Local human impacts decouple and alter biophysical relationships on Pacific coral reefs

Background/Question/Methods

Human impacts homogenize and simplify ecosystems, disrupting natural biophysical relationships that would otherwise shape ecological communities. The result is a community whose appearance no longer reflects adaptation to long-term background environmental conditions but instead echoes chronic human pressure. Here we use coral reefs as a case study to explicitly test this concept as they provide example of both the most human-degraded and the most pristine ecosystems on our planet. This, together with their wide geographical distribution spanning multiple environmental gradients, makes them an ideal ecosystem in which to test whether local human impacts are capable of disrupting natural biophysical relationships. Using 39 Pacific coral reef atolls and islands spanning 45° of latitude and 65° of longitude we ask, what are “natural” biophysical relationships on coral reefs, and are local human impacts capable of disrupting them? We assembled data at an island-mean scale; we used towed-diver surveys to quantify benthic communities, and satellite-derived and modeled data to quantify sea surface temperature (SST), irradiance, wave energy, chlorophyll-a (as a proxy for phytoplankton biomass), and the frequency/magnitude of anomalous events for each. We used bootstrapping and generalized additive mixed-effects models to characterize benthic variation and biophysical relationships.

Results/Conclusions

At remote (unpopulated) islands, hard coral cover peaked where mean SST was higher, where wave energy was lower (particularly the frequency and magnitude of anomalies), and where mean chlorophyll-a was higher. This model provided strong explanatory power, but performed poorly across populated islands; the relationships with SST and chlorophyll-a broke down, and wave energy provided little explanatory power. The cover of crustose coralline algae at remote islands was positively related to both the frequency and magnitude of temporal anomalies in chlorophyll-a. At populated islands these relationships broke down and reversed, respectively, with a complete loss of explanatory power. At remote islands, macroalgae was negatively related to the frequency of chlorophyll-a anomalies; the direction of the relationship held at populated islands, but explanatory power was lost. In conclusion, local human impacts can break down and, on occasion, reverse natural biophysical relationships on coral reefs; novel relationships may even develop. We hypothesize this occurs through ecological homogenization, whereby human impacts act to reorganize community structure and effectively lock the system in a regime that no longer reacts to background environmental conditions. Our findings highlight the ability of human impacts to fundamentally modify ecological community dynamics and relationships with their surrounding environment.