An individual-based model for southern Lake Superior wolves: A tool to explore the effect of human-caused mortality on a landscape of risk

Abstract

Gray wolves (Canis lupus) have complex life-histories due, in part, to mating systems that depend on intra-group dominance hierarchies set within an inter-group (pack) social structure linked to philopatric territories. In addition to this spatially oriented social structure, mortality risk associated with interactions with humans varies spatially. We developed an individual-based spatially explicit (IBSE) model for the southern Lake Superior wolf population to better capture the life-history of wolves in a harvest model. Simulated wolves underwent an annual cycle of life-history stage-dependent mate-finding, dispersal, reproduction, and aging on a simulated landscape reflecting spatially explicit state and water boundaries, Indian reservation boundaries and ceded territories, wolf harvest zones, livestock depredation areas, and a spatial mortality risk surface. The latter 3 surfaces were linked to mortality events for simulated wolves. We assessed our IBSE model and conducted a sensitivity analysis of the most uncertain parameters with a categorical calibration of patterns observed at the individual, pack, population, and landscape level. We found that without recreational harvest, the Wisconsin wolf population grew to an average carrying capacity of 1242 wolves after 50 years and breeding pairs persisted for a mean 1.8 years. We simulated 6 recreational harvest scenarios with varying rates and timings of harvest and assessed effects on population size, pack sizes, age ratios, dispersal and immigration rates, and breeding pair tenures of the Wisconsin wolf population. The simulated harvest with rates of 14% which corresponded to the 2012 harvest in Wisconsin reduced the populations 4% in the first year of harvest and equilibrated to the pre-harvest population size after 20 years of harvest, on average. A 30% harvest rate across the simulation on average reduced the populations by 65% after 20 years with some populations going extinct before 100 years. In general, harvest increased the proportion of pups in the simulated populations and decreased breeding pair tenure. Targeted lethal control was more effective than harvest for reducing the number of wolves near known livestock depredation sites. Our model facilitates prediction of important population patterns that is simultaneously dependent on complexities associated with spatially structured life history and mortality.