Many groups of animals are provided with unique and intrinsic self-regulatory mechanism such as failure of reproduction and self-inflicted mortality for controlling the size of the population. Populations grow when natality exceeds mortality, and the), decline when mortality exceeds natality.
However, limitation animal number in a population is brought about by of two basic regulatory processes namely density and density dependent factors. Density independent facetis the extrinsic factors which tend to regulate the density population under different conditions, appearing to act on the population and inflict loss of individuals irrespective of the population density.
Variations in space or cover, favorable weather and food occur independently of population densities and may cause drastic changes in the abundance of animals. Such ecological or environmental factors influence negatively or positively all the individuals of a population irrespective of density.
The density dependent factors are intrinsic or biotic factors and they depend on coactions between individuals within the same population or between populations of different species. Density- dependent factors may stabilize populations at an asymptote, the level of which is determined by the carrying capacity of the environment.
Some of the important density dependent factors competition reproductively, predation, emigration and disease the combination offaotors or any specific factor involved in the density dependent action may vary from species to species. Further these species have been known to display the property of intercompensation.
According to intercompensation, if there is a change in the environment to relieve the population of pressure from an existing effect, then the population increases till it reaches a level when the second effect takes over.
Thus, if the predators that normally keep herbivorous animals down are removed, the population of herbivores may increase to become overcrowded and result is starvation. A supply of abundant food at this junction would make the individuals susceptible to diseases due to the intensity of crowding (Wilson and Bossert, 1971).
Generally, the population fluctuations controlled by extrinsic factors tend to be irregular and correlated with the variation in one or more major physical limiting factors such as temperature, food, water, etc., while fluctuations of populations controlled by intrinsic factors exhibit regularity and population cycles.
Populations are said to be cyclic when they alternatively cropland subside in a more or less uniform manner between high and low levels of density. Different animal’s exhibit population cycles at different times. The best established cycles of population density of fluctuations are those of periodicities of 3-4 years and 9-10 years.
The 3-4 year cycles are most commonly observed in many birds such as snowy owl, willow ptarmigan, and capercailzie. Black game, Hazel grouse etc.; mammals such as lemmings, voles, arctic foxes, etc., and fishes. 9-10 years cycles of population density fluctuations have been observed in birds such as ruffed-grouse, sharp-tailed grouse, willow Ptarmigan, etc., and mammals such as snow-shoe muskrats panada lynx, etc.
Among invertebrates, insect pests of coniferons forests in Germany fluctuate in periods variously from 6 through 18 years. Asterias forkesi is found to contain periodicity of 14 years.
Attempts to explain these vast oscillations in numbers on the basis of climatic changes have been unsuccessful. At one time it was believed that these were caused by sunspots, and the sunspots and lynx cycles do appear to correspond during the early part of the nineteenth century.
However, the cycles are of slightly different lengths and byT920 were completely out of phase, sunspot maxima corresponding to lynx minima. Attempts to correlate these cycles with other periodic weather changes and with cycles of disease organisms have been unsuccessful.
The snowshoes hares die off cyclically even in the absence of predators and in the absence of known disease organisms. The animals apparently die of “shock” characterized by low blood sugar (hypoglycemia), exhaustion, convulsions and death, symptoms which resemble the “alarm responses” induced in laboratory animals subjected to physiologic stress.
Visualizing this similarity, J. J. Christian (1950) proposed that their death, like the alarm response is the result of some upset in the adrenal-pituitary system. As population density increases; there is increasing physiological strees on individual hares owing to crowding and competition for food. Some individuals are forced into poorer habitats where food is less abundant and predators more abundant.
The physiologic stresses stimulate the adrenal medulla to secrete epinephrine, which stimulates pituitary via the hypothalamus to secrete more ACTH (adrenocorticotropic hormone). This, in turn, stimulates the adrenal cortex to produce corticosteroids, an excess or imbalance of which produces the alarm response or physiological shock.
In the latter part of the winter of a year of peak abundance, with the stress of cold weather, lack of food and the onset of the new reproductive season putting additional demands on the pituitary to secrete gonadotropins, the adrenal-pituitary system fails, becomes unable to maintain its normal control on carbohydrate metabolism, and low blood sugar (hypoglycemia), convulsion and death ensue.
According to Kendeigh (1974) several intrinsic factors such as disease, predation, food factor and natural selection affect animal cycles in addition to several extrinsic factors such as weather, solar radiation, and so on.