Dispersal limitation at poleward range boundaries: Consequences under climate change and the role of rarity

Background/Question/Methods

Climate change is affecting the persistence and geographic distribution of species. Therefore, we must understand limitations to range boundaries and predict the potential for poleward range shifts to manage future biodiversity values. Species differ in their dynamics at range boundaries, however. Rare species produce fewer potential colonists than common species, and factors related to rarity may limit establishment when dispersal is successful. To investigate variation in range dynamics among species, we studied three Lomatium species at their common northern range limit on Vancouver Island, British Columbia, Canada. These species differ in abundance from common to exceedingly rare but share many life history characteristics. We executed a field experiment using 1m² study plots placed inside and outside the current species range. Our design included a competitor reduction treatment and an herbivore exclosure treatment. Competitors were reduced at the start of the experiment by mechanical removal, and mammals were prevented access by caging.

Results/Conclusions

After one growing season (i.e., seed germination and initial growth), the species differed in their responses to the region and competitor-reduction treatments. Lomatium utriculatum, the most common species, grew equally well outside the range as inside the range and had higher germination in plots with competitors (i.e., in undisturbed plots). Lomatium nudicaule, the species intermediate in abundance, showed a significant interaction between region and competition treatment. Inside the current range, germination was higher in plots with reduced competitors, but outside the range the competitor reduction treatment had no effect. Lomatium dissectum, the rare species, showed no effect of the competitor reduction treatment inside the range but germinated better outside the range with competitors intact. In all three species, the herbivore exclosure treatment had no effect. Over all treatment combinations, average germination for each species ranged from 37-52%, with the rare L. dissectum having lowest germination. We will compare these germination results (2007) with data on survival collected in 2008. Because plants germinated as well in sites outside the current range as inside it, we conclude that conditions outside the range are potentially suitable for germination and early growth. We also infer dispersal limitation as an explanation for the absence of these species in what otherwise appears to be suitable habitat. Finally, our results suggest that rare taxa may suffer difficulties of establishment due to fewer seeds and reduced germination relative to more common taxa. Human intervention could be a useful tool to enable poleward range expansion in these species.