Fecundity
Fecundity comes from the Latin word fecundus, meaning fruitful, rich, or abundant. It is the rate at which individual organisms in the population produce offspring. Although the term can apply to plants, it is typically restricted to animals.
There are two aspects of reproduction: 1) fertility, referring to the physiological ability to breed, and 2) fecundity, referring to the ecological ability to produce offspring. Thus, higher fecundity is dependent on advantageous conditions in the environment that favor reproduction (e.g., abundant food, space, water and mates; limited predation, parasitism, and competition). The intrinsic rate of increase (denoted as "r") equals the birth rate minus the death rate. It is a population characteristic that takes into account that not all individuals have equal birth rates and death rates. It therefore refers to the reproductive capacity in the population made up of individual organisms. Fecundity, on the other hand, is an individual characteristic. It can be further subdivided into potential and realized fecundity. For example, deer can potentially produce four or more fawns per year, but they typically give birth to only one or two per year. In good years with ample food, they often have only two fawns.
Animals in nature are limited by environmental conditions that control their life history characteristics such as birth, survivorship, and death. A graph of the number of offspring per female per age class (e.g., year) is a fecundity curve. This can then be used to interpret the individuals of a certain age class who contribute more to the population growth than others. In other words, certain age classes have a greater reproductive output than others. Wildlife managers often use this type of information in deciding which individuals in a population can be hunted verses those that should be protected so they can reproduce.
As the number of animals increase, competition for food may become more intense and, therefore, growth and reproduction may decrease. The result is an example of density-dependent fecundity. Fecundity in predators typically increases with an increase in the prey population. Conversely, fecundity in prey species typically increases when predation pressure is low.
Some scientists have found that fecundity is inversely related to the amount of parental care given to the young. In other words, small organisms such as insects and fish which typically invest less time and energy into caring for the young usually have higher fecundity. Larger organisms such as birds and mammals which expend a lot of energy on caring for the young through building of nests, feeding, protecting, and caring have lower fecundity rates.
Resources
Books
Colinvaux, P. A. Ecology. New York: Wiley, 1986.
Smith, R. E. Ecology and Field Biology. 4th ed. New York: Harper and Row, 1990.
Ricklefs, R. E. Ecology. 3rd ed. New York: W. H. Freeman, 1990.
Krebs, C. J. Ecology: The Experimental Analysis of Distribution and Abundance. 3rd ed. New York: Harper and Row, 1985.
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