Background/Question/Methods We investigate how local and global stressors affect coral reefs in situ by taking advantage of a longitudinal gradient in the central equatorial Pacific driven by El Niño / Southern Oscillation, where the past frequency of heat stress decreases away from the equator. We compare the benthic communities at 40 sites across four atolls in the Gilbert Islands of Kiribati (Tarawa and Abaiang Atolls) and the Republic of the Marshall Islands (Majuro and Arno Atolls), representing gradients in local human disturbance and past frequency of bleaching-level heat stress.
Results/Conclusions A hierarchical clustering analysis found three groupings of benthic communities, which correspond with sites with (1) low human influence and frequency of heat stress, (2) low human influence and high frequency of heat stress, and (3) high human influence. The frequency of heat stress explained 8.27% of the differences in community compositions across all sites (PERMANOVA, p < 0.01), while local human disturbance explained 15.84% (p < 0.01), and the combined effects explained 6.99% (p < 0.01). Interactions between stressors were multiplicative and acted synergistically to increase the percent cover of macroalgae and hard corals. The high prevalence of the weedy coral P. rus at sites with high human influence drove the positive effects of these stressors on live coral overall. We also find that at the taxa level, half of the multiplicative interactions were antagonistic. These findings suggest that actions that reduce local stressors may help some coral taxa respond to climate stress, but possibly at the expense of other taxa.
