The Norway lemming (Lemmus lemmus) is a compact rodent native to the arctic and alpine tundra regions of Fennoscandia and the Kola Peninsula. These small mammals, typically weighing between 20 and 130 grams, possess a striking coat of black, brown, and golden-yellow fur, which they retain year-round. They thrive in moist habitats like bogs and fells, surviving the harsh winters by remaining active beneath the snowpack. The species is known for its population fluctuations, which occur on a cycle of approximately three to five years. This periodic rise and fall in abundance gives the Norway lemming an outsized influence on its environment, leading ecologists to recognize it as a keystone species.
Defining the Keystone Role
A keystone species is defined by its disproportionately large effect on its natural environment relative to its total biomass. The presence or absence of this species dramatically alters the ecosystem, which is why the Norway lemming fits this ecological classification. Its influence is not derived from its sheer numbers alone, but from the structural changes it imposes on the tundra food web and physical landscape. If the lemming were removed, the entire biological and physical system would undergo radical reorganization.
The lemming’s status differs significantly from that of a dominant species, which controls the ecosystem primarily through high biomass. Instead, the lemming acts as a structural force by linking the lowest trophic level (plants) with the highest (predators). This small rodent acts as a powerful energy conduit, channeling resources from the tundra vegetation into the broader animal community. The ecological consequences of its periodic population explosions and subsequent crashes illustrate a fundamental control mechanism within the Arctic biome.
Driving the Arctic Trophic Cascade
The most immediate consequence of the lemming’s presence is its function as the central energy source in the Arctic trophic cascade. The species is the primary prey for a suite of specialized predators whose survival and reproductive success are intrinsically tied to the lemming’s cyclical abundance. These population cycles typically oscillate every three to five years, dictating the reproductive output of high-level consumers.
During years of lemming abundance, predators like the Arctic Fox, Snowy Owl, ermines, and Rough-legged Buzzards experience intense breeding success. Female Arctic Foxes, for example, will produce larger litters, and Snowy Owls may only nest during these periods of ample food supply. The abundance of lemmings also temporarily shields other prey species, such as ptarmigan and geese, from heavy predation pressure as hunters focus on the easily accessible rodents.
When the lemming population inevitably crashes, the effects cascade rapidly up the food web. Specialized predators suddenly face a severe food shortage, forcing them to either migrate long distances or forgo reproduction entirely. The decline in lemmings shifts predation pressure onto alternative prey, causing a corresponding dip in the populations of ground-nesting birds like the grouse. This tight, cyclical dependence illustrates the lemming’s role as a biological engine that drives the dynamics of the entire vertebrate food web across the Fennoscandian tundra.
Shaping the Arctic Tundra
Beyond its role in the food web, the Norway lemming acts as an ecosystem engineer, physically modifying the Arctic tundra through its actions. A primary mechanism is intense herbivory, particularly during population peaks, when lemming densities can reach hundreds per hectare. These vast numbers of rodents consume enormous quantities of vegetation, feeding mainly on mosses, grasses, sedges, and lichens. This heavy grazing prevents certain dominant plant species from monopolizing resources, promoting a higher degree of plant biodiversity.
The consumption of biomass is so intense during a peak year that the vegetation composition can be altered for several years afterward. Long-term exclosure studies show that the exclusion of these rodents leads to a significant increase in total plant biomass. This highlights how lemmings maintain the structure of the tundra plant community through continuous pressure.
The lemming’s activity in the subnivean space—the layer between the ground and the snowpack—also physically modifies the soil structure. They create extensive tunnel systems as they forage and nest during the winter months, an activity that aerates the frozen soil. This digging and tunneling mixes organic matter and affects nutrient cycling processes, creating microhabitats beneficial for the germination of certain plants. By disturbing the soil layer, the lemming’s behavior also influences the thermal regime of the active layer above the permafrost.