Species' distributions are thought to be in disequilibrium because of changing climate, and anticipating how they will respond has become a major objective of 21st century biodiversity science. However, species’ distributions are influenced by more than climate, including biotic interactions like competition, disturbance, as well as their biogeographic and evolutionary histories. Because of the many processes involved, their within- and cross-scale interactions, and possible feedbacks, our understanding of what factors primarily control species’ ranges remains limited. We took a demography-driven approach to better understand the factors shaping a species’ distribution: we modeled underlying vital rates (birth, growth, and death) explicitly as a function of climate and competition to evaluate them as direct causes of that species’ geographic distribution. Demographic data were used for 15,950 trees occurring in 1,941 forest inventory plots systematically sampled across the distribution of Pinus edulis (common pinon) in the southwestern United States. Vital rate models were combined to form competing integral projection models (IPMs) – reflecting climate, competition, and their combination as alternative explanations for the distribution of common pinon.
Results/Conclusions
Geographic projection of asymptotic population growth rate as a function of landscape variation in climatic and competitive conditions, compared against occurrence data, showed that climate effects provide a very good fit to the lower-elevation limit of the distribution of common pinon, but they failed to explain its upper-elevation range limit. Positive effects of precipitation and negative effects of temperature lead to high projected population growth rates at high-elevation locations where pinon is absent. An alternative IPM that predicted vital rates as a function of competitive conditions marginally outperformed the climate-only model, because of its ability to better predict common pinon occurrence at higher elevations. Disturbance (i.e., fire) or heightened sensitivity of early life stages to competition may best explain pinon’s high-elevation range limit. Correlative species distribution models, based only on climate, risk representing range dynamics poorly, particularly in cases where climate is an indirect control on other processes that more directly determine a species’ distribution, and those processes include feedbacks and thus complex, nonlinear (threshold) behavior. Disturbance processes and biotic interactions (as mediated by disturbance) are direct drivers at a local scale, and are necessary for predicting common pinon’s distribution.