Temporal stability of ecosystem function on coral reefs: Roles of density-dependence, species interactions, and the portfolio effect

Background/Question/Methods Species perform diverse ecological functions, some of which are critical for maintaining ecosystems in a desirable state. Of particular interest is the temporal stability of these essential functions. Three processes are known to increase the temporal stability of ecosystem functions: density dependence, compensatory species interactions (such as competition), and the Portfolio Effect. The aim of this study is to quantify the relative importance of these processes in nature using long term monitoring data. We investigated herbivorous reef fishes’ community stability over an eleven-year period on 42 sites of the outer Great Barrier Reef, Australia. Herbivorous reef fishes were classified into three functional groups: territorial grazers, roving grazers and scrapers.  We estimated the strength of density dependence within groups and the strength of interactions between groups, by fitting a multivariate autoregressive model (MAR) to the data.  We quantified response diversity by estimating the mean correlation coefficient between responses of the groups to environmental fluctuations.

Results/Conclusions We found strong evidence for density dependence in every group at every site. Interactions between groups were detected in at least 50% of the sites; however, the average interaction effect over all sites is zero: in other words, local positive and negative interaction effects average out at the regional scale.  Species responses to environmental fluctuations were nearly independent (correlation coefficient ~ 0.1), indicating high response diversity and a potentially strong Portfolio Effect. Within functional groups response diversity among territorial grazers was much higher that response diversity among scrapers. These findings show that intraspecific density dependence and the Portfolio Effect are likely to be the principal drivers of stability in ecosystem function on coral reefs, at least for herbivory.