Factors Affecting Population Growth

There are different factors or mechanisms under different circumstances which limit the population size. They are as follows: 1. Density-dependent regulation and 2. Density independent regulation.

.A. Density-dependent regulation

(i) Density-dependent factors are those that vary in the intensity of their action with the size or density of the population.

(ii) They increase in intensity as the population level rises and decrease as the population level declines.

(iii) Thus, density-dependent factors are biotic as they depend on intraspecific or interspecific coactions.

(iv) The concept of density-dependent factors indicates that all populations are regulated automatically.

 (V) Populations are self-governing systems and regulate their densities in relation to their own properties and those of their environments.

B.  Population Fluctuations

It refers to the change in size and density of a natural population in a time period. Fluctuations may be of the following kinds:

(a) Flat.........minute fluctuations

(b) Cyclic-large and regular fluctuations

(i) Seasonal.........more individuals in the breeding season.

 (ii) Annual........increase or decrease per year.

(c) Irruptive-Irregular fluctuation.

C. Population Dispersal

The movement of individuals in or out of a population area is called population dispersal.

Population dispersal can be due to:

(a) Emigration..........permanent outward movement of individuals.

(b) immigration..........inward movement of individuals leading to overpopulation.

(c) Migration.......... departure and return of individuals in a population.

The population dispersal is observed in search of food, to avoid predators, prevent overcrowding, and under the influence of various environmental factors. Dispersal facilitates the interchange of genetic material and helps in the evolution of species.

 The most important density-dependent factors are as follows:

(a) Competition

(1) The size of a population is determined by interactions either among its members or with other species of competitors, predators, or parasites.

(1) The role of competition in regulating population is directly effective by causing mortalities, nest destruction, and loss of food supplies.

(iii) It also results in lowered rates of reproduction.

(iv) Gause's principle of competitive exclusion states that no two species with identical niche requirements can continue to coexist.

(b) Predation

(1) The predator-prey relationship provides an example of density-dependent regulation.

(ii) Eg. The removal of mountain lions was followed by an increase in populations of deer at some places.

(c) Reproductivity

() It is also a density-stabilizing factor. The birth and death rates have their important roles in the regulation of population size.

(d) Territoriality

Another situation in which the population size is regulated by a density-dependent factor is the phenomenon of territoriality.

(e) Emigration

The pressure of overpopulations can be relieved by mass emigrations of individuals from particular localities. Eg. Emigrations under normal conditions occurs when there is overcrowding in the migratory locust, lemming, grouse, snowy owl and snowshoe rabbit.

(f) Disease and physiological stress

(i) Although the infectious disease in some forms is a common cause of mortality, it is less important as a stabilizing factor than the others already described above, because it reduces the population size in an important manner only when epidemics or epizootics occur.

(ii) The mortality may then be extreme so that the population falls below the level of stabilization and a period of recovery follows.

(iii) Physiological stress may also bring considerable mortality in the population. It seems probable that shock disease in snowshoe rabbits is a manifestation of the stress syndrome.

D. Density-Independent regulation

(1) The level at which populations become stabilized is determined by such factors as space or cover, prevailing weather, and food supply.

(ii) These factors are largely density-independent since their magnitude is primarily determined by the physical or abiotic conditions of the environment.

(iii) Maximum population size in plants could be determined by the physical environment.

i(v) At one extreme there are organisms living under very harsh conditions, such as small insects. Their populations grow during the short wet season until the drought kills the plants upon which they feed, and insects disappear. Thus, they are completely at the mercy of their physical environment and the size of their population at the end of the wet season depends on their intrinsic rate of increase.

E. Population gene pool. The aggregate of all the genes and their alleles present in an interbreeding population is called a gene pool.