For the purpose of comparing age distributions, a useful means of portraying age is as a percentage of total life span. The maximum observed life span of a species is determined, and ages are presented as’ percentages of that maximum.
This presentation format for age data can be applied equally well to any species of organism, legardless of its life span. A related means of presentation is percentage of deviation from mean life span. In this format, the average age at death is computed and ages -presented in terms of deviation from this mean age.
Both of these formats require that the maximum or mean life span, respectively, be shown for each species, because this information is not presented in data converted to percentage form.
A somewhat different type of age portrayal is the ecological or functional age, where the life span of a species can be divided up into various functional units, such as prcreproductive, reproductive, and post reproductive, or egg, larva, nymph, and adult.
The appearance of individuals of the various functional ages may be quite similar, as in man, or quite different, as in animals that undergo metamorphosis. Functional age breakdowns can be extremely useful in comparing certain features of different populations, such as the relative importance of specific environmental factors on the population as a whole.
If we know both the percentage of time spent by an organism in specific functional ages and the environmental factors that have the greatest effect on each age, we have gained considerable insight into the characteristics of the species in question.
This knowledge is essential if we are trying to design a procedure to control a best species on an economically important crop, is necessary to ascertain which stage in the life most vulnerable to attack by which means.
The first means of portraying age distribution is the age pyramid. An age pyramid is a vertical bar graph in which the number or proportion of individuals in various age ranges at any given time is shown from youngest at the bottom of the graph to oldest at the top (Fig. 15.2). It is very useful in monitoring the commercial exploitation of food species by man among other things.
Ages may be shown in any of the formats we outlined. An Age Pyramids represents the age distribution of a living population at a specific moment; – thus, as the population age distribution changes over a period of time, the form of the age pyramid does.
From an age pyramid, one can collect the information that whether a population is expanding, contracting, or stable. When compared to the age pyramid of a stable population, if age pyramid shows an increased number of juveniles (i.e., a broader base), it is expanding, if it shows a decreased number of juveniles (i.e., a narrower base), it is contracting.
Consequently, a triangular pyramid will represent a growing population and an urn-shaped pyramid will indicate an increased number of middle aped and old organisms compared to the young. However a bell-shaped pyramid will denote a stationary or stable population having an equal number of young and middle-aged classes.
Besides the age distribution, the ratio of organisms of different sexes also affects the size of population. Most of the plants are bisexual and in relatively few cases the male and female plants are separate.
Among the animals, there exist great variations in (the number of male and female individuals. For example, among social insects like in honeybees, a single female is responsible for reproduction out of a large population which is mostly neutral. The sex ratio also changes with the age because of differences in the mortality of the male and female individuals.
Biotic Potential and Environmental Resistance:
The term biotic potential or reproductive potential refers to the inherent power of a population to increase in numbers when the age ratio is stable and all environmental conditions are optimal. The biotic potential is defined mathematically as the slope of the population growth curve during the logarithmic phase of growth.
When environmental conditions are less than optimal; the rate of population growth is less. The difference between the potential ability of a population to increase and the actual change in size of the population is a measure of environmental resistance.
The sum of the physical and biologic factors which prevent a species from reproducing at its maximum rate is termed the environmental resistance. Environmental resistance is often low when a species is first introduced into a new territory so that the species increases in number at a fantastic rate as when the rabbit was introduced into Australia and English sparrow and Japanese bettle were brought into the United States.
But as a species increases in number the environmental resistance to it also increases, in the form of organisms that prey upon it or parasitize it and the competition between the members of the species for food and living space and consequently, population decreases. A cauilibrium will be reached either by decreasing the birth rate (natality) or by increasing the mortality rate.