The effectiveness of marine reserves is compromised by human proximity
The ecological benefits of marine reserves located near human settlements are limited. This is demonstrated by researchers from the University of Montpellier, the IRD, and the University of New Caledonia, in collaboration with the CNRS, who studied 1,800 coral reefs, including 106 located in 20 marine reserves. This research is published in PNAS.
A grey reef shark swims over a reef in a large protected area far from humans. Photo credit: Nick Graham
Marine reserves: a tool for combating overfishing
Coral reefs are among the marine ecosystems most severely impacted by human activity, resulting in a loss of biodiversity and biomass (the total mass of organisms). To combat direct human pressures, particularly overfishing, more than 2,000 marine reserves—covering over 6% of the world’s coral reefs—have been established. Some are established on reefs very close to human settlements or coastal communities, while others are on reefs isolated from any human presence. This second conservation strategy often draws criticism: why preserve isolated ecological systems rather than protect—and thus restore—heavily degraded systems in contact with humans?
It is against this backdrop that this scientific study has been conducted by an international consortium since 2016. Its objective: to estimate fish biomass and the presence of top predators—such as sharks or large fish-eating species—in marine reserves in order to assess their effectiveness. These species play an essential role in the proper functioning of coral reefs: as an indispensable link in the food chain, they accelerate nutrient recycling and eliminate diseased individuals.
Partial protection of ecosystems close to human settlements
This global study was conducted across 1,800 coral reefs, 106 of which are located within marine reserves. It shows that establishing reserves near human settlements mitigates human pressures, but does not eliminate them entirely—particularly those related to fishing.
“Overall, top predators are absent from more than 70% of coral reefs. They are present on less than 1% of reefs near human settlements. In contrast, their presence rate on isolated reefs is 59%,” explains David Mouillot, one of the researchers who coordinated the study. Restoring populations of top predators such as sharks through small reserves located near human-impacted coasts therefore seems unrealistic. The best way to preserve them remains the implementation of protective measures at isolated sites.
When it comes to fish biomass, the situation is somewhat different. Protection appears to be optimal in reserves where human pressure is at an intermediate level.
“Different conservation strategies can benefit coral reefs,” concludes David Mouillot. “Our study suggests maintaining a wide variety of reserves with diverse objectives and not neglecting the protection of sites remote from human activity.”
Laboratories involved in the study
- ENTROPIE: Tropical Marine Ecology of the Pacific and Indian Oceans (IRD, University of Réunion, CNRS)
- LIVE: Island Laboratory for Life and the Environment (University of New Caledonia)
- MARBEC: Center for Marine Biodiversity, Exploitation, and Conservation (University of Montpellier, IRD, Ifremer, CNRS)