Barring dispersal limitation, species’ distributions are regulated by variation in environmental variables across the landscape. Historically, researchers have focused on effects of abiotic clines on geographic distributions, but there is growing evidence that biotic interactions can have similarly large effects on range limits. Here I explore causes of the geographic range limit of a narrowly endemic California annual plant, Clarkia xantiana ssp. xantiana (Onagraceae). Transplant experiments suggest a geographic range limited by adaptation, but pinpointing the environmental clines underlying this limit remains elusive. For example, though there is a strong aridity gradient across the subspecies’ range, during transplant experiments we have also observed severe mammalian herbivory on plants. To determine whether fatal mammalian herbivory may constrain range expansion in C. x. xantiana, I first characterize the spatial gradient in this species interaction across and beyond the subspecies’ range using experimental populations of C. x. xantiana. I then use data from a two-year transplant experiment to ask, what are the effects of herbivory on C. x. xantiana population growth rates within and beyond its range edge? Finally, I explore how susceptibility to herbivory is mediated by a specific plant trait, phenology.
Results/Conclusions
Probability of herbivory was low at C. x. xantiana’s range center and increased sharply near the range limit. Overall, the odds of a plant being eaten increased nine percent for every kilometer eastward, with the gradient in probability of herbivory becoming very steep near the range limit, reaching 0.7 two km from the range border. When we used simulations to “remove” the effect of herbivory on transplant populations, population growth rates increased two and five fold at the range edge and beyond-edge sites, respectively, while at range center, removal of herbivory had only small effects. A plant’s susceptibility to herbivory was strongly mediated by its phenology, though this relationship varied across space. For each day later that flowering occurred, a plant’s odds of herbivory in the range Center, Edge, and Beyond-Edge sites increased by two, five, and 14 percent, respectively. These results give a comprehensive, empirical characterization of how a biotic interaction can contribute to a species’ distributional limit, and how that interaction is linked to an important plant trait. The talk will also include discussion of temporal variation in range-limiting factors, and how complex, multivariate environmental gradients influence the formation of range limits.