| The effects of climate change on the range limits of hybridizing species depends on the relative fitness of those species under changing conditions. However, fitness is rarely linked to both climatic conditions and hybrid zone movement, such that the relationship between climate change and hybrid zone dynamics remains tenuous. We report how interactions between climate (seasonal precipitation) and competitor densities result in steep differentials in survival, which in turn drive hybrid zone movement for two woodrat species (Neotoma fuscipes and N. macrotis) in central California. Using six years of capture-mark-recapture data, we found that the smaller-bodied species, N. macrotis, and hybrids had survival advantages over the larger-bodied N. fuscipes in the contact region during dry winters and wet springs. This pattern of differential survival, with N. macrotis having a consistent advantage over N. fuscipes during our study period, matched the spatial dynamics of the hybrid zone, which moved steadily north into N. fuscipes territory (~150 meters in six years). Although all study site areas experienced the same climatic conditions, competitive effects created a complex spatial pattern of survival differentials, which in turn influenced hybrid zone movement. Characterization of fitness differentials between competitors should greatly improve our ability to understand and forecast climate-driven range dynamics. |