Woody plant encroachment restructures bird communities in semiarid grasslands

Abstract

The abundance and distribution of woody plants have increased in grassland ecosystems worldwide. Robust generalizations about the consequences of this transformative process on animal communities have been elusive, especially in semiarid regions where populations of many species have declined. We evaluated how distributions and species richness of breeding birds responded to woody plant encroachment by using spatial variation in woody cover as a proxy for the temporal process by which grasslands transform into shrub savannas. Specifically, we surveyed breeding birds and vegetation on 140 10-ha plots in semiarid grasslands that spanned the gradient of cover by Prosopis (mesquite), a genus of shrubs that has proliferated in semiarid grasslands worldwide. We used a multispecies occupancy model to characterize distributions of breeding bird species along the encroachment gradient. Distributions of 29 of 35 species changed markedly in response to encroachment, with distributions of most obligate grassland species contracting and most facultative grassland species expanding. Species richness increased sharply as cover of woody plants increased and peaked at ∼22% cover; this increase was driven by recruitment of generalist and shrub-associated species, many of which are common at regional scales. Lastly, we identified thresholds of woody cover where distributions contracted or expanded markedly, which provide targets for conservation and restoration efforts. Our results highlight the importance of understanding species-specific responses to woody plant encroachment as the basis for explaining community-level patterns because increases in diversity at local scales might ultimately reduce diversity at broader scales as grassland specialists are displaced.