31 March 2026Policy analyses often treat ecosystems in isolation, overlooking the complex interconnections that define socio-ecological systems. However, conservation measures in one ecosystem may produce unintended spillover effects in another-a phenomenon that remains poorly understood. This dynamic could be particularly important in coastal regions, where one billion people live, many of whom rely on access to both fisheries and land resources for their livelihoods. In Madagascar, where agricultural expansion commonly occurs through forest conversion, we wonder if increasing the protection of one of these resources, fisheries, results in greater use and pressure on another resource, forests. We conducted a household survey of 1,120 households across 41 villages. The results show that spillovers between marine and terrestrial resources are likely to occur but that increasing the regulation of marine resources can have an ambiguous effect on deforestation. Next, we combine matching and difference-in-differences models to quantify the effect of creating 200 locally managed marine areas on deforestation across the entire country over 17 years. We find that spillovers do occur in terrestrial areas following the interventions and, on average, lead to a 68% increase in forest cover loss. This increase in deforestation is more pronounced in remote areas, in regions with greater baseline forest cover, and in places that receive less precipitation.