Animal Ecology To-Day

By F. S. Bodenheimer; W. W. Weisbach | Go to book overview

I
PHYSIOLOGICAL AND ECOLOGICAL LIFE-TABLES AND CONNECTED PROBLEMS
(LIFE-INTENSITY, AGE-STRUCTURE OF ANIMAL POPULATIONS, RELATION BETWEEN THE ECOLOGICAL AGES)
1. Physiological Life-Expectation
Every analysis of life-expectation begins with a study and definition of physiological longevity. Physiological longevity may be defined as that life duration which a healthy individual may expect to live under optimal environmental conditions until dying by senescence. Such an individual, having exhausted its inborn vital potentialities, dies because death is the end of every organized life.Any individual out of an animal population bred under optimal conditions or under an optimal sequence of conditions (weather, food, density, parasites, diseases, episites, etc., and accidents excluded) and selected from a thoroughly healthy and genetically homogeneous stock will die a physiological death, conditioned only by senescence. Assuming that conditions are always optimal and that genetically the stock is healthy and homogeneous, an ideal curve of longevity, or death, must be expected. All individuals should survive until senescence and death set in, within a short period following a sharp decline. The main difference would be due to the different longevity of the sexes and to the minor individual differences which are always met with, even in a genetically very similar stock. Ecological conditions induce a heavy infant mortality and a permanent but lower mortality until senescence, when the last individuals die.However, no animal lives throughout life under optimal conditions, and the physiological life-expectation is not verified in nature. Also, no two individuals of the same population live under exactly the same conditions during their life, and no two individuals of gamogenic origin are exactly alike in their hereditary constitution, inborn resistance, and vitality*. Whatever is said here for individuals holds still more for populations of the same species.Even in the absence of enemies and diseases, the following factors induce a deviation from the ideal death curve:
A. Genetic: different genetic constitution; not thoroughly healthy constitutions.
B. Ecological: deviation from optimal conditions during one, some, or all stages (concerning weather, food, density, enemies, etc.).
____________________
*
See note a, p. 267

-12-

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