Philip B. Stark Steven M. Ostoja Roland A. Knapp Heather C. McKenny Matthew L. Brooks William T. Langford Lucas N. Joppa Eric L. Berlow John R. Matchett 2015 <p><span>Statistical models often use observational data to predict phenomena; however, interpreting model terms to understand their influence can be problematic. This issue poses a challenge in species conservation where setting priorities requires estimating influences of potential stressors using observational data. We present a novel approach for inferring influence of a rare stressor on a rare species by blending predictive models with nonparametric permutation tests. We illustrate the approach with two case studies involving rare amphibians in Yosemite National Park, USA. The endangered frog,<span class="Apple-converted-space">&nbsp;</span></span><i>Rana sierrae</i><span>, is known to be negatively impacted by non-native fish, while the threatened toad,<span class="Apple-converted-space">&nbsp;</span></span><i>Anaxyrus canorus</i><span>, is potentially affected by packstock. Both stressors and amphibians are rare, occurring in ~10% of potential habitat patches. We first predict amphibian occupancy with a statistical model that includes all predictors but the stressor to stratify potential habitat by predicted suitability. A stratified permutation test then evaluates the association between stressor and amphibian, all else equal. Our approach confirms the known negative relationship between fish and<span class="Apple-converted-space">&nbsp;</span></span><i>R. sierrae</i><span>, but finds no evidence of a negative relationship between current packstock use and<span class="Apple-converted-space">&nbsp;</span></span><i>A. canorus</i><span><span class="Apple-converted-space">&nbsp;</span>breeding. Our statistical approach has potential broad application for deriving understanding (not just prediction) from observational data.</span></p> application/pdf 10.1038/srep10702 en Nature Publishing Group Detecting the influence of rare stressors on rare species in Yosemite National Park using a novel stratified permutation test article