Date of this Version
Wildlife Monographs 198:1–30; 2017; DOI: 10.1002/wmon.1026
Long-term wolf (Canis lupus) research programs have provided many insights into wolf population dynamics. Understanding the mechanisms controlling responses of wolf populations to changes in density, environmental conditions, and human-caused mortality are important as wolf management becomes increasingly intensive. Competition with humans for ungulate prey has led to large-scale wolf control programs, particularly in Alaska, and although wolf populations may sustain relatively high (e.g., 22–29%) rates of conventional harvest, control programs are specifically designed to have lasting population-level effects.
Understanding the broader impacts of wolf control efforts on the surrounding area is of particular concern for conservation agencies such as the United States National Park Service, whose mandates generally preclude the artificial reduction of populations of native predators, particularly for the primary purpose of increasing available prey biomass for human harvest. Detailed assessments of the factors influencing population vital rates (i.e., survival, natality, dispersal) and population trajectory in the context of control efforts are critical for understanding complex ecological relationships between wolves and their prey and informing management of each. Using a long-term dataset and a powerful new integrated modeling approach, we assessed the effects of wolf control on the dynamics of a monitored wolf population residing primarily within an adjacent protected area where wolf control activities were prohibited.
We monitored wolf population dynamics in Yukon-Charley Rivers National Preserve (YUCH) in interior Alaska, USA for 22 years (1993–2014). During our study, 2 large-scale wolf control programs were implemented in the surrounding area with the primary goal of increasing the size of the Fortymile caribou herd. We used known-fate data based on relocations of marked wolves and repeated counts of associated pack mates to estimate survival, dispersal, and natality rates. We jointly analyzed these data using an integrated modeling approach, thereby providing inference to the entire resident, pack-dwelling population of wolves using YUCH. Apparent survival (i.e., including mortalities and dispersals) was lower in the study area during the lethal control period, indicating a direct additive effect of control despite the prohibition of control efforts inside YUCH boundaries. Apparent survival was higher in years following winters with above-average snowfall, corresponding with a predicted increase in ungulate prey vulnerability the following year. Extraterritorial forays were associated with lower apparent survival rates, particularly after the initiation of lethal wolf control in the surrounding area. In general, mortalities tended to occur evenly throughout the year, whereas dispersal rates increased during late winter and early spring. Dispersals accounted for approximately half of the observed losses in our collared sample across all age classes (excluding known breeders), although yearlings were the most likely to disperse.
Sustained reductions in wolf densities outside the YUCH boundary during both wolf control programs also allowed us to directly assess the effects of reduced density on vital rates. Natality rates (estimated number of individuals added to each pack over the May–Aug interval) increased sharply over the course of each control program, suggesting a strong reproductive response to large-scale reductions in wolf densities in the surrounding area. Natality rates dropped rapidly between the 2 control programs, further supporting this conclusion. Smaller pack sizes and losses of known breeders were associated with lower natality rates per pack in the following year, suggesting human-caused mortality could have direct short-term effects on productivity by reducing pack sizes and removing breeders. However, although control can reduce the fecundity of individual packs in the short term, adjacent populations quickly respond to reduced wolf density by increasing natality rates.
Estimates of wolf density based on relocations of marked individuals within packs were dependent on sample size and could not be used to reliably estimate population growth rate (λ). As an alternative, we developed a new metric, λ*, which assessed whether natality was sufficient to offset population losses on an annual basis, under the assumption that the minimum functional unit in a wolf population is a breeding pair. When λ* decreased below 1.0 because of a combination of loss of individuals and the dissolution of packs, the population of interest effectively became a population sink reliant on immigrants from surrounding areas for maintenance. Based on estimates of λ*, we determined the YUCH study population was a source of wolves for the surrounding area in most years before implementation of lethal wolf control but became a population sink largely reliant on immigration from surrounding areas afterwards, despite the prohibition on control activities within YUCH. This finding has important implications for the management of protected areas, particularly in areas such as Alaska where wolf control is commonly implemented at large spatial scales. We expect λ* will be a useful tool for understanding wolf ecology and managing populations in other areas as well.
Overall, wolf vital rates were quite dynamic and responded quickly to changing conditions. The rapid increase in natality in an apparent response to decreased density strongly suggests that density dependence plays an important role in regulating wolf populations. Flexibility in dispersal and natality rates likely allow wolf populations to respond to variation in prey resources, wolf density, and mortality. These findings also suggest that sustainable harvest rates depend on annual variation in population vital rates. The clear impacts of management schemes in the area adjacent to YUCH suggests that effective conservation of protected areas may require more active management decisions than are often employed, particularly if the maintenance of unaltered system dynamics is a primary objective.