Metapopulation-level associations in positively interacting stream fishes

Positive biotic interactions are recognized as important factors determining species distributions. Although effects of positive interactions have often been observed at local scales, much less is known about consequences at larger spatial scales. Here, we study nest associations of stream fishes - widespread reproductive facilitation between host (nest-builder) and beneficiary (nest associate) species in North America - as a model system to examine the role of positive interactions in determining the metapopulation-level relationship between host and beneficiary species. Using regional data of fish distribution in the Midwestern US, we found that watershed-level occupancy of host species (i.e., metapopulation occupancy) remarkably increased that of nest associates. Our results illustrated that the effects of positive biotic interactions at the metapopulation level were comparable or even stronger than environmental drivers, i.e., factors that have been studied most extensively in metapopulation studies. Further, our model supported the hypothesis that the metapopulation-level relationship between hosts and nest associates was mediated by a gradient of environmental conditions: strong associations occurred under stressful habitats. This study provides insightful evidence that positive biotic interactions have larger scale consequences for distributions of organisms than previously thought. Successful biodiversity conservation may need a broader framework that appreciates the role of positive biotic interactions at larger spatial scales.