A global analysis of the effectiveness of marine protected areas in preventing coral loss

Abstract

Background: A variety of human activities have led to the recent global decline of reef-building corals. The ecological, social, and economic value of coral reefs has made them an international conservation priority. The success of Marine Protected Areas (MPAs) in restoring fish populations has led to optimism that they could also benefit corals by indirectly reducing threats like overfishing, which cause coral degradation and mortality. However, the general efficacy of MPAs in increasing coral reef resilience has never been tested.

Methodology/principal findings: We compiled a global database of 8534 live coral cover surveys from 1969-2006 to compare annual changes in coral cover inside 310 MPAs to unprotected areas. We found that on average, coral cover within MPAs remained constant, while coral cover on unprotected reefs declined. Although the short-term differences between unprotected and protected reefs are modest, they could be significant over the long-term if the effects are temporally consistent. Our results also suggest that older MPAs were generally more effective in preventing coral loss. Initially, coral cover continued to decrease after MPA establishment. Several years later, however, rates of coral cover decline slowed and then stabilized so that further losses stopped.

Conclusions/significance: These findings suggest that MPAs can be a useful tool not only for fisheries management, but also for maintaining coral cover. Furthermore, the benefits of MPAs appear to increase with the number of years since MPA establishment. Given the time needed to maximize MPA benefits, there should be increased emphasis on implementing new MPAs and strengthening the enforcement of existing MPAs.

Figure 1. Location of unprotected (orange) and protected (purple) reef coral cover survey sites.

Figure 2. Comparisons of the average coral cover per year as predicted by the models.

Simulated data sets (light grey lines) with the observed mean coral cover per year (thick black line) for (A) MPA versus control model, (B) MPA-only Caribbean years of protection model, and (C) MPA-only Indo-Pacific years of protection model. The histograms at the bottom of the figures display the relative sample sizes at each year for the actual data. In all models, the earlier years had less data and therefore exhibit more variation in behavior. In the ‘year of protection’ models there was not sufficient data to accurately estimate percent coral cover so the simulation results begin in later years. Note that the right y-axes are different in each of the plots due to the varying number of observations in each model.

Figure 3. The change in percent coral cover from 2004 to 2005 inside and outside of MPAs.

The 95% credibility intervals (error bars) are also shown. Reefs protected in MPAs had slightly positive changes in percent coral cover, although not significantly different from zero (dashed line). Percent coral cover was obtained by back-transforming the predicted logit from the model.

Figure 4. The effect of the number of years of protection on the 1-year change in coral cover and number of observation.

Coral cover change rates are shown in the (A) Caribbean and (B) Indo-Pacific with the 95% credibility intervals (light grey bands) and the 50% credibility intervals (dark grey bands) as well as the median (white line) of the posterior distributions of the year of protection models using all years of data and 2005 coral cover change rates. The number of years of protection by observations (surveys) in the (C) Caribbean and (D) Indo-Pacific show that most surveys have been performed when MPAs have been established for 15 years or less.