Assessing Extinction Risks of Corals in a Changing Climate

In October 2009, NOAA was petitioned to list 83 coral species as endangered or threatened under the U.S. Endangered Species Act. A Biological Review Team (BRT) was established to assess the global status of and provide estimates of the extinction risks for each of the 82 candidate coral species. The BRT considered the ecological character of each candidate species—particularly life history, taxonomy, and abundance, and the interactions of natural phenomena and a comprehensive suite of local and global change stresses that could potentially contribute to coral extinction. After a comprehensive literature review, ocean warming, disease, and ocean acidification were considered to be the most influential threats in posing extinction risks. Local threats having widespread impacts, such as sedimentation, nutrient over-enrichment, and fishing, were considered of medium importance in determining extinction risks. While there was relatively high confidence in the general progression of ocean warming and ocean acidification, there was little certainty in their timing and spatial patterns. Likewise, there was little certainty in the relative abilities of the corals to respond or adapt to each of the threats individually or interactively. Observed and currently predicted increases in human population and their collective consumption were considered the root drivers of most of the threats to the 82 coral species.

There were similar uncertainties in abundance and trend data, taxonomic status, complex life cycles, and threat interactions.  These complicated the BRT’s assessments of the status and extinction risks facing these corals. The BRT relied upon the best available information on the spatial extent of the species ranges and on their understanding of the likely impacts of the suite of threats on these corals. Extinction risks were estimated as the likelihood of each species status falling below a Critical Risk Threshold (CRT) by the year 2100, a time frame over which climate projections were readily available and sufficiently reliable for decision-making. Based on consideration of depensatory processes, environmental stochasticity, and catastrophic events, the CRT describes a condition where a species is of such low abundance, or so spatially fragmented, or at such reduced genetic and genotypic diversity that extinction is extremely likely. The likelihood of the status of each coral species falling below the CRT by 2100 was independently estimated by each of the 7 BRT members and then tallied to determine the risk likelihood for each candidate coral species.