What determines species range limits? Climate is often thought to play a critical role in establishing range limits, however distributions are also determined by biotic interactions with competitors, consumers, prey, and mutualists. For example, if seed production is low due to insufficient pollinator services at range limits, it is likely plant-pollinator interactions, rather than climate that constrains population growth. Distinguishing between direct climatic vs. biotic controls on range limits is particularly relevant because this will influence how species respond to changing climates. To address this question, we investigated plant-pollinator dynamics of Erythronium montanum (Liliaceae) across its altitudinal range limit and asked whether pollen transfer limits fruit set and whether the degree of limitation changes as we approach the range limits of the species. E. montanum is a particularly interesting species because it is one of the first flowers to bloom in the spring on the south side of Mount Rainier. As such, there is reason to suspect that this species will be among the most sensitive to a changing climate.
Results/Conclusions
Our results suggest that plant-pollinator interactions play an important and complex role in driving range limits of this species. First, insect exclusions indicated that this species requires outcrossing to set seed, suggesting that pollinators play and important role in determining plant fitness. We found that seed set is pollen limited at the upper elevational range but not at the lower elevational range of this species. However, overall reproduction was significantly decreased at lower range limits (regardless of pollen addition), suggesting that other factors (potentially abiotic, potentially biotic) constrain reproduction there. Surprisingly, visitation rates by pollinators did not vary by elevation, suggesting that pollinators may differ in their efficacy across the species range (due perhaps to the pollen load carried). In all, these findings suggest it is essential to understand the complexity of plant-pollinator interactions, as influenced by climate, to forecast changes to plant species distributions with warming.