Hindcasts of passerine density in arctic and subarctic Alaska suggest noncomplementary responses to shrub expansion by tundra- and shrub-adapted species

Abstract

ABSTRACT Shrub expansion is among the most conspicuous of the warming-related phenomena occurring within tundra regions, but its effects on vertebrates are not well understood. Among passerines, the habitats of shrub- and tundra-adapted species are expected to increase and decrease, respectively, by roughly equivalent amounts. However, shrub expansion is highly discontinuous, and these predictions are typically based on coarse-scale models of vegetation that permit limited spatial heterogeneity. We used a hindcasting approach to investigate spatial and interspecific heterogeneity in passerine densities through time, as a function of changes in Normalized Difference Vegetation Index (NDVI). We conducted line transect surveys of passerines between 2015 and 2019 at four sites in arctic and subarctic Alaska. Collecting the encounter locations permitted fine-scale assessment of covariates of density. We fit spatial distance sampling models to these data and then used the predicted relationship between passerine density and NDVI to hindcast changes in density since 1999. Our hindcasts suggested limited areal changes in the habitat of tundra-adapted species over the last twenty years while the habitat corresponding to tall shrub–adapted species has likely expanded. Thus, our results contrast with previous predictions due partially to the finer spatial scale of our inference, which allowed for portions of the landscape to resist shrub expansion.