Biotic interactions limit the geographic range of an annual plant: Herbivory and phenology mediate fitness beyond a range margin

Species9 range limits offer powerful opportunities to study environmental factors regulating distributions and probe the limits of adaptation. However, we rarely know what aspects of the environment are actually constraining range expansion, much less which traits are mediating the organisms9 response to these environmental gradients. Though most studies focus on climatic limits to species9 distributions, biotic interactions may be just as important. We used field experiments and simulations to estimate contributions of mammal herbivory to a range boundary in the annual plant Clarkia xantiana ssp. xantiana . A steep gradient of increasing probability of herbivory occurs across the boundary, and herbivory drives several-fold declines in lifetime fitness at and beyond the boundary. By including in our analyses data from a sister taxon with more rapid phenology, we show that delayed phenology drives C. xantiana ssp. xantiana 9s susceptibility to herbivory and low fitness beyond its border.