Abstract
Population fragmentation affects numerous species throughout the world by reducing large, continuous populations into small remnant populations separated by barriers or unsuitable habitat. Small, fragmented populations face elevated risks of extinction and lose genetic diversity more quickly than larger populations. However, genetic diversity may be maintained if migration occurs between fragmented populations. Although fragmentation affects many species, large carnivores are particularly susceptible to fragmentation due to their large home ranges and potential for negative interactions with humans. Grizzly bears (Ursus arctos) in North America are one such example. In the lower 48 United States (U.S.), this species was reduced from a contiguous population across the western U.S. to five fragmented populations occupying just 2% of the species’ former range. The smallest of these populations are the Selkirk, Yaak, and Cabinet populations of Montana, Idaho, Washington, and British Columbia. These populations of approximately 100 bears or fewer were isolated for generations, but they have recently been experiencing increased gene flow due to natural connectivity and a population augmentation program. Due to the small size of these populations, they are reliant on connectivity to maintain genetic diversity, but the specific levels of migration and augmentation needed are not well understood. Long-term genetic sampling conducted in these populations presented a unique opportunity to evaluate trends in genetic diversity over approximately five bear generations, then model future genetic diversity and population growth rates under varying levels of migration and augmentation. In Chapter 1, we tested the hypothesis that genetic diversity increased as a result of gene flow into the Selkirk, Yaak, and Cabinet populations. We also estimated effective population size, an important parameter that determines how quickly a population will lose diversity due to genetic drift. Post gene flow, allelic richness increased in the Selkirk and Cabinet populations and pairwise relatedness decreased in all three populations, while heterozygosity did not significantly change in any population. Effective population size was 15.2-15.8, 15.4-17.5, and 5.6-8.9 for the Selkirk, Yaak, and Cabinet populations, respectively. Despite small effective population sizes, current levels of gene flow and augmentation appear to be maintaining or increasing genetic diversity in these populations. In Chapter 2, we used population viability analysis to model the effect of different migration and augmentation rates on genetic diversity and population growth rates in the Selkirk, Yaak, and Cabinet populations 100 years into the future. We found that genetic diversity was likely to increase and growth rates remained positive under baseline conditions in all three populations. Genetic diversity declined over the 100-year simulation period in the Selkirk and Yaak populations when migration rates were reduced below half of baseline rates, while population growth in the Yaak became negative when migration rates decreased below baseline. Population augmentation benefited genetic diversity and population growth in the Cabinet population, and we found that replacing augmentation with migration did not fully compensate for the loss of augmentation. Overall, our results demonstrate the beneficial impact of gene flow to small populations and provide a comprehensive genetic baseline for the Selkirk and Cabinet-Yaak grizzly bear populations that will be a useful comparison for future genetic monitoring.