What controls species' distributions? A demographic analysis and a new
hypothesis
Abstract
Ecologists have long sought to understand the controls of species’
geographic distributions. Two important hypotheses have been that range
limits are determined 1) predominantly by climate or 2) by competition
in addition to climate, with competitive interactions dominating where
climate is benign. If the first hypothesis is correct, the effect of
changing climate on species’ distributions should be adequately
predicted by climate-only models. Here, we used demographic range models
(DRMs) to evaluate the influence of climate and competition on the
geographic distribution of Pinus edulis, a desert pine in the
southwestern U.S. We parameterized DRMs with data on 23,426 trees in
1,941 forest inventory plots. Vital rate responses were consistent with
the predictions of the second hypothesis: negative effects of warm-dry
conditions at low-elevation sites and competition at cool-humid,
high-elevation sites. Further, including both climate and competition in
the DRM yielded a better fit of DRM-predicted population growth rate to
the observed distribution than a climate-only DRM. However, at
cool-humid sites, negative effects of competition were too weak to
offset positive climate effects, resulting in a mismatch between
predicted population growth rate and observed distribution. This result
leads us to formulate a new hypothesis: climate has complex influences
on species’ distributions through its effects on range-limiting
processes at a variety of scales, for example, biotic and disturbance
feedbacks at community, ecosystem, landscape, and macrosystem scales.
The implications of this complex systems hypothesis are that range
dynamics may be poorly predictable from static, climate-only models, and
may be characterized by sudden changes in abundance, even distribution
collapse, with climate change.