PATTERNS OF PREDATION ON NONCYCLIC LEMMINGS

Noncyclic populations of microtine rodents may be limited within a rel atively constant range of densities by generalist predators with a pre y base sufficiently diverse to sustain them when rodents are scarce (g eneralist predator hypothesis). Collared lemmings (Dicrostonyx kilangm iutak) at Pearce Point, Northwest Territories, Canada, are noncyclic a nd limited to fairly constant low densities in summer by predation, pr incipally by red fox and Rough-legged Hawks. We tested four prediction s of the generalist predator hypothesis as a possible explanation for relatively constant lemming densities: (1) predators do not show stron g numerical responses to lemming density; (2) the proportion of lemmin g biomass in predator diets declines with declining lemming abundance, compensated for by increased consumption of alternative prey; (3) pre dators show a type-III functional response to lemming density; and (4) at low densities, predation on lemmings ceases. The first prediction was not satisfied by the principal predators: at very low lemming dens ities, Rough-legged Hawks did not settle, and breeding success of red foxes and hawks was limited by lemming abundance. However, a number of generalist predators (Golden Eagle, grizzly bear, arctic ground squir rel, Peregrine Falcon, and Gyrfalcon) did not respond numerically to t he lowest lemming densities. The second prediction was partly supporte d: all predators consumed lemmings at a lower rate as lemming densitie s declined. However, Rough-legged Hawks were not able to compensate fu lly for the declining consumption by increasing their use of alternati ve prey, and red foxes were able to do so in one of three years. Regar ding the third prediction, foxes showed some evidence of a type-III fu nctional response but hawks did not. As for the fourth prediction, mos t predators still consumed lemmings at very low densities; lemmings la cked a secure refuge. The Pearce Point system differs from those where microtine dynamics are relatively constant and nonirruptive because o f persistent predation by generalists. In terms of biomass, lemmings a re the principal prey for their dominant predators. These predators (t he semigeneralist red fox and the specialist Rough-legged Hawk) rely o n lemmings to breed, but drive them to densities too low to sustain br eeding by these same predators in the subsequent spring. In this regar d, the system is similar to one driven by specialists. In some winters , however, populations recover because lemmings breed under the snow a nd most summer predators are absent. As a result, lemming densities in Spring are often high enough for specialists and semigeneralists to i nitiate breeding. When winter breeding and survival fail to allow popu lation growth, hawks and foxes may fail to breed and then leave the sy stem. Even so, summer generalists still persist and continue to consum e lemmings, curtailing potential irruptive growth. In this regard, the system is similar to one where prey are relatively constant because o f generalists. Community dynamics at Pearce Point can best be understo od as a combination of three dominant processes. Summer predation by s pecialists and semigeneralists results in destabilizing declines. Wint er breeding, coupled with good survival, can lead to destabilizing gro wth. However, this growth is curtailed in the following summer by eith er destabilizing specialist predation or the stabilizing influence of generalist predation. When lemmings are scarce, the semigeneralist red fox and some generalist predators rely on arctic ground squirrels as their primary prey or their principal alternative prey. The ground squ irrel appears to be the critical species maintaining this relatively d iverse arctic tundra predator community and the relatively constant le mming densities.