Population trends influence climate niche breadth, geographic distribution, and response to climate change in North American birds
Ecological niche theory states that occupied niche breadth should increase with population density. This relationship has been studied extensively in the context of habitat niche breadth, and much evidence supports that populations at higher density occupy a wider range of habitats. However, no previous studies have investigated the relationship between population trend and breadth of other environmental niche components, for example climate niche breadth. Given the importance of climate niche in determining species distributions, if population trends influence occupied climate niche breadth, this could have important implications for understanding dynamic species distributions, responses to climate change, and our ability to model species distributions into the future. We compare trends in abundance to changes in distributional extent and climate niche breadth between 1980 and 2012 for 60 species of birds, estimated using occurrence data from the North American Breeding Bird Survey and 8 bioclimatic variables. We use path analysis to determine what role changes in climate niche breadth play in the widely documented positive correlation between abundance and distribution. Lastly, we characterize shifts in geographic and climate niche space to determine whether trends in abundance influence species’ responses to climate change.
We demonstrate for the first time that trends in abundance are significantly and positively correlated with changes in climate niche breadth. Path analyses revealed that the direct influence of trends in abundance on distribution was non-significant compared to a highly significant indirect pathway through change in niche breadth, indicating that climate niche breadth is likely an important mechanism linking abundance and distribution. Additionally, we found that population trend influenced species’ responses to climate change. Species with increasing trends were more likely to expand their niche into warmer climate space than were declining species, consistent with increasing species geographically maintaining their southern boundary despite climates warming. Conversely, species with declining trends were more likely to unfill from cooler niche space, indicating a failure to geographically “track” climates northward at the northern periphery. These results indicate that occupied climate niche can change within species on an ecological timescale as a function of population trends, and that changes in climate niche are important for understanding dynamic distributions especially in the face of climate change.