Description:
<p>Aim A species' distribution reflects its ecological niche when all of the habitats that support population growth are occupied. However, organisms with limited dispersal potential may not colonize all suitable habitats and may consist of differentially adapted populations that have different niche requirements. Here, we evaluate the factors determining the northern range limit in a fern species that is endemic to a patchily distributed habitat throughout its geographic range. Location Non-calcareous rock shelters that provide habitat for six separate populations spanning the geographic range of Vittaria appalachiana, an asexual, gametophytic fern species, in the Appalachian Mountains and Appalachian Plateau of eastern North America. Methods We transplanted V. appalachiana gametophytes from six different populations to a rock shelter habitat 19 km north of the contemporary northern range boundary and 48 km away from the most northern population and back into their home locations as experimental controls. We compared the fitness of transplants at 'home' versus 'beyond the northern range limit' to test whether the northern range boundary is defined by physiological limits or dispersal limitation. Furthermore, we tested whether transplant success beyond the northern range boundary varied among individuals collected from different populations in the species' range. Results Overall, transplant performance was similar between the home locations and the site beyond the northern range limit. However, populations differed in their response to transplantation beyond the northern range boundary, with individuals from the northernmost population having higher fitness beyond the range limit than all other populations. Main Conclusions We conclude that the northern range limit of V. appalachiana is heavily influenced by dispersal limitation because transplant performance was, on average, similar between home sites and beyond the northern range limit. However, because populations varied in their relative performance beyond the range limit, we also conclude that the ecological niches of these populations are not equivalent.</p>