Coral Reef Disturbance and Recovery Dynamics Differ across Gradients of Localized Stressors in the Mariana Islands

<div><p>The individual contribution of natural disturbances, localized stressors, and environmental regimes upon longer-term reef dynamics remains poorly resolved for many locales despite its significance for management. This study examined coral reefs in the Commonwealth of the Northern Mariana Islands across a 12-year period that included elevated Crown-of-Thorns Starfish densities (COTS) and tropical storms that were drivers of spatially-inconsistent disturbance and recovery patterns. At the island scale, disturbance impacts were highest on Saipan with reduced fish sizes, grazing urchins, and water quality, despite having a more favorable geological foundation for coral growth compared with Rota. However, individual drivers of reef dynamics were better quantified through site-level investigations that built upon island generalizations. While COTS densities were the strongest predictors of coral decline as expected, interactive terms that included wave exposure and size of the overall fish assemblages improved models (R² and AIC values). Both wave exposure and fish size diminished disturbance impacts and had negative associations with COTS. However, contrasting findings emerged when examining net ecological change across the 12-year period. Wave exposure had a ubiquitous, positive influence upon the net change in favorable benthic substrates (i.e. corals and other heavily calcifying substrates, R² = 0.17 for all reeftypes grouped), yet including interactive terms for herbivore size and grazing urchin densities, as well as stratifying by major reeftypes, substantially improved models (R² = 0.21 to 0.89, lower AIC scores). Net changes in coral assemblages (i.e., coral ordination scores) were more sensitive to herbivore size or the water quality proxy acting independently (R² = 0.28 to 0.44). We conclude that COTS densities were the strongest drivers of coral decline, however, net ecological change was most influenced by localized stressors, especially herbivore sizes and grazing urchin densities. Interestingly, fish size, rather than biomass, was consistently a better predictor, supporting allometric, size-and-function relationships of fish assemblages. Management implications are discussed.</p></div

Comparisons of fish biomass and numeric density for several functional fish groups on Saipan and Rota.

<p>(*indicates P<0.05, comparative tests described in methods).</p

Dynamics of coral colony-size distributions and population densities across the disturbance periods.

<p>Rota had a non-significant decline and recovery in colony-size across the study periods (a), as well as a non-significant, sequential increase in population density (b). Reductions in colony-size were evident for Saipan (c) during the COTS period, accompanied by increases in population densities (d), attributed to the emergence of numerous small faviid and <i>Porites</i> corals (*indicates P<0.05, repeat measures ANOVA with post-hoc tests, see also Fig. 6).</p

Stepwise regression models predicting coral decline.

<p>Stepwise regression models predicting coral decline.</p

Methods used to calculate the percent decline and net change in (a) the benthic substrate ratio and (b) multivariate measures of the coral assemblages.

<p>B, D, A – before, during, and after the disturbance period, respectively. Percent declines were calculated by taking the difference between the minimum and maximum values during and before the disturbance period, and dividing by the pre-disturbance values. Net change values were calculated by taking the difference between maximum values before and after the disturbance period. Values for coral assemblages were calculated based upon their vector magnitude from the origin (0,0), with positive values given for PCO movement towards <i>Acropora</i>, <i>Montipora</i>, and <i>Pocillopora</i> assemblages, and negative values for PCO movement towards tolerant faviids and sparse <i>Porites</i>. See methods and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0105731#pone-0105731-g006" target="_blank">Figure 6</a> for a better description of the corals depicted on the PCO plot.</p

Principle components ordination of coral assemblages for six representative monitoring sites around Saipan (a) and Rota (b).

<p>See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0105731#pone-0105731-g001" target="_blank">Figure 1</a> for site identification and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0105731#pone.0105731.s003" target="_blank">Table S3</a> for summary statistics. Pre-disturbance assemblages are indicated with an asterisk (*), while vectors depict directional change through time. Sparse <i>Porites</i> refers to a dominance of <i>P. lichen</i>, <i>P. vaughani</i>, and small colonies of other massive species. Tolerant faviids consisted of <i>Leptastrea purpurea</i>, <i>Goniastrea retiformis</i>, <i>G. edwardsi</i>, <i>Favia matthaii</i>, <i>F. pallida</i>, and <i>F. favus</i>. Other faviids consisted of <i>Favia stelligera</i>, <i>Platygyra</i> spp., <i>Cyphastrea</i> spp., and <i>Favites abdita</i>.</p

Benthic substrate ratio dynamics for representative monitoring sites around Saipan (sites 1, 9, 8, and 6) and Rota (sites 19 and 16).

<p>See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0105731#pone-0105731-g001" target="_blank">Figure 1</a> for site identification and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0105731#pone.0105731.s003" target="_blank">Table S3</a> for trends from all sites. Grey bars indicate the disturbance period. Benthic substrate ratios indicate the proportion of heavily-calcifying versus less-or-non-calcifying substrates (<i>see methods</i>).</p

Stepwise regression models predicting net ecological change.

<p>Stepwise regression models predicting net ecological change.</p