As climate has warmed, many species have moved up mountains as physiological limits to their distributions have ameliorated. These distribution shifts are creating novel communities, begging the question: What happens to species at the tops of mountains as potential antagonists encroach upwards? Theory predicts that upward migrations will cause range contractions for high-elevation species because of novel interactions with encroaching antagonists. My dissertation work is one of the most comprehensive tests of this question to date, using a combination of ecological niche modeling (ENM), experiments, and demographic and trait-based modeling approaches. I created novel ENMs that suggest context-dependency of biotic interactions, where predictions of biotic interactions change from positive to negative over environmental gradients, is common over elevation gradients. Additionally, ENMs suggested the current focus on plant-plant interactions in niche modeling targets the most important biotic interaction for many species. I then constructed space-for-time experiments that transplanted alpine species into novel low elevation plant and mammal communities expected to encroach upwards, as well as into their native high elevation communities. Plant competition was manipulated by vegetation removals and mammals were excluded in a separate factorial experiment using below- and aboveground fencing. In both experiments, low elevation plant and mammal communities suppressed growth of alpine species to a greater extent than those antagonists found in their home range. However, demographic models suggested that environmental factors (e.g. temperature) other than novel plant and mammal communities are more consequential for determining population fate. The experiments validated a novel trait-based model of competitive interactions that can be broadly applied to other systems and conservation needs. My dissertation work found that alpine plants are unlikely to remain โking of the hillโ under climate change, in part due to the upward encroachment of novel competitors and intensification of herbivore pressure
