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.

Rising anthropogenic CO 2 in the atmosphere is accompanied by an increase in oceanic CO 2 and a concomitant decline in seawater pH (ref. 1). This phenomenon, known as ocean acidification (OA), has been experimentally shown to impact the biology and ecology of numerous animals and plants 2 , most notably those that precipitate calcium carbonate skeletons, such as reef-building corals 3 . Volcanically acidified water at Maug, Commonwealth of the Northern Mariana Islands (CNMI) is equivalent to near-future predictions for what coral reef ecosystems will experience worldwide due to OA. We provide the first chemical and ecological assessment of this unique site and show that acidification-related stress significantly influences the abundance and diversity of coral reef taxa, leading to the often-predicted shift from a coral to an algae-dominated state 4,5 . This study provides field evidence that acidification can lead to macroalgae dominance on reefs.Coral reefs contain the highest concentration of biodiversity in the marine realm, with abundant flora and fauna that form the backbone of complex and dynamic ecosystems 6 . From an anthropocentric standpoint, coral reefs provide valuable goods and services, supporting fisheries and tourism, and protect shorelines from storms 7 . Recently, widespread coral mortality has led to the flattening of reef frameworks and the loss of essential habitat 4 . This trend will be accelerated by ocean acidification (OA), as calcification is impaired, and dissolution is accelerated 8,9 . Furthermore, experimental evidence suggests that OA could enhance the growth 10 and competitive ability of fleshy macroalgae 11 . This OA-induced shift in the competitive balance between corals and algae could exacerbate direct effects of OA on calcifying reef species 12 and lead to ecosystem shifts favouring non-reef-forming algae over coral 4,5 . Understanding the individual responses of taxa to OA, as well as alteration of multi-species assemblages, is therefore critical to predicting ecosystem persistence and managing reef health in an era of global change.At present, much of what is known concerning the impacts of OA on coral reef biota has been laboratory-based experimental work focused on the responses of select taxa 2 . This has been expanded to mesocosm-based studies, allowing manipulation of groups of organisms and investigation of community responses 13 .Although these multi-species experimental studies are vital, they cannot recreate the variability (physical, chemical, biological) of real-world reef systems 14 . In an effort to overcome the limitations of laboratory studies, real-world low-saturation-state (Ω) sites have been investigated. In the eastern Pacific, nutrient and CO 2 -enriched upwelled waters impact coral calcification and the precipitation of carbonate cements, influencing the distribution of reefs 15 . In Mexico, freshwater springs depress Ω, influencing coral calcification and species distributions 16 . In Palau, restricted circulation and biological activity contribute to ...

For sessile organisms, such as reef building corals, differences in the degree of dispersion of individuals across a landscape may result from important differences in life history strategies or may reflect patterns of habitat availability. Descriptions of spatial patterns can thus be useful not only for the identification of key biological and physical mechanisms structuring a given ecosystem, but also by providing the data necessary to generate and test ecological theory. Here, we used an in situ imaging technique to create large-area photomosaics of 16 plots at Palmyra Atoll, central Pacific, each covering 100 m 2 of benthic habitat. We mapped the location of 44,007 coral colonies and identified each to the lowest taxonomic level possible. Using metrics of spatial dispersion, we tested for departures from spatial randomness and also used targeted model fitting to explore candidate processes leading to differences in spatial patterns among taxa.Most taxa were clustered and the degree of clustering varied by taxon. A small number of taxa did not significantly depart from randomness and none revealed evidence of spatial uniformity.Importantly, taxa which readily fragment or tolerate stress through partial mortality were found to be more clustered. With little exception, clustering patterns were consistent with models of fragmentation and dispersal limitation. For some taxa, dispersion levels were related linearly to abundance, suggesting density dependence of spatial patterning. The spatial patterns of stony corals are non-random and reflect fundamental life historical characteristics of the taxa, suggesting that the reef landscape may, in many cases, have important elements of spatial predictability.