Decadal turnover of thermally stressed coral taxa support a risk-spreading approach to marine reserve design

Abstract

Coral reef communities exposed to rapid temperature rises and frequent thermal anomalies were evaluated for taxonomic turnover via presence/absence information over a 27-year period experiencing large changes in the dominant taxa. Temporal turnover of the taxa within sites was consistently high (~ 40%) due to both inter-annual episodic and directional changes. Turnover with time displayed a rapid increase and slow decline after sequential cool and warm thermal anomalies between 1996 and 1998. Subsequent warm temperature anomalies caused fewer broad-scale changes. Directional change for all sites combined indicated three overall gains and losses in taxa—Montipora being the only dominant taxon that declined in both abundance and presence/absence. The studied marine reserves had higher local but lower between-site taxonomic richness than fished reefs. Despite similar mean turnover, there were fewer gains than losses in marine reserves (7 gains and 20 losses in 5 sites) than fished sites (16 gains and 15 losses in 7 sites). Changes in taxonomic cover and presence/absence turnover data were not correlated, indicating that turnover detects finer scale taxonomic change likely to be missed when the cover of the dominant taxa is evaluated—especially in the higher richness marine reserves. High spatial richness, community change, and thermal acclimation in these shallow reef lagoons may have prevented higher net losses of taxa. Consequently, the probabilities of reducing local extirpations of taxa may be best achieved by planning and management that promotes spreading more evenly-spread access restrictions to reef areas with high between-site diversity rather than focusing restrictions to sites with high within-site diversity.