Date Published or Written
Aim: To evaluate changes in breeding bird communities and assess implications for conservation.
Location: Madrean Sky Islands and northern Sierra Madre Occidental, Mexico. Methods: I compared observations from recent fieldwork (2009–2012) with an extensive historical data set (1887–1954) and used modelling and multivariate techniques to assess spatiotemporal changes in species occurrence, richness and assemblage composition, and associations with climate, land use and landscape factors.
Results: Breeding species richness peaked in larger ranges often in the south and east, and regional beta diversity was attributable much more to turnover (0.80) than nestedness (0.07). Although richness increased across time, spatial assemblage heterogeneity declined due only to changes in nestedness, with temporal shifts in beta diversity equally attributable to variation in nestedness and turnover. Community change was associated with variation in climate, land use and landscape factors in ways that depended on species’ traits. Major gains in Madrean and lowland Neotropical species from the south contrasted little change in Nearctic species, and there was some evidence lowland species expanded eastward into the higher-elevation interior, suggesting poleward and up-elevation shifts now occurring globally. Some such patterns were associated with increasing temperature and summer–fall precipitation typical of the south suggesting climate forcing. Despite regional gains in pine-dependent species linked to post-logging forest recovery, losses and turnover were greater in smaller more arid ranges due likely to climate-mediated habitat loss. High regional losses of cavity-nesting species in forest were greater in ranges subjected to past logging indicating effects of historical habitat degradation persist today.
Main conclusion: Despite ongoing forest recovery and potential for northward range expansion to offset climate-mediated losses of montane species, further assemblage homogenization seems likely. Active forest restoration that promotes large old trees, snags and mature forest conditions combined with increasing the capacity of local communities to implement best management practices will enhance conservation.