Using HexSim to assess potential reintroduction sites for a native ungulate

Background/Question/Methods

The tule elk (Cervus elaphus nannodes), a subspecies of ungulate endemic to central California, was nearly brought to extinction in the 19^th century and is still extirpated from most of its natural range. As part of an ongoing restoration program, we evaluated a portion of its former range in the Central Valley for potential reintroduction of a free-ranging herd. We used a new spatially explicit population model (HexSim) to analyze four different elk release scenarios.  Each scenario corresponded to a different release location, and the model was used to compare simulated elk population dynamics 25 years into the future. We also used HexSim to identify likely locations of human-elk conflict, the major cause of elk mortality in California’s tule elk herds. These sources of conflict were: urban areas, roads, and concrete-lined canals. We varied the maximum dispersal distance to test the sensitivity of the model to uncertainty surrounding this parameter, running each scenario three times using different values.

Results/Conclusions

Population forecasts at the four release locations after the 25-year period ranged from a mean of 84.2 (females only) to 169.4. Mean barrier interactions per iteration per release location ranged from 8.6 to 2,837.5. Populations were highest and potentially harmful barrier interactions were lowest at the East Bear Creek site. Conversely, populations were lowest and barrier interactions dramatically highest at the Kesterson site. These relationships held regardless of the maximum dispersal distance used. These results point to the East Bear Creek site release scenario as the most likely to result in a successful elk reintroduction. Changes in maximum dispersal distance had minimal effects on resulting population and barrier interaction numbers. We found HexSim to be a very useful tool for this type of reintroduction planning and believe that it will prove to be successful for other conservation planning studies as well. HexSim enabled us to rank management scenarios and to identify more likely future locations of detrimental human-elk interactions. The herd movement component of HexSim was especially critical for this study.