Modeling disjunct gray wolf populations in semi-wild landscapes

Gray wolves (Canis lupus) in parts of the United States and Europe live in networks of disjunct populations, many of which are close to human settlement. Because wolf management goals include sustaining disjunct populations, it is important to ask what types of areas and protections are needed for population survival. To predict the effects of different levels of human-caused mortality, we created a simulation model for a disjunct wolf population living in a semi-wild landscape with abundant, well distributed prey. The landscape included a maximum of 16 territories divided into core and peripheral range. The mortality rate in the core range was 20%, whereas the mortality rate in peripheral range (40%) was higher because of human-caused deaths. We examined the relationship between the proportions of core and peripheral range and the 50-year occupancy of that range by wolf packs, given different assumptions about pup and dispersal mortality and immigration. Simulations showed that occupancy increased as the number of core sites increased, but curve location depended on parameter assumptions. With pup and dispersal mortality rates consistent with those for disease-free and legally protected populations, wolves saturated a 16-territory cluster with as few as two core sites, regardless of immigration rate. When populations had high pup or dispersal mortality, as few as two immigrants per year helped maintain high (>80%) site occupancy in clusters with four or more core sites. Small numbers of immigrants were also important for sustaining colonizing populations and buffering the negative effects of increased environmental variation. The simulations supported the claim that wolves can survive in disjunct populations provided that wolves can move between populations, human persecution is not excessive, and prey is abundant.