Authors: Jones, Julia; Wauchope, Hannah ;

International policy is focused on increasing the proportion of the Earth's surface protected for nature. While studies show that protected areas prevent habitat loss, there is a surprising lack of evidence for their impact on species’ populations: existing studies are local scale or use simple designs that lack appropriate controls. We explore how 1506 protected areas have impacted the trajectories of 27,055 waterbird populations using a robust Before-After-Control-Intervention study design, which compares protected and unprotected populations in the years before and after protection. We show that the simpler study designs typically used to assess protected area effectiveness (before-after and control-intervention) incorrectly estimate impact for 37-50% of populations. Using our robust study design, we find that protected areas have a decidedly mixed impact on waterbirds, with a strong signal that areas managed for waterbirds or their habitat are more likely to benefit populations, and a weak signal that larger areas are more beneficial than smaller ones. Calls to conserve 30% of the Earth’s surface by 2030 are gathering pace, but protection does not guarantee good biodiversity outcomes. As countries gather to agree the new Global Biodiversity Framework, targets must focus on creating and supporting protected areas that measurably benefit populations.