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August 2000
There are several factors affecting populations of wildlife. Not surprisingly, they are the factors that affect human populations but we may miss the similarities because the veneer of civilization disguises that we too are part of nature.
All life needs food, cover, water and space to survive. A species' habitat is an area that provides food, cover and water in the appropriate forms, proportion and arrangement for that species' adaptations. The physical space required by an individual animal of a species is affected by amount of food, cover and water present.
The maximum number of individuals of a species that can be supported by an ecosystem over time is called its carrying capacity. It is a function of the quantity, quality, distribution and seasonal variation of food, water and cover in the ecosystem. When the carrying capacity of a species' habitat is high, but the population there is low, the number of live births per female per year is high. When a species' population is at or exceeds the habitat's carrying capacity, birth rates are low and the death rates are high.
When food, cover or water is scarce, it is considered a limiting factor in regard to population. A reduction in any of these habitat elements may prevent the increase of a particular species' population in an area. For example, poor pollination of berry-bearing shrubs will result in less food available for those species needing fruits, such as cedar waxwings. A disease killing white pine trees will reduce winter cover. The lack of protection from cold, ice and snow may increase deaths among species using white pine. Limiting factors affect birth rates or death rates, or both, and so affect the population of a species in that habitat.
Environmental constraints, or decimating factors, include predation by wild animals, disease, weather, hunting and trapping. In habitats of high quality, the effects of decimating factors may be alleviated and populations can usually rebound. The drought last summer and the ice storm of recent memory are two good examples of decimating factors.
Some species produce more offspring than others. Each species has a maximum genetic reproductive potential, also called biotic potential. In general, species at the higher end of the food chain have fewer offspring and species at the lower end have more offspring. A broad-winged hawk, a predator fairly high in the food chain, usually has two or three young each year. Shrews are a staple of their diet, and the northern short-tailed shrew has two or three litters a year, with about five young in each litter. So the biotic potential of this shrew is higher than that of the broad-winged hawk.
It is interesting to note that environmental contamination, habitat destruction by development and predation by free-ranging domestic cats are not included in the definitions of either limiting or decimating factors. Wildlife biologists do not agree amongst themselves how, or if, these should be incorporated into theories of population dynamics. Ironically, these three ultimate causes of mortality are today the major factors affecting wildlife populations, not just in Maine but also in the world.
By Nancy Coverstone (ncstone@umext.maine.edu), University of Maine Extension Educator in Androscoggin and Sagadahoc Counties
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