16 



Ecological Processes 



and Community 



Dynamics: 



Regulation of 

 Population Size 



It is seldom possible to measure the total world- 

 wide population of any species, unless it is one of 

 restricted distribution and is readily accessible to 

 censusing. The gannet, a sea bird, nests in only 22 

 island and sea cliff colonies. From 1819 to 1929 the 

 annual population level of this species was about 

 340.000 birds. Because of molestation by man, the 

 population dropped to about 106,000 in 1894, but, 

 after some protection of the colonies was established, 

 rose to about 165,600 in 1939 (Fisher and Vevers 

 1944). For most species population can only be e.\- 

 pressed in terms of number per unit area (population 

 density). 



The abimdance of a species in a geographic re- 

 gion is termed the average or regional density of it. 

 A region, however, usually includes unfavorable habi- 

 tats, from which the species is absent, as well as suit- 

 able niches in which it is populous. The abundance 

 of a species within its niche is called its economic or 

 niche density, never less and almost always higher 

 than its regional density. 



The regional density of a species depends on the 

 prevalence of its favored niche in the area and the 

 density which the species maintains within its niche. 

 Muskrats may be very numerous in a marsh, but if 

 there are few marshes in the region, their average 

 density will be low. We will here be primarily con- 

 cerned with why an animal species attains a particu- 

 lar level of abundance within its niche. 



Species obviously vary in the level of abundance 

 that they attain. Springtails may occur in hundreds 

 per square meter, large mature snails as one per 

 square meter, and white-footed mice as only one in- 

 dividual per 400 m-. Principles involved in under- 

 standing these differences in population levels are the 

 size classes of the animals and their position in food 

 chains, pyramids, and trophic levels. 



The population of any one species may be said 

 to be stabilised when it fluctuates in an irregular but 

 restricted manner from the mean. If environmental 

 conditions temporarily become unusually favorable 

 or unfavorable, population size may fluctuate accord- 

 ingly, but with stabilization there is always the tend- 

 ency to revert again to the average level when the 

 unusual conditions have disappeared. The dynamic 

 resiliency of populations is evident in the high rates 

 of increase that occur with the beginning of recovery 

 after a population has been depleted, and the progres- 

 sive diminution in the rate of increase as the popula- 

 tion approaches its characteristic level. Although at 

 the beginning of the population growth curve, in- 

 creasing numbers may sometimes bring a cooperative 

 effect evident in increased rates of growth and re- 

 production (Odum and Allee 1954), cooperation is 

 soon replaced by disoperation in that the reproductive 

 rate then varies inversely and the mortality rate 

 varies directly with the density of the population in 



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