Population Limiting Factors | Learn Science At Scitable - Nature

Figure 1: Lemming population numbers during the period 1988–2002 (Gilg et al. 2003).Lemmings were captured following the spring snowmelt by live trapping and during winter nest counts.© 2011 Nature Education All rights reserved. No population can increase without limitation. Instead, populations in natural ecosystems increase or decrease in response to the changes in the factors that restrict growth. Many factors influence population densities and growth, and these factors may lead to oscillations in population size over time. It is also often difficult to determine the exact factor limiting growth. Many different factors may combine to produce unexpected results.

One organism that experiences rapid oscillations in population density in response to growth limiting factors is the lemming. Lemmings are small rodents that live in the high-Arctic tundra of Greenland and in other arctic environments across the world. One species, the collard lemming (Dycrostonyx groenlandicus), is a chubby-looking rodent living in the arctic of North America and Greenland. It is food for a number of vertebrate predators, including the stoat (a short-tailed weasel), the arctic fox, the snowy owl, and the long-tailed skua (a seabird). Because of the simplicity of this system, lemming population dynamics make an excellent case study for examining the factors regulating population growth. Gilg et al. (2003) studied this system in the Karup Valley of northeast Greenland. The lemming population increased and decreased in a regular four-year cycle during the study period, 1988–2002 (Figure 1). The number of lemmings increased to as many as ten per hectare. Gilg et al. (2003) found that the single most important factor limiting lemming population size was the predation pressure affecting those populations. The owl, fox, and skua switched to lemming predation as the lemming numbers increased, preventing rapid population growth. As the lemmings provided the stoat with additional food, their reproductive success increased, allowing an increased stoat population. Stoat population expansion eventually overran lemming population growth, and the lemming population collapsed, soon followed by a collapse in the stoat population, and the cycle repeated itself.