The evolution of stable hybrid zones in ecologically similar species can be partially driven by differential responses of survival rates to fluctuating ecological conditions. We investigated potential stabilizing mechanisms for the coexistence and hybridization of two woodrat species (Neotoma fuscipes and N. macrotis) in southern coastal California. We hypothesized that, despite their similarities, species and hybrids would have higher survival rates under different ecological conditions, thus allowing for stability of the hybrid zone. Using standard survival estimation models (e.g., Cormack Jolly Seber models) can not necessarily separate survival and movement processes, and the latter may also affect hybrid zone dynamics. We used a Bayesian spatial capture-recapture model to estimate survival rates (while accounting for movements and spatial variability in capture probability). The hybrid zone and surrounding areas were extensively trapped, with approximately 45,000 trap-nights from 2008-2013. All captured individuals (almost 2,000 individuals) were marked with ear tags and genotyped using 15 microsatellite markers. The incredible richness of the dataset posed unique challenges in model fitting, forcing us to simplify the spatial capture data into 12 zones across the study area. We fit the model using survival covariates of individual characteristics, conspecific density, and environmental conditions.
Results/Conclusions
Model results indicated substantial differences in survival probabilities among genotypes, with adult female N. fuscipes having a lower survival rate (0.45-0.57 95% credible interval [CI]) than N. macrotis (0.54-0.71 95% CI), with hybrids having an intermediate survival rate (0.44-0.64 95% CI). Survival rates were also spatially variable with N. macrotis having higher survival rates in the zone where conspecifics were least dense, potentially indicating a stronger effect of intraspecific competition than interspecific competition with the larger-bodied N. fuscipes. Conversely, N. fuscipes and hybrids both had higher survival rates where conspecific density was highest. Both species had their highest survival in years with dry winters and dry springs, but N. fuscipes was more negatively affected by wet springs (N. macrotis was not affected by wet springs), and N. macrotis was more negatively affected by wet winters (N. fuscipes was not as affected). This results in a large discrepancy in survival rates in years with a dry winter followed by a wet spring, providing a potential (infrequent) survival advantage for N. macrotis. These preliminary results based on a modified spatial capture-recapture model provide compelling clues into how hybrid zone stability is maintained between these two species.