Large Marine Protected Areas (MPAs) have been shown to be far more effective than smaller ones in conserving biodiversity and ecosystems. New research from the Chagos Archipelago in the Indian Ocean shows that size matters: large reserves can fully encompass the movements of wide-ranging species and secure critical feeding and breeding grounds.

Key findings

By tracking manta rays, hawksbill turtles and seabirds, scientists found that most of these animals remained almost entirely within the boundaries of the 640,000 km² reserve. For instance, all of the monitored turtles and manta rays remained within the MPA, while the seabirds ranged more widely but still concentrated around protected waters. Such findings emphasise the fact that large areas can act as the core habitat of very different species.

Despite their different behaviours, all three groups converged on the Great Chagos Bank, a vast reef system within the reserve. This overlap shows that big MPAs not only protect individual species but also preserve multi-species hotspots, which are essential for maintaining biodiversity at scale.

Advantages of scale

Smaller MPAs often fail to protect migratory species that travel hundreds of kilometres. By contrast, larger protected reserves offer better buffer zones against the effects of climate change, such as heatwaves and ocean acidification. They help to preserve genetic diversity by enabling populations to flourish, reproduce and repopulate nearby areas. Large MPAs also provide ecosystem services, such as carbon sequestration and coastal protection on a scale that small MPAs cannot match.

Challenges and trade-offs

Creating and managing large MPAs is not without difficulty. These include enforcing regulations over vast distances, monitoring costs, the complexity of governance, and balancing human activities, such as fishing and tourism. There is also a risk that large MPAs will be ineffective if protection is weak or laws are not enforced.

For MPAs to reach their full potential, design must consider ecological size, placement, connectivity and strong legal protection. Policymakers should prioritise expanding MPAs, ensuring they are not just vast in area but also effective in regulation and management. Communities must be engaged and resources must be committed to enforcement and science-based monitoring.