Temperature is often implicated as a primary mechanism driving species turnover across elevation gradients. Janzen’s climate variability hypothesis posits that the seasonal temperature variation experienced across temperate mountains should favor the evolution of species with broad thermal tolerances, greater dispersal ability, and wider elevation range sizes. Tropical mountain species should evolve narrower thermal tolerances in response to stable temperatures, restricted dispersal across elevation, and narrower range limits. Previously, we demonstrated that tropical aquatic insects have narrower thermal breadths (CTmax – CTmin) and lowered acclimation ability compared to their temperate relatives. However, thermal breadth alone does not adequately explain the abrupt cessation of one species’ occurrence and the beginning of another’s in certain streams sites. We therefore propose that important biotic interactions, such as predation, mediated by temperature, may play a role in determining mayfly elevation range limits, and explain the observed pattern of species turnover. We tested this hypothesis by artificially simulating range expansions of low elevation mayflies to high elevation streams and high elevation mayflies to low elevation streams, in a lab setting. “Transplanted” mayflies were exposed to suboptimal temperatures in the presence of native stonefly predators. Survival of transplanted mayflies was measured after 24h and compared to that of native mayflies in the same testing arena. We predicted that transplanted mayflies would be preyed upon more heavily than native mayflies because lowered performance in response to suboptimal temperatures would exacerbate their susceptibility to the predators. We also predicted that this effect would be stronger in the tropical species where thermal breadths are even narrower.