Regulation, cycles and stability in northern carnivore-herbivore systems: back to first principles

In studies on dynamics of northern predator-prey systems. two assumptions a re often made. First. the bifurcation from stable to cyclic dynamics is see n as a consequence of changing generalist-specialist ratio. ultimately due to reduced prey diversity at high latitudes and the negative impact of snow on the efficiency of generalists as predators of small, folivorous mammals . Supposedly, the primary mechanism is the qualitative difference between t he functional response of specialist and generalist predators. Second, the interaction between large predators and ungulates is supposed to be prone t o lead to two alternative equilibria, one where predation regulates ungulat es at a relatively low equilibrium and another. where ungulate densities ar e close to carrying capacity, In the first-mentioned issue, our analysis co rroborates the general idea of snow favoring specialists and leading to cyc les. However, differences in functional response appear to be of secondary importance only, and rather special conditions are required for generalists to have a stabilizing type III functional response. A destabilizing type I f functional response or a slight modification of it should be common in ge neralists, too, as also indicated by the classical experiments. Stability o f generalist dominated systems seems primarily to derive from their relativ e inefficiency, allowing prey's density-dependent mechanisms to play a bigg er role in the neighborhood of the equilibrium, Moreover. the main destabil izing impact of deep, long-lasting snow cover appears to lie in the protect ion it offers to the efficient but vulnerable specialists, which are elimin ated or marginalized by intraguild predation in areas with snow-free winter s, unless the habitat offers some other form of efficient protection. As fo r the conjecture of multiple equilibria in northern wolf-ungulate systems. it seems to be derived from an erroneous operational definition of numerica l response and has little if any empirical support. Available data suggest that predation limitation of folivorous mammals prevails along the entire g radient from relatively productive low arctic habitats to the humid parts o f the temperate zone. provided that the numbers of predators are not contro lled by man.