Grassland fragmentation affects declining tallgrass prairie birds most where large amounts of grassland remain

Abstract

Habitat fragmentation can exacerbate the negative effects of habitat loss for some species. Mitigating fragmentation is difficult, however, because population responses depend on species-level traits (e.g., dispersal ability, edge sensitivity) and landscape context (e.g., habitat amount). Thus, conservation requires determining not only if, but also where and why, fragmentation matters. We aimed to determine if and where grassland fragmentation affects tallgrass prairie birds, which have declined precipitously due to land-use change. We surveyed four edge-sensitive species at 2250 sites (10,291 total surveys) across eastern Kansas, USA, over two breeding seasons. We assessed how the occurrence of each species varied with different levels of fragmentation in local landscapes comprising different grassland amounts. Fragmentation clearly mediated positive relationships between occurrence probability and grassland area for all four species. The direct effect of fragmentation was greater than that of grassland area for two species. Moreover, fragmentation reduced the occurrence of each species by at least half in some contexts. Fragmentation effects were most pronounced in landscapes comprising ~ 50–90% grassland, and less pronounced or absent in landscapes comprising < 50% grassland, which were occupied relatively infrequently. Conservation efforts should minimize ‘perforation’ of large grasslands by woody vegetation and land development, which not only replace grassland, but also often create disproportionately large amounts of grassland edge. Identifying mechanisms responsible for edge effects could further inform species-level conservation. Our results counter assertions that fragmentation does not matter or only matters when habitat is scarce or for species that are dispersal limited.