Reproduction, predation, and population decline in collared lemmings

One of ecology’s major challenges is explaining how animal populations fluctuate in time. We tested whether reduced reproduction was a contributing cause of a cyclic decline in a collared lemming ( Dicrostonyx groenlandicus (Traill, 1983)) population in northern Canada. The decline began in autumn and winter (1996/97) and continued the following summer (1997). We concluded previously that predation was a primary cause of the summer lemming decline, and limited summer population density. Here, we recorded changes in lemming reproductive condition and estimated winter predation on lemmings from predators’ functional and numerical responses. Most lemming reproduction stopped late in peak summer 1996 but recovered in decline summer 1997. Estimated winter predation during the decline was similar to summer radio-telemetry mortality estimates. We conclude (1) in autumn and winter, a decreased reproductive rate and increased predation by Arctic foxes and avian predators before snowfall may have contributed to winter decline; (2) in summer 1997 reproduction recovered; hence, a low summer reproductive rate was not a factor in the continuing decline. Winter declines from high collared lemming density may be driven by absence of winter reproduction, along with mortality caused by predation and harsh winter weather.