Shifts in species’ geographic ranges towards higher latitudes and elevations are among the most frequently reported consequences of climate change. However, the role of species interactions in setting range margins remains poorly understood. It is commonly hypothesized that species interactions like competition should be less important for determining range margins in abiotically stressful habitats. Thus, we would expect the strength of competition to decrease as elevation increases. However, theory also suggests that as environmental conditions become more stressful, species should become more vulnerable to antagonistic interactions. Consequently, we may observe that species interactions become stronger, and therefore more important for structuring ranges, as you move toward the abiotically hash edge of a species distribution. To test these competing hypotheses about the role of abiotic and biotic mechanisms for structuring range boundaries we used cage experiments on caddisflies in ponds at both montane (2900 m a.s.l) and subalpine (3400 m a.s.l.) elevations. In this experiment we examined how the strength of competition between a range-shifting species Limnephilus picturatus and the cosmopolitan species Limnephilus externus varied across their elevational extent. Specifically, we quantified how competition between these two species influenced several demographic parameters, including survival, emergence, and adult body condition.
Results/Conclusions
We found that competition with L. externus significantly decreased L. picturatus survival and adult emergence at high elevations compared to lower elevations. These data support the notion that species interactions play a critical role in determining up-slope range limits. Yet, in the absence of competitors, L. picturatus survival was greater at high-elevation than low-elevation sites indicating that these high-elevation sites may not be more stressful for this species. However, L. picturatus adults at high elevations were smaller than their lower elevation counterparts regardless of the presence or absence of competitors, which has implications adult fecundity. We found no effect of competition on L. externus, but elevation reduced body mass of L. externus larva. Thus, our results suggest that the stress associated with high-elevation environments decreases the ability for L. picturatus to respond demographically to competition for resources. These findings lend support for the hypothesis that species interactions should be more important for structuring range margins in abiotically stressful habitats. Ultimately, while we show that species interactions are important for setting up-slope range margins, our mechanistic understanding of range margins may vary depending on the demographic variables examined.