Understanding how biotic and abiotic interactions determine species distributions is a fundamental question in ecology with important implications for how distributions are expected to shift with climate change. Here, we examine the relative importance of biotic and abiotic factors in explaining temporal occupancy for 143 bird species across North America. Unlike previous studies which tend to model distributions in terms of presence/absence, we take advantage of a geographically extensive dataset of community time series to calculate the temporal occupancy of species at sites throughout their expected range. We first characterize the extent to which temporal occupancy varies across the geographic range, and then use variance partitioning to evaluate the relative importance of biotic variables (the abundance of potential competitors) and abiotic variables (elevation, temperature, precipitation, a vegetation index) for explaining patterns of temporal occupancy.
Results/Conclusions
On average, species exhibited a striking bimodal distribution of temporal occupancy across the range, where some sites supported persistent populations and other sites where members of the species were only occasionally observed. At each site, we compared whether species’ occupancy was driven more strongly by competitor abundance or environmental variables. Interspecific interactions were a strong predictor of focal species occupancy (p = 0.002) and the slope was negative as predicted (-0.02), indicating that as abundance of the competitor species increased, occupancy of the focal species decreased. Elevation and precipitation were also strong predictors of focal occupancy (p = 0.002 & 0.056), while temperature and the vegetation index were not (p = 0.146 & 0.207). Environmental variables explained slightly more variation observed in focal species occupancies than competitor abundance (20% versus 14%). These results can be used improve biodiversity modeling, gain a better understanding of what factors influence species distributions and temporal persistence, and identify species at risk for habitat loss due to climate change.