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International policy is focused on increasing the proportion of the Earth's surface that is protected for nature 1,2 . Although studies show that protected areas prevent habitat loss 3-6 , there is a lack of evidence for their effect on species' populations: existing studies are at local scale or use simple designs that lack appropriate controls 7-13 . Here we explore how 1,506 protected areas have affected the trajectories of 27,055 waterbird populations across the globe 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 or control-intervention) incorrectly estimate effects for 37-50% of populations-for instance misclassifying positively impacted populations as negatively impacted, and vice versa. Using our robust study design, we find that protected areas have a 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 14 , but we show that protection alone does not guarantee good biodiversity outcomes. As countries gather to agree the new Global Biodiversity Framework, targets must focus on creating and supporting well-managed protected and conserved areas that measurably benefit populations., (© 2022. The Author(s), under exclusive licence to Springer Nature Limited.)

Climate change is forcing the redistribution of life on Earth at an unprecedented velocity 1,2 . Migratory birds are thought to help plants to track climate change through long-distance seed dispersal 3,4 . However, seeds may be consistently dispersed towards cooler or warmer latitudes depending on whether the fruiting period of a plant species coincides with northward or southward migrations. Here we assess the potential of plant communities to keep pace with climate change through long-distance seed dispersal by migratory birds. To do so, we combine phenological and migration information with data on 949 seed-dispersal interactions between 46 bird and 81 plant species from 13 woodland communities across Europe. Most of the plant species (86%) in these communities are dispersed by birds migrating south, whereas only 35% are dispersed by birds migrating north; the latter subset is phylogenetically clustered in lineages that have fruiting periods that overlap with the spring migration. Moreover, the majority of this critical dispersal service northwards is provided by only a few Palaearctic migrant species. The potential of migratory birds to assist a small, non-random sample of plants to track climate change latitudinally is expected to strongly influence the formation of novel plant communities, and thus affect their ecosystem functions and community assembly at higher trophic levels.