“In a world without humans…” may sound like the gravelly voiced opening of your typical Hollywood blockbuster trailer. But for marine ecologists at Scripps Institution of Oceanography at UC San Diego, envisioning nature in an undisturbed state set the stage for a sobering study of how humans dislodge the very foundation of coral reefs.
Human activity has wreaked havoc on coral reef habitats around the world. In case after case, from the Caribbean to the Indo-Pacific, coral reef health has declined wherever the human footprint is stamped. Overfishing, pollution, and other anthropogenic forces have led to declining reef health and the loss of reef-building organisms, such as corals, and an increase in competitive fleshy seaweeds.
But exactly how does this happen?
Scripps marine ecologist Gareth Williams and his colleagues sifted through an extensive cache of coral reef data and examined conditions on coral reefs at 39 islands spread across the Pacific Ocean, from northwest of Hawaii south to American Samoa and from the central equatorial region west to the Mariana Archipelago.
The new study is the first to include such an extensive swath of reef data. Twenty-four of the islands are unpopulated, allowing Williams and his coauthors to compare human-free conditions against damaged reefs at 15 populated islands. The results of the study generate valuable insights for understanding human impacts on reefs and managing protection against future threats.
Ultimately the scientists found that local human disturbances lead to a “decoupling” of the natural biological and physical relationships inside coral reefs. The foundations of coral reef ecosystems, called benthic communities, at populated islands were cut off from their natural relationship with the surrounding environment —and many of the relationships broke down altogether, the study showed.
For example, around people-free islands, coral coverage was seen to naturally thrive in warmer, more productive waters where damaging wave energy was low. At human-impacted reefs, however, these relationships were no longer seen; it was as if coral cover was no longer responding to these background environmental conditions. Williams and his colleagues termed this phenomenon biophysical decoupling – the natural links between the biological (in this case the benthic cover made up of corals and algae) and the physical (temperature, productivity, wave energy) becoming disrupted in the presence of human impacts.
“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,” the authors say in the report, published in the journal Ecography.
“Given that unpredictability itself slows economic growth, the financial consequences of environmental degradation are multiplied—we show that unregulated exploitation on coral reefs can lead to fewer resources in a disturbed ecosystem and more volatility of these resources,” said Professor Stuart Sandin of the Center for Marine Biodiversity and Conservation at Scripps and coauthor of the paper. “This is a recipe for economic disaster, and is a solid reason to assure that we manage ecosystems in a manner that maintains basic health and functionality, and as a critical benefit, maintains relative predictability of the ecosystem looking forward.”
Another key finding of the study: Not all reefs are the same, regardless of human intervention.
Williams and his colleagues found that the core benthic communities vary in the absence or presence of humans. The researchers say that coral cover—the organisms building the reefs at the base level—peaked in warmer, more productive waters where damaging wave energy is low. On the flipside, even with humans removed from the equation, coral cover in certain cases was low if the islands were located in colder, less productive waters with high-wave energy.
Thus the study leads the way for marine resource managers to protect coral reefs in a more tailored fashion, rather than a one-conservation-plan-fits-all approach.
“Management targets for coral reefs need to be context-specific and consider the upper limit set by the surrounding natural environment,” said Williams.
Scripps researchers led by Sandin have been visiting pristine coral habitats such as regions of the Line Islands since 2004 in an effort to establish a baseline of human impacts.
“While the academic insights gained from this research are large, there is a direct and fundamental importance to the fields of resource management and economics,” said Sandin. “Prediction of natural patterns and phenomena is at the foundation of resource management and these results suggest that there is an intimate linkage between the predictability of natural ecosystems and their state of health. More ‘healthy’ ecosystems are more predictable.”
In addition to Williams and Sandin, coauthors include Jamison Grove of NOAA Pacific Islands Fisheries Science Center, and Yoan Eynaud and Brian Zgliczynski of Scripps Oceanography.
The study was supported by the NOAA Coral Reef Conservation Program and the NOAA/NSF CAMEO (Comparative Analysis of Marine Ecosystem Organization) program.
The paper is dedicated to Rachel Morrison, the late Scripps graduate student and Williams’s friend and colleague. Morrison tragically died on March 28, 2014, after being struck by a vehicle.
— Mario C. Aguilera