Thus, it was not all individuals suffering from reduced ability to reproduce (e.g., fecundity) due to the density increase. When squirrel densities were low, no females occupied the low-quality territory. They found that when squirrel densities were high, territoriality relegated some females to poor quality territory, which in turn reduced their reproductive success. They studied squirrels in both coniferous and deciduous woodlands and investigated how limitations in food resulted in limitations in reproduction as population densities increased.
The red squirrel ( Sciurus vulgaris) is a small rodent inhabiting forests in Europe and Asia. Wauters & Lens (1995) studied how food availability and density combine to limit red squirrel reproduction rates. With a negative relationship, population growth is limited at low densities and becomes less limited as it grows.ĭensity-dependant factors may influence the size of the population by changes in reproduction or survival. With a positive relationship, these limiting factors increase with the size of the population and limit growth as population size increases. Density-dependant factors can have either a positive or a negative correlation to population size. Density-dependent factors include disease, competition, and predation. Limitations to population growth are either density-dependant or density-independent. 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. As the lemmings provided the stoat with additional food, their reproductive success increased, allowing an increased stoat population. The owl, fox, and skua switched to lemming predation as the lemming numbers increased, preventing rapid population growth. (2003) found that the single most important factor limiting lemming population size was the predation pressure affecting those populations. The number of lemmings increased to as many as ten per hectare. The lemming population increased and decreased in a regular four-year cycle during the study period, 1988–2002 (Figure 1). (2003) studied this system in the Karup Valley of northeast Greenland. Because of the simplicity of this system, lemming population dynamics make an excellent case study for examining the factors regulating population growth. 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). One species, the collard lemming ( Dycrostonyx groenlandicus), is a chubby-looking rodent living in the arctic of North America and Greenland.
Lemmings are small rodents that live in the high-Arctic tundra of Greenland and in other arctic environments across the world. One organism that experiences rapid oscillations in population density in response to growth limiting factors is the lemming. Many different factors may combine to produce unexpected results. It is also often difficult to determine the exact factor limiting growth. Many factors influence population densities and growth, and these factors may lead to oscillations in population size over time. Instead, populations in natural ecosystems increase or decrease in response to the changes in the factors that restrict growth. No population can increase without limitation. At carrying capacity, because population size is approximately constant, birthrates must equal death rates, and population growth is zero.
The population size at which growth stops is generally called the carrying capacity (K), which is the number of individuals of a particular population that the environment can support. As resources are depleted, population growth rate slows and eventually stops: This is known as logistic growth. In nature, population growth must eventually slow, and population size ceases to increase. However, geometrical or exponential growth cannot continue indefinitely. Because of this, exponential growth may apply to populations establishing new environments, during transient, favorable conditions, and by populations with low initial population density. All populations begin exponential growth in favorable environments and at low population densities. Exponential populations grow continuously, with reproduction occurring at any time, such as among humans. Geometric populations grow through pulsed reproduction (e.g., the annual reproduction of deer, which have a constrained mating and reproduction season). Populations grow at geometric or exponential rates in the presence of unlimited resources.