Effectiveness of terrestrial protected areas in reducing habitat loss and population declines

Protected Areas (PAs) are a critical tool for maintaining habitat integrity and species diversity, and now cover more than 12.7% of the planetís land surface area. However, there is considerable debate on the extent to which PAs deliver conservation outcomes in terms of habitat and species protection. A systematic review approach is applied to investigate the evidence from peer reviewed and grey literature on the effectiveness of PAs focusing on two outcomes: (a) habitat cover and (b) species populations. We only include studies that causally link conservation inputs to outcomes against appropriate counterfactuals. From 2599 publications we found 76 studies from 51 papers that evaluated impacts on habitat cover, and 42 studies from 35 papers on species populations. Three conclusions emerged: first, there is good evidence that PAs have conserved forest habitat; second, evidence remains inconclusive that PAs have been effective at maintaining species populations, although more positive than negative results are reported in the literature; third, causal connections between management inputs and conservation outcomes in PAs are rarely evaluated in the literature. Overall, available evidence suggests that PAs deliver positive outcomes, but there remains a limited evidence base, and weak understanding of the conditions under which PAs succeed or fail to deliver conservation outcomes.