Protected areas can provide net benefits without reducing the loss of ecosystem area

Evaluations of protected areas (PAs) commonly measure PA effectiveness by the avoided loss of ecosystem area, with many reporting a lack of effectiveness according to this measure. Here, we present evidence that avoided area loss can be a misleading measure of PA effectiveness, because it does not account for spatial variation in conservation benefits and costs. We evaluate PAs in India’s coastal mangrove forests against both an avoided deforestation measure and alternative measures that account for spatially varying carbon stocks and agricultural land values. We find that the PA system did not significantly reduce overall deforestation during 1990–2015, yet it provided significant net climate benefits because carbon stocks were greater in areas where forest cover increased than in areas where it decreased. This favorable climate impact depends, however, on the carbon price used in the evaluation being sufficiently high to offset protection’s opportunity cost. Our findings imply that PA evaluations should shift toward using more complete measures of effectiveness than avoided area loss.