Escape from the predator pit: 30 years of high wolf and moose densities following wolf control in Alaska

Background/Question/Methods

Prey populations may be regulated by predation at low densities. In theory, if a prey population is released from predation, a shift to a high density equilibrium determined by food supply is possible. However, empirical evidence of state changes in ungulate populations from low to high density are scarce despite numerous predator control experiments. We present a case study from interior Alaska as a unique example of a multi-ungulate, multi-predator system that shifted to an alternate “stable” state partly due to a wolf control program and harvest management. Moose and caribou numbers sharply declined during the 1960’s and early 1970’s in game management unit 20A due to a combination of severe weather, overharvest, and predation. Wolf populations and ungulate harvest rates were reduced for a 7-year period (1976-1982), and the response of wolves and ungulates were monitored using repeated aerial surveys and radio-telemetry. We synthesized data on wolf, moose, and caribou populations from 1963-2013 to examine the dynamics of this predator-prey system.

Results/Conclusions

Wolf density was reduced from 14.1 per 1,000 km² to between 4.4-8.4 wolves per 1,000 km² during the 7-year wolf control program. During this time, moose density increased from 0.18 to 0.48 moose per km², and the Delta caribou herd increased from 2,200 to 7,335 individuals. Wolf density rapidly increased following wolf control and has remained fairly stable for 28 years (12.8 wolves per 1,000 km², 95% CI = 11.9-13.6). Moose density continued to increase during and after wolf recovery, reaching a peak of 1.36 moose per km² in 2003. The moose population showed clear signs of approaching a nutritional carrying capacity, as evidenced by the lowest recorded reproductive rates among non-insular North American moose and the highest browse removal rates measured in Alaska. Ungulate population growth rates were negatively correlated with wolf density before and during wolf control (1963-1982, r = -0.89 and -0.84 for moose and caribou, respectively) and were uncorrelated with wolf density after wolf control (1983-2013, r = 0.17 and -0.29, p > 0.25). In comparison to 7 other telemetry studies in Alaska and the Yukon, rates of bear predation on moose calves were lower in 20A (24% vs. 42-67%). A combination of low bear predation rates, > 14 years of mild winters, and adequate habitat (including improvements via wildfires) likely facilitated the release of moose to a high density state despite a recovered wolf population.