Demography of an increasing caribou herd with restricted wolf control

Abstract

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 overshoo...

Modeling multispecies predator–prey dynamics: predicting the outcomes of conservation actions for woodland caribou

“…, Boertje et al. ). Similar efforts have been directed at black bears and coyotes ( C. latrans ; Mosnier et al.…”

“…There are numerous examples of the lethal reduction of wolves as a strategy to increase or stabilize declining populations of caribou (Bergerud and Elliot 1986, Hayes et al 2003, Hervieux et al 2014, Boertje et al 2017). Similar efforts have been directed at black bears and coyotes (C. latrans; Mosnier et al 2008, Lewis et al 2017.…”

Effects of control on the dynamics of an adjacent protected wolf population in interior Alaska

“…In fall 1920, Murie () estimated the Fortymile caribou herd's size at 568,000 caribou; however, the methods used to generate this number were rudimentary, leading to repeated revisions of this estimate (e.g., Valkenburg et al , McDonald and Cooley , Boertje et al ). Since the 1930s, numbers have fluctuated from approximately 5,000 to 50,000, most recently increasing nearly 9‐fold during the period 1973–2010 from approximately 6,000 to approximately 52,000 individuals (Valkenburg and Davis ; Valkenburg et al ; Gross ; Boertje et al , ). Although the absolute value of Murie's estimate has been questioned, it continues to be used as a justification for the implementation of lengthy and intensive predator control programs in the area in an attempt to achieve higher caribou numbers.…”

“…A control program was also conducted in the Fortymile area by the ADF&G in the early 1980s (Gasaway et al ). During our study, 2 wolf control programs, sterilization and translocation (non‐lethal control) and aerial shooting (lethal control), were implemented at separate times by the ADF&G on the borders of YUCH with the goal of increasing the size of the Fortymile caribou herd to provide more opportunity for human harvest (Boertje and Gardner ; Boertje et al , ; Harvest Management Coalition ). Much of the caribou calving area lies within YUCH, and one of the goals of these programs was to reduce wolf predation on calves (Boertje and Gardner , Boertje et al ).…”

Lifetime, known‐age moose reproduction in a nutritionally stressed population

“…Several moose populations with the lowest reproductive metrics were successfully managed through implementation of temporary liberal harvests of female moose to curtail growth of the populations to prevent long-term overstocking and associated shifts in forest composition to browse-resistant species (Boertje et al 2010, Gingras et al 2014, Charron and Hermanutz 2017. The importance of linking increasing herbivore numbers with declining reproductive metrics should not be underestimated, particularly when faced with the task of convincing stakeholders to allow liberal harvests of female ungulates (Jorgenson et al 1993;Forchhammer et al 1998;McCullough 1999;Boertje et al 2009Boertje et al , 2017. Boertje et al (2009) used preliminary reproductive indices in our study area to help model population trend and Boertje et al (2007) identified several indices suggesting moose were reaching unsustainable, high densities.…”