1855 1865 1875 1885 1895 1905 Year

Two kinds of limiting factors, which control population size, have been identified. Density-independent factors, such as weather, floods, and fires, reduce the population by the same proportion, regardless of the population's size. For example, if a forest fire destroys a population of chipmunks, it does not matter if the population of chipmunks is 1 or 100. An unseasonable cold snap is a density-independent factor because its severity and duration are completely independent of population size. Density-dependent factors include resource limitations, such as shortages of food or nesting sites, and are triggered by increasing population density. With density-dependent factors, an individual's chance of surviving or reproducing depends on the number of individuals in the same area.

Population Fluctuations

All populations fluctuate in size. Some population fluctuations are clearly linked to environmental changes. For example, a drought may reduce a population of deer living in a forest. Some population fluctuations are not obviously connected to environmental fluctuations, and explaining their occurrence is much more difficult. For example, consider the population changes shown in Figure 19-10. These cycles of change were first described by Charles S. Elton (1900-1991), one of the pioneers of ecology. Elton obtained more than 70 years of records showing the number of snowshoe hare pelts the Hudson's Bay Company of Canada purchased from trappers. He assumed that the number of pelts purchased in a year indicated the size of the snow-shoe hare population. The records showed that the hare population underwent a very regular cycle, with about 10 years between peaks in population size. When Elton examined the records for the number of lynx pelts purchased, he found that the lynx, a medium-sized species of cat that preys on snowshoe hares, also followed a population cycle. The peaks in the lynx population usually occurred near the peaks in the hare population.

Elton thought that each species was the cause of the other's cycle. Thus, when the population of snow-shoe hares increased, providing more food for the lynxes, the lynx population also increased. The increased lynx population then ate more hares, so the hare population decreased. With less food, more lynxes starved and the lynx population declined, allowing the hare population to increase and start the cycle over again. However, the observation that the same cycles occur in snowshoe hare populations living on islands without lynxes indicates that this explanation is insufficient. Another possible explanation is that the lynx cycle is dependent on the hare population but the hare cycle is dependent on some other factor.

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