Hobbies And Interests
Basis of Study
Lemmings, an order of rodent, populate the tundra worldwide. Their population fluctuates every four years on average. Researchers have carried out numerous studies in an effort to discover what brings about the fluctuation. Three population ecology models have been considered: the bottom-up model says food supply is the most important factor; the top-down model shows predation or disease to be dominant; and the social behavior model indicates that competition for territory or infanticide play key roles. Though researchers have not agreed upon a definitive explanation, their hypotheses all depend on one or more density-dependent factors.
Predation
In a study done in Greenland over a 15-year period, researchers showed in "Cyclic Dynamics in a Simple Vertebrate Predator-Prey Community," published in "Science," that lemmings have four major predators: the snowy owl, the arctic fox, the long-tailed skua and the stoat. Populations of the first three tend to rise and fall in general synchrony with the lemming population. In contrast, the stoat population rises and falls with a one-year lag. Competition for space and food is not a density-dependent factor among the lemmings; there is space and food available to them regardless of how big their population becomes. What causes the lemming population to fall is predation. As the lemming population grows, the predators begin to eat more of them, which causes the population to fall. As the population falls, the birds and fox look for other food sources. The stoat population, though, whose sole prey is the lemming, declines in numbers shortly after the lemming does. Competition is, therefore, a density-dependent factor for stoats.
Predation and Reproductive Competition
In "Limit Cycles in Norwegian Lemmings: Tensions Between Phase-Dependence and Density-Dependence," published by The Royal Society in 1997, researchers found predation and reproductive competition to be significant density-dependent factors in the cyclic nature of lemming survival. It determined that the high population density periods were caused by two factors. Some of their predators did not find lemmings palatable and ate them only when there was no other option. Also, lemmings are capable of rapid reproduction. A female lemming has an average of six young in a litter, may produce three or more litters in a summer, and may continue to reproduce during the winter.
Indirect Density-Dependency Through Predation
"Trophic Interactions in a High Arctic Snow Goose Colony," published by Oxford Journals, examined the influence of the greater snow goose on the lemming population. It found that predation by foxes was a density-dependent factor in both lemming and goose population growth or decline. Researchers found that a decrease in the lemming population affected the reproduction of the fox, which then increased its consumption of goose eggs and caused a decline in goose productivity. Thus, there was an indirect density-dependent relationship between the lemming and the goose by means of the shared predator, the fox.
Further Study Needed
In "Of Lemmings and Snowshoe Hares: The Ecology of Northern Canada," published by The Royal Society, a study conducted in the Yukon Territory, the researcher raises a number of questions. He writes that, due to the difficulty of collecting accurate field data, few conclusions can be drawn about the density-dependent factors that may affect lemming populations. Two species of lemmings were studied, and it was determined that populations of both species rose and fell at the same time over large geographical areas and reproduced most rapidly during the winter. The researcher, therefore, records that reproduction plays an important part in population cycles. Yet, little research has been done during the winter, so its full significance is not known. The researcher adds that more study is needed on food competition and social behavior as possible density-dependent factors.