Climate-driven, but dynamic and complex? A reconciliation of competing hypotheses for species’ distributions
Abstract
Estimates of the percentage of species “committed to extinction” by climate change range from 15% to 37%. The question is whether factors other than climate need to be included in models predicting species’ range change. We created demographic range models that include climate vs. climate‐plus‐competition, evaluating their influence on the geographic distribution of Pinus edulis, a pine endemic to the semiarid southwestern U.S. Analyses of data on 23,426 trees in 1941 forest inventory plots support the inclusion of competition in range models. However, climate and competition together only partially explain this species’ distribution. Instead, the evidence suggests that climate affects other range‐limiting processes, including landscape‐scale, spatial processes such as disturbances and antagonistic biotic interactions. Complex effects of climate on species distributions—through indirect effects, interactions, and feedbacks—are likely to cause sudden changes in abundance and distribution that are not predictable from a climate‐only perspective.
What the paper shows and why it matters
(AI-generated)
Static climate-envelope models are used to project future ranges and extinction risk. They rely on the view that climate sets where species occur and that a species’ needs can be read from its occurrence records. Biotic interactions such as competition should limit ranges too, yet the study notes that evidence for this at the scale of whole distributions is sparse. Demographic range models, which use survival, growth and reproduction to ask where populations can replace themselves, avoid relying on occurrence records, but none to the authors’ knowledge had included competition. The open question is whether climate, or climate plus competition, is enough to explain where a species occurs.
The study argues that climate remains the dominant influence on where species occur, but that it does not act alone: it also shapes other processes that limit ranges, so climate-only models are likely to miss range dynamics. Its evidence is a pair of demographic range models of the piñon pine Pinus edulis, one with climate only and one with competition added, built from data on 23,426 trees in 1941 forest inventory plots in the south-western U.S. Adding competition fitted the pine’s presence and absence better than climate alone. Yet both predicted persistent populations at cool, wet, high-elevation sites where the species is absent. The study traces this to survival, the demographic rate with the greatest influence on population growth, which competition did not detectably affect. It therefore looks for a missing source of mortality and proposes fire, to which this pine is not adapted. Precipitation helps the trees directly but also promotes fire across the landscape. Fire was rare in the study period, and the idea is proposed, not tested.
The study reads its results as a reconciliation of the climate-only and the climate-plus-competition views of range limits. Climate and competition both matter, competition only weakly, but together they do not explain the distribution. The study proposes that climate also acts indirectly, through disturbances and antagonistic species, and through interactions and feedbacks involving them. Such effects could produce sudden changes in abundance and distribution, so that a species’ persistence or extinction under climate change may not be predictable from climate-envelope models. The test covers one species; the study names a stochastic analysis of climate, fire and insect outbreaks as the next step.