CONCEPTS IN BIOLOGY

PART IV. EVOLUTION AND ECOLOGY

 

17. Population Ecology

 

17.4. Limits to Population Size

 

Populations cannot continue to increase indefinitely. Eventually, a factor or combination of factors limits their size. The factors that prevent unlimited population growth are known as limiting factors. All the limiting factors that act on a population are collectively known as environmental resistance.

 

Extrinsic and Intrinsic Limiting Factors

Some factors that control populations come from outside the population and are known as extrinsic limiting factors. Predators, the loss of a food source, a lack of sunlight, and accidents of nature are all extrinsic factors. However, the populations of many kinds of organisms appear to be regulated by factors from within the populations themselves. Such limiting factors are called intrinsic limiting factors. For example, a study of rats under crowded living conditions showed that, as conditions became more crowded, abnormal social behavior became common. There was a decrease in litter size, fewer litters per year were produced, the mothers were more likely to ignore their young, and many young were killed by adults. Thus, changes in the rats’ behavior resulted in lower birthrates and higher death rates, which limit population size. In another example, the reproductive success of white-tailed deer is reduced when the deer experience a series of severe winters. When times are bad, the female deer are more likely to have single offspring than twins.

 

Density-Dependent and Density-Independent Limiting Factors

Density-dependent limiting factors are those that become more effective as the density of the population increases. For example, the larger a population becomes, the more likely that predators will have a chance to catch some of the individuals. A prolonged period of increasing population allows the size of the predator population to increase as well. Disease epidemics are also more common in large, dense populations, because dense populations allow for the easy spread of parasites from one individual to another. The rat example previously mentioned also illustrates a density-dependent limiting factor in operation—the amount of abnormal behavior increased as the density of the population increased. In general, whenever there is competition among the members of a population, the intensity of competition increases as the population density increases. Large organisms that tend to live a long time and have relatively few young (K-strategists) are most likely to be controlled by density-dependent limiting factors.

Density-independent limiting factors are populationcontrolling influences that are not related to the density of the population. They are usually accidental or occasional extrinsic factors in nature that happen regardless of the density of a population. A sudden rainstorm may drown many small plant seedlings and soil organisms. Many plants and animals are killed by frosts in late spring or early fall. A small pond may dry up, resulting in the death of many organisms. The organisms most likely to be controlled by density-independent limiting factors are small, short-lived organisms that can reproduce very rapidly (r-strategists).

 

17.4. CONCEPT REVIEW

10. Differentiate between density-dependent and density-independent limiting factors. Give an example of each.

11. Differentiate between intrinsic and extrinsic limiting factors. Give an example of each.