Understanding how various biotic and abiotic factors interact to produce the variation found in species’ abundances across large areas is an important challenge for ecologists today. In this paper we assess the extent to which competition, climate, predation, productivity and land cover variables permit us to accurately predict species' abundances across breeding ranges. We used these variables to explain the variation in an individual species’ abundance across its breeding range, for 74 non-rare resident land bird species of the US (using North American Breeding Bird Survey data). We chose resident birds because changes in their abundances will not be due to environmental changes on wintering grounds that are located elsewhere. For each focal species in turn, we regressed abundances against measures of local climate conditions, productivity (as an index of resource availability), distance to nearest preferred habitat of the focal species, competition pressure and predation pressure, in a multi-model inference framework, to assess the relative strengths of these factors (using Akaike weights) as drivers of abundance.
Across these species (and at this scale of analysis), the amount of variation in abundance explained was between 0 and 0.75: further analyses show that species with uniformly low abundances are not well served by our approach. For the rest: some species’ local abundances are predictable with climate information only, some species require both climate and competitive effects, and a few species’ local population sizes are predictable with competition (or less frequently, predation pressure or proximity to preferred habitat) alone. However, when tallying the number of species for whom a climate variable is selected in > 99% of models of local population size, the tally is equal to that for the number of species for whom competition pressure was selected in > 99% of models of local population size. Predation pressure, as measured here, was a less important predictor of local population size than competition pressure or climatic conditions. These results have broad-ranging implications, in that a number of projections of climate change impacts on species will be rendered of little use, unless we can in future a) establish for which species climate information is sufficient to predict abundances, and b) achieve a synthetic understanding of how climate and competition pressure impact local population sizes across large areas.