Rareness as an Antipredator Strategy to Reduce Predation Risk for Moose and Caribou

The densities of moose (Alces alces) and woodland caribou (Rangifer tarandus) are low in Canada and Alaska (caribou- -0.03-0.13/km2, moose- -0.1-0.5/km2) in systems where wolves (Canis lupus) are unexploited; hence these ungulates are far below a food carrying capacity (K) of ≥1.5/km2. Cow moose and caribou migrate prior to parturition seeking birth sites to reduce predation risk and show philopatry. This spacing-out results in damping the predation of neonates and can provide a stabilizing recruitment (R s ), where R s equals the annual natural mortality (M) of adults (R = M,λ = 1.00). These stabilizing recruitments at 6–9 months-of-age for moose were ~ 25–26 calves/100 females, and for caribou were 15–16% calves of the population (25–26 calves/100 females). The R s values resulted only when the densities of ungulates were low in systems where wolves were common and fluctuations in R (greater and less than R s ) occurred around a mean density defined as the stabilizing density (D s ). A phase plane analysis of the isocline of the abundance of wolves on prey was oblique, and when moose were the prey, was tangent to the moose isocline in the range of 0.2–.5/km2. Such predator-prey interactions should result in low stable equilibria for moose populations with reduced chances for extinctions. However, for caribou coexisting with moose populations, extinction is a distinct possibility, especially in isolated populations where recolonization is precluded.