, those where third-quartile coral cover exceeded 17% showed reduced hyperabundance of parasites while 9 out of the 13 remaining plots exhibited higher parasite hyperabundance. Combined with earlier data from laboratory studies, we suggest that live coral is the principal control of gnathiid abundance on these Eastern Caribbean reef plots. This 17% third-quartile coral cover threshold—roughly a 10% mean coral cover—corresponds to critical transitions in ecosystem function identified in the literature. We suggest that these generalist ectoparasites provide an inverse metric of ecosystem function that complements conventional positive metrics, providing enhanced sensitivity in ecosystem functional assessment. Future work should focus on cryptobenthic invertebrates whose ecosystem functional significance is little understood.