Integrated trait-based and demographic approaches to predict how biotic interactions constrain the range limits of plant species

Background/Question/Methods

Species are shifting their ranges upward in elevation with climate change, potentially creating novel communities. If theory is correct, and species’ low elevation range limits are more constrained by antagonistic interactions than by abiotic stress, then the upward encroachment of novel competitors could drive range contractions and local extinctions of alpine populations. We tested this theory for three alpine-restricted species in the Colorado Rocky Mountains, USA. We asked: Do novel competitors at low elevations reduce population growth more than similar interactions in the center of alpine plants’ ranges? And do larger trait asymmetries between alpine plant species and encroaching competitors predict larger declines in alpine plant population growth? We created experimental populations with either intact or removed competitors in the core of the species’ range (core) or ~430m below the plants’ low elevation range limits (novel). We applied experimental effects on growth, survival, flowering probability, and inflorescence number to matrix projection models of focal alpine species to predict population growth (lambda). Additionally, we fit models of trait hierarchies, which predicted alpine plant performance based on the differences between their traits and competitors’ traits and predicted the effects of competitor trait variation on focal species population growth.

Results/Conclusions

Models predicted eventual extinction (lambda< 1) of all three focal alpine species when transplanted into novel, low elevation environments with competition. The magnitude of the effect of competition was similar between novel and core sites. However, lambda was on average lower in the novel sites than core sites, regardless of competition treatment, suggesting either the novel abiotic environment or unaccounted for biotic interactions, such as herbivory, also determine low elevation range limits. Competitor height and specific leaf area (SLA) increased with air temperature across sites, indicating that upward colonizing species with climate change will be taller and potentially more resources acquisitive (i.e., more competitive). Across focal alpine species, survival probability and vegetative growth increased as their height and SLA increased from being less than to greater than their competitors. Together, the results support the hypothesis that upward migration of taller, more resource-acquisitive (greater SLA) plant species will amplify trait asymmetries that accelerate the local extinction (lambda < 1) of alpine plant species. This work marks a novel integration of demographic and trait-based approaches to gain generalizable predictions on novel species interactions in a changing world.