Species that shift their ranges upward in elevation with climate encounter novel species of previously non-overlapping ranges in alpine elevations. If past theory is right and species’ low elevation/latitude range limits are more constrained by antagonistic interactions than by abiotic stress, then the upward encroachment of novel antagonisms could be a potent constraint on the trailing range edges of alpine residents. We tested this theory for three alpine-restricted species in the Colorado Rocky Mountains, USA. We asked: Do novel biotic interactions (competition and mammalian herbivory) at low elevations reduce population growth more than similar interactions in the center of alpine plants’ ranges? We created experimental populations with either intact or removed vegetation to manipulate competition in the core of the species’ range (core) or ~430m below the low elevation range limits (novel). Similarly, we factorially excluded above- and belowground mammalian herbivores with fences around experimental populations below (novel), at the edge (limit), or in the center (core) of current elevational range. For each experiment, we fit vital rate models to growth, survival, flowering probability, flower number, and recruitment, with model intercepts that varied among treatments. We then to populated matrix projection models of population growth (lambda) and performed life table response experiments (LTREs) to determine how each vital rate contributed to the response of plant population growth to competition or herbivory.
Results/Conclusions
Models predicted the eventual extinction (lambda<1) of all three alpine species transplanted into novel, low elevation environments, regardless of competition treatment. Thus, abiotic conditions at low elevations may be more important than competition for population persistence below elevation range limits. However, herbivore exclusions suggested that increased herbivore pressure in both novel and range limit sites restricted population growth of the alpine species more than in their core range. This result implicates herbivores, rather than abiotic limits, as constraints on low elevation range edges. Together, our results suggest that increased herbivore pressure under climate warming may cause larger population declines for alpine plants than upward encroaching competitors. Results also highlight the importance of integrating across life history events to predict the fate of populations, rather than focusing on fitness proxies like biomass production. Novel biotic interactions will act in conjunction with increased abiotic stress under climate change to drive future population declines at species trailing range edges, but a focus on competition may lead to inaccurate predictions of range dynamics.