A spatially-explicit population viability analysis for the desert tortoise

Background/Question/Methods

Population viability analysis is useful for evaluating factors that influence population persistence and rate of population change. Its spatial applications, however, have been limited to considering multiple discrete populations or generalizing over broad regions. We developed a spatially-explicit population viability analysis for the Sonoran and eastern Mojave populations of the desert tortoise (Gopherus agassizii). We divided the area encompassing these populations into a regular grid and for each grid cell we empirically estimated annual rates of adult and juvenile survival, juvenile to adult transition, and adult reproductive output. We estimated survival and transition rates based on mark-recapture data gathered on 25 1.3-3.9 km² plots surveyed between 1977 and 2008.  We used Bayesian hierarchical multi-state population models in conjunction with conditional autoregressive models to estimate these rates for each grid cell. For reproductive output, we integrated data on female reproduction and hatchling success from 10 studies at 14 sites throughout the tortoise’s entire range to produce aspatial estimates of the number of females produced per female each year for Sonoran and Mojave populations. We used these demographic rates to create a stage-structured population projection matrix for each grid cell from which we derived estimates of rate of population change (λ).

Results/Conclusions

In general, spatial patterns in demographic rates were complex. Adult survival rates varied spatially from 0.85 to 0.96 and decreased along a southeast to northwest gradient. Transition rates varied from 0.07 to 0.11 and increased along the same gradient.  Juvenile survival rates ranged from 0.71 to 0.93 and showed no consistent spatial pattern.  Annual reproductive output was twice as high in the Mojave population (1.58 females female^-1 year^-1) as in the Sonoran population (0.72). In combination, these demographic rates suggest that the rate of population change varied from stable to increasing (λ = 1.02 - 1.13) across the range of the Sonoran population and was increasing (λ = 1.11 - 1.24) across the range of the eastern Mojave population; differences in rates of change between the two populations were largely a result of the differences in reproductive output. Our results will help to understand spatial patterns in demographic processes, provide insight into factors that drive population dynamics, and identify areas where management action is needed most to recover the desert tortoise.  Importantly, this methodology can also be applied to explore spatial variation in demography and population viability in other species where there are sufficient data.