Interspecific interactions shift toward facilitative with increasing proximity to range margins

Authors: Daryn Hardwick*, University of Oklahoma, Bruce Hoagland, University of Oklahoma
Topics: Biogeography, Spatial Analysis & Modeling, Environment
Keywords: Stress Gradient Hypothesis, species distributions, biogeography, ecology, trees, forests
Session Type: Paper
Day: 4/11/2018
Start / End Time: 5:20 PM / 7:00 PM
Room: Muses, Sheraton, 8th Floor
Presentation File: No File Uploaded

The Stress Gradient Hypothesis (SGH) posits that interspecific interactions become more facilitative under stressful environments. Many studies have found contradictory evidence to the SGH. However, almost all SGH research has analyzed the direct interactions between a few species over small spatial scales. We analyzed the effect of biogeography on indirect interspecific interactions across regional scales to provide a unique perspective on the SGH. Upland and bottomland forest tree species were used to assess species importance across 307 sites. We analyzed the difference in interaction intensity over space using a nearest neighbor analysis iteratively for each site. The most competitive net interactions occur farthest away from range margins, while the most facilitative net interactions occur beyond range margins. The trend in net interactions is monotonic and shifts to more facilitative with increasing proximity to range margins. Net interactions between species in the null communities are significantly more competitive across the entirety of the stress gradient. Our results support the original hypothesis of the SGH. However, analyzing species separately produced different results with approximately equal proportions of species exhibiting unimodal, monotonic, and multimodal distributions. Geographic context is therefore important in interpreting how interspecific interactions change across stress gradients and is an adequate measure of stress for many species. These results, along with how geographic range shifts are altered by interspecific interactions should be utilized to inform and improve species distribution modeling efforts.

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