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What controls species' distributions? A demographic analysis and a new hypothesis
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  • Emily Schultz,
  • Lisa Huelsmann,
  • Michiel Pillet,
  • Florian Hartig,
  • David Breshears,
  • Sydne Record,
  • John Shaw,
  • Robert DeRose,
  • Pieter Zuidema,
  • Margaret Evans
Emily Schultz
Rice University

Corresponding Author:[email protected]

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Lisa Huelsmann
University of Regensburg
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Michiel Pillet
Montana State University Bozeman
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Florian Hartig
University of Regensburg
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David Breshears
University of Arizona
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Sydne Record
Bryn Mawr College
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John Shaw
USDA Forest Service Rocky Mountain Research Station
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Robert DeRose
University of Utah
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Pieter Zuidema
University of Utrecht
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Margaret Evans
The University of Arizona College of Science
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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.