Coral reef degradation is not correlated with local human population density

Abstract

The global decline of reef-building corals is understood to be due to a combination of local and global stressors. However, many reef scientists assume that local factors predominate and that isolated reefs, far from human activities, are generally healthier and more resilient. Here we show that coral reef degradation is not correlated with human population density. This suggests that local factors such as fishing and pollution are having minimal effects or that their impacts are masked by global drivers such as ocean warming. Our results also suggest that the effects of local and global stressors are antagonistic, rather than synergistic as widely assumed. These findings indicate that local management alone cannot restore coral populations or increase the resilience of reefs to large-scale impacts. They also highlight the truly global reach of anthropogenic warming and the immediate need for drastic and sustained cuts in carbon emissions.

Figure 1. Location of reef sites by subregion.

Map created in R 3.2.0 , https://www.R-project.org.

Figure 2. The global relationship between coral and macroalgae cover and the logarithm of human population density within 50 km of each reef location.

Responses in figures (A,B) are raw coral and macroalgal cover data. Responses in (C,D) are smoothed with partial residuals from the generalized additive mixed effect models (GAMM, Table 1) to account for spatial autocorrelation. Human population density was used as a proxy for local impacts (e.g., fishing, development, and pollution). Colors correspond to different subregions. Subregional relationships and analyses are shown in Fig. S2 and S3 and Tables S1 ansd S2 respectively.

Figure 3

Density histograms of coral cover (A) and macroalgae cover (B) for sites with no human inhabitants within a 50 km radius. Average coral cover in these relatively isolated reefs was 32.5% (median 31.3% with 19.7–44.4%, 1^st–3^rd quantile), slightly higher than the global average of 27.6% (median 24. 5% with 13.1–39.5% 1^st–3^rd quantile). Mean macroalgal cover on these reefs (10.2%, median 6.9% with 1.6–15.5%, 1^st–3^rd quantile) was somewhat lower than the global mean of 15.1% (median 10.0% with 3.1–22.4% 1^st–3^rd quantile).