Learning from models: Individual-based models inform field data collection priorities for the gopher tortoise

Background/Question/Methods

The gopher tortoise (Gopherus polyphemus) is federally threatened in the western portion of its range and under review for listing range-wide. Gopher tortoises are long-lived and exhibit complex social interactions that are not well understood, especially in poor quality habitat where tortoise densities are low and intensive observation is difficult.  In order to target conservation measures and prevent further decline of the species, it is critical to understand how habitat quality affects the tortoises’ abilities to maintain social contact and reproduce.  We developed an individual-based spatially explicit population model (IB-SEPM) to provide a hypothesis testing framework that can be applied across many locations. IB-SEPMs are mechanistic models that simulate the explicit link between multiple biological processes (e.g., recruitment, migration, competition, and predation) and land cover patterns. Model development was guided by an extensive literature review as well as analysis of a multi-year, multi-site database of gopher tortoise telemetry data. 

Results/Conclusions

Dispersal and home range use are poorly understood in this species.  Our results indicate that these parameters have a strong impact on population persistence.  We found that observed patterns of home range overlap were only reproduced with the inclusion of a parameter allowing individuals to avoid members of the same sex, lending support to hypotheses of intra-sex competition that have been proposed but not confirmed for the gopher tortoise.  We present model projections from several alternative scenarios that show that differences in these parameters have important consequences for gopher tortoise populations in the future.  By identifying the range of parameter values that lead to population persistence, we highlight how field biologists could focus their efforts on the ground.