US National Wildlife Refuge System likely to see regional and seasonal species turnover in bird assemblages under a 2 degrees C warming scenario

The National Wildlife Refuge System (NWRS) provides one of the United States' greatest protected area networks for wildlife conservation. As climate changes beyond historical ranges of variability, refuge managers are confronted with assessing the utility of refuges, including how to best manage refuges both individually and as a system to help species cope with rapid change. Using published species distribution models, we projected species-specific changes in environmental suitability for 590 native North American bird species under a 2 degrees C future warming scenario (similar to 2050s under RCP8.5) at 525 refuges. For each species, we classified projected changes in suitability (i.e., improving, stable, or worsening suitability) and whether they crossed a model-derived persistence threshold at a refuge (i.e., potential colonization or potential extirpation). Overall, we found that a quarter of species (23% in summer, 26% in winter) could be different (i.e., turnover) across the refuge system despite protections. Summer and winter communities are not equally affected, so managers should consider different strategies for the different seasons. We forecast a slight net species loss in summer, from a mean of 109.0 +/- 0.8 to 102.0 +/- 0.7 species per refuge. In winter, we forecast a net gain in species, from a mean of 97.1 +/- 2.2 to 118.5 +/- 1.8 species per refuge. This includes an average of 12 species per refuge that may overwinter rather than migrate south. Refuges at northern latitudes will see relatively more turnover in species, while southern and coastal refuges will see fewer changes. Despite these species changes, dominant habitat association groups (e.g., waterbirds, forest birds) will generally stay the same across most of the NWRS. Some species may be lost from the entire NWRS and can benefit in the near term from targeted management. Regions of high extirpation and colonization (i.e., at northern latitudes) can be prioritized for strategic additions of new refuges.