Demography of an increasing caribou herd with restricted wolf control
Authors: Boertje Rodney D. , Gardner Craig L. , Ellis Martha M. , Bentzen Torsten W. , Gross Jeffrey A. 
Published: January 2017

Understanding the limiting factors of a prey population is important before and during predator control programs, and optimal intensive management of an increasing prey population requires formal recognition of a sustainable population size. The migratory Fortymile caribou (Rangifer tarandus) herd in Alaska reached a low of approximately 6,000 caribou during 1973–1975. To regain peak numbers of approximately 50,000 caribou estimated in the 1960s, stakeholder groups gained approval for conservative harvest rates (1973–2013) and periods of restricted nonlethal (1998–2004) and lethal wolf (Canis lupus) control (2005–2013). We studied demography of the herd using radio‐telemetry during 1990–2014, when herd size increased from about 22,000 to 52,000 caribou. Parturition rates in the early 1990s were among the highest reported, but parturition rates of primiparous females subsequently declined to a level indicating resource‐limitation as caribou numbers approached and then exceeded 50,000. This and companion studies documented several other cautionary signals to an eventual decline, including declining October calf weights, early summer movement off the alpine and subalpine tundra to lower elevation spruce–moss taiga, relatively high caribou densities, a nearly 40‐year history of increasing caribou numbers, and a return to previous peak numbers. We studied mortality of calves and older females during the 4 years before wolf control and the first 5 years of nonlethal wolf control. During those 9 years, annual mortality rates averaged 54% for calves and 9% for adult females. We detected no convincing support for decreased wolf predation during nonlethal control. We also detected no support for increased caribou survival during nonlethal or lethal wolf control. Based on counts of caribou during summer aggregations using a total search photocensus technique, rate of herd increase (λr) was negligible (λr = 1.00) during 1990–1995, highest during the 3 years immediately before nonlethal wolf control (λr = 1.11, 1995–1998), moderate during nonlethal wolf control (λr = 1.07, 1998–2003), and low during the period that included the first 5 years of lethal wolf control (λr = 1.02, 2003–2010). We combined observed cause of death with the 9 annual modeled starting populations (all newborn calves and adults) and estimated that wolves killed 10–15% of the populations annually, grizzly bears (Ursus arctos) killed 4–7%, other predators killed 2–4%, nonpredation factors killed 1–2%, and hunters killed ≤2%. Wolves killed 5–9% of the annual populations as calves and 5–6% as adults. In retrospect, nonlethal wolf control efforts were too localized to decrease wolf numbers (e.g., adjacent untreated wolf packs reached max. mean numbers). Lethal wolf control efforts had only seasonal and localized effects on wolf numbers. It is important that stakeholders focus on describing a preferred, sustainable herd size, or nutritional status and proceed toward managing this increasing herd in a sustainable manner because, when ungulates overshoot carrying capacity, the effects of high density, adverse weather, and increased predation can have synergistic negative effects on prey numbers and long‐lasting negative effects on sustainable yields, contrary to the intended purpose of the wolf control programs. © 2017 The Wildlife Society.