BIOLOGY Volume 1 - A Guide to General Biology - 2004

10. ORGANISMS AND THE ENVIRONMENT

10.7. Population Ecology

10.7.4. Density-Dependent Factors Affecting Population Size

After a population completes its initial phase of growth, its size continues to fluctuate over time. These fluctuations reflect the instability of environmental factors, including climate (Temperature), food availability, and predator activity. Sometimes these fluctuations occur regularly, in which case they are referred to as cycles.

Population sizes can change As a result of shifts in parameters such as birth and death rates. The factors driving these changes often operate more effectively as population density increases; consequently, these factors are density-dependent. Food depletion and intensified predation clearly have an impact on mortality rates. Birth rates are regulated by more subtle mechanisms and may also depend on territorial behavior and the Physiological effects of overcrowding.

Territorial behavior

Territorial behavior, or territoriality, is observed in A wide variety of animals, including certain fish, reptiles, birds, mammals, and social insects. It has been studied most thoroughly in birds. Either a single male or a breeding pair establishes the boundaries of its nesting territory, excluding other individuals of the same species. The signal that an area is "occupied" is usually delivered through a specific song or a threatening posture, such as the display of a red breast in robins. A boundary trespasser typically retreats, sometimes after a brief "ritualized fight" in which neither competitor is seriously harmed. Within the same species, territories overlap very little or not at all; if they also serve as "feeding territories," they provide an adequate food supply for the breeding pair and its brood (Fig. 10.20). As the population grows, territories generally shrink, and fewer chicks survive. Eventually, some adult birds fail to find a suitable site and consequently do not reproduce. Thus, population size is regulated by the spatial interactions of its members.

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Fig. 10.20. Territorial display.

Overcrowding

Another factor capable of influencing birth rates is overcrowding. Laboratory experiments demonstrate that as population density increases, the fecundity of rats declines even when food is abundant. This occurs because the animals experience stress, triggering the release of specific Hormones that affect reproductive behavior. Phenomena such as mating failure, Infertility, miscarriages, and filial cannibalism become more widespread, while parental care declines. Young animals leave the nest earlier, consequently reducing their chances of survival. Animal aggressiveness also increases. Such patterns are characteristic of a range of mammals and may well operate under natural conditions as well as in the laboratory. A similar type of population regulation is observed, for instance, in natural populations of voles.

Among the density-dependent factors affecting population size, dispersal (emigration) should also be mentioned. For example, when aphids are highly abundant, not only does their reproduction rate drop, but the proportion of winged individuals migrating to other host plants increases as well.

Certain factors that regulate population sizes, such as climatic ones, are generally considered density-independent; however, they inevitably interact with density-dependent influences (for instance, weather conditions affect food availability). Therefore, although making a methodological distinction between the Two Types of factors is useful, they should not be viewed in opposition to one another.