Avian abundance thresholds, human-altered landscapes, and the challenge of assemblage-level conservation

Land-use change is a global phenomenon with potential to generate abrupt spatial changes in species' distributions. We assessed whether theory about the internal structure of bird species' geographic ranges can be refined to reflect abrupt changes in distribution and abundance associated with human influences on landscapes, and whether the prevalence and diversity of bird-landscape threshold relationships may significantly complicate assemblage-level avian conservation. For three large regions in the United States, we used the North American Breeding Bird Survey, U.S. National Land Cover Data, and multivariate adaptive regression splines to assess whether land bird species' abundances were associated with landscape composition and configuration in a threshold fashion. Threshold relationships between abundance and landscape characteristics were exhibited by 42-60 % of the species studied. The relationships were evident for five land types and five habitat guilds. We observed threshold relationships for more taxonomically diverse groups of bird species, a broader set of land types, and larger geographic extents than have been considered to date. Avian distribution and abundance theory can be refined by articulating that characteristics of human-altered landscapes have the potential to be widespread and biologically important contributors to abrupt spatial change in species' abundances. Our findings also expose bird-landscape threshold relationships as pervasive and diverse patterns that impose a much more complicated set of circumstances for assemblage-level conservation of birds than has been widely recognized. To cope with these complications, landscape planners and managers can use optimization analyses, multispecies frameworks, regulatory limits, and multivariate change-point analyses.