Local human impacts decouple natural biophysical relationships on Pacific coral reefs

Abstract

Human impacts can homogenize and simplify ecosystems, favoring communities that are no longer naturally coupled with (or reflective of) the background environmental regimes in which they are found. Such a process of biophysical decoupling has been explored little in the marine environment due to a lack of replication across the intact‐to‐degraded ecosystem spectrum. Coral reefs lacking local human impacts provide critical baseline scenarios in which to explore natural biophysical relationships, and provide a template against which to test for their human‐induced decoupling. Using 39 Pacific islands, 24 unpopulated (relatively free from local human impacts) and 15 populated (with local human impacts present), spanning 45° of latitude and 65° of longitude, we ask, what are ‘natural’ biophysical relationships on coral reefs and do we see evidence for their human‐induced decoupling? Estimates of the percent cover of benthic groups were related to multiple physical environmental drivers (sea surface temperature, irradiance, chlorophyll‐a, and wave energy) using mixed‐effects models and island mean condition as the unit of replication. Models across unpopulated islands had high explanatory power, identifying key physical environmental drivers of variations in benthic cover in the absence of local human impacts. These same models performed poorly and lost explanatory power when fitted anew to populated (human impacted) islands; biophysical decoupling was clearly evident. Furthermore, key biophysical relationships at populated islands (i.e. those relationships driving benthic variation across space in conjunction with chronic human impact) bore little resemblance to the baseline scenarios identified from unpopulated islands. Our results highlight the ability of local human impacts to decouple biophysical relationships in the marine environment and fundamentally restructure the natural rules of nature.

Energetic differences between bacterioplankton trophic groups and coral reef resistance

“…Even though six of eight locations are from the same regional group (MHI), the intra-group proximity within cluster 2 is relatively low. This finding supports the hypothesis that chronic human impacts probably alter Pacific coral reefs in such a way that they are no longer reflective of the background environments in which they reside [52].…”

A test of trophic cascade theory: fish and benthic assemblages across a predator density gradient on coral reefs

“…Our findings demonstrate the importance of distinguishing between fishing-induced trophic cascades that are solely instigated by the loss of predators rather than broader anthropogenic-induced ecosystem collapse on coral reefs, which is often due to a multitude of factors in addition to fishing pressure. However, given the high variability of environmental and biological factors such as productivity, habitat complexity, and species richness on coral reefs, the detectability of trophic cascades may significantly differ among geographically distinct reef systems (Salomon et al 2010;Edwards et al 2010;Williams et al 2015a).…”

“…Regional effects may exist due to differences in environmental conditions such as nutrient input and reef geomorphology across the GBR (Hutchings et al 2008) or biophysical factors (i.e. sea surface temperature, wave energy, system productivity; Williams et al 2015a). In a post hoc evaluation of nutrient availability on the reefs included in this study (Online Resource Table S22), we found no difference in nutrient availability among the management zones (ANOVA: F (2,12) = 0.64, p = 0.544) or between the regions (paired two-tailed t test: t = −1.207, df = 12.971, p = 0.249).…”

Seasonal dynamics and disturbance of phytoplankton biomass in the wake of Tahiti as observed by Biogeochemical-Argo floats

“…In oligotrophic environments, where ecosystems are predominantly based on nutrient-depleted waters, the increase in phytoplankton biomass driven by the IME is critical for coral reef ecosystems (Williams et al, 2015) and fishery productivity but also contributes to carbon vertical flow (Heywood et al, 1996). Finally, such biological enhancement has to be more carefully described and studied in order to be integrated into global biogeochemical models, and hopefully to better predict the evolution of nearshore marine ecosystems.…”

“…In these surface nutrient depleted waters, the biological enrichment reported near many islands, the so-called "Island Mass Effect" (IME; Doty and Oguri, 1956), locally enhances to the productivity and potential fisheries (Gove et al, 2016). By supporting a greater abundance of fish and reef-building organisms 20 in comparison with more oligotrophic waters, the IME-induced nearshore biological enhancement is also critical for coral reef ecosystems development and sustainability (Williams et al, 2015). The IME may result from many processes such as the vertical transport of nutrient-rich water masses (e.g.…”