Some of the most important characteristics of population are as follows:
1. Population density 2. Natality 3. Mortality 4. Population growth 5. Age distribution of population 6. Population fluctuations.
1. Population Density:
Population density refers to the size of any population in relation to some unit of space. It is expressed in terms of the number of individuals or biomass per unit area or volume, as for example, 500 teak trees per hectare; 40 lions per 100 km2, 5 million diatoms per cubic meter of water. Population density is seldom static and it changes with time and space.
Population size can be measured by several methods:
Absolute number of individuals in population.
(ii) Numerical Density:
Number of individuals per unit area or volume. It is expressed when the size of individuals in the population is relatively uniform, as in mammals, insects and birds.
(iii) Biomass Density:
Biomass density is expressed in terms of wet weight, dry weight, volume, and carbon and nitrogen weight per unit area or volume.
Population density can be expressed in two ways:
(i) Crude Population Density:
When the density is expressed with reference to total area at a particular time.
(ii) Ecological Density:
When the density is expressed with reference to total area of habitat available to the species. The distribution between crude density and ecological density becomes important because the patterns of distribution of individuals in nature are different and individuals of some species like Cassia tora, Oplismenus burmanni are found more crowded in shady places than in other parts of the same area. Thus population density calculated in total area would be crude density and the densities for the shade areas and open areas separately would be ecological densities.
Population density can be calculated by the following equation:
D = n/a/t
Where D is population density; n is the number of individuals; a is area and t is unit time. Density of human population can be obtained by dividing the total number of persons in the area by the total land area of the region. Density of population of a country can be obtained by dividing the total number of persons living in the given region by total land area of that region. Average population density in developing countries is more as compared to those in developed countries.
Netherlands is smaller than India but its population density is greater (319/km2 in Netherlands and les/km2 in India). Area of India is 2.5% of the world but 15% population of the world lives in India alone. The population density of India is 4% higher than that of Europe and more than 7 times that of U.S.A. Population density is affected by a number of environmental factors, such as geographical factors, mortality, natality, emigration and immigration and socio-economic factors.
Natality refers to the rate of reproduction or birth per unit time. It is an expression of the production of new individuals in the population by birth, hatching, germination or fission.
Natality is calculated by the following formula:
Birth rate or Natality (B) = Number of births per unit time/Average population.
The maximum number of births produced per individual under ideal conditions of environment is called potential natality. It is also called reproductive or biotic potential, absolute natality or maximum natality.
Natality varies from organism to organism. It depends upon the population density and environmental factors. It is a general rule that if the population density is usually low, the birth rate is also low. This is so because the chances of mating between males and females are low. If population density is unusually high, the birth rate may also be low due to poor nutrition or physiological or psychological problems related to crowding.
The maximum or absolute natality is observed when the species exists under ideal ecological and genetic conditions. The actual number of births occurring under the existing environmental conditions is much less as compared to absolute natality. It is referred to as ecological natality or realized natality. It is not constant for population and may vary with the size of population as well as with the time.
Mortality refers to the number of deaths in population per unit time.
Mortality rate = D/t where D is the number of deaths in the time t.
Mortality can be expressed in the following two ways:
(i) Minimum or Specific or Potential Mortality:
It represents the minimum of theoretical loss of individuals under ideal or non-limiting condition. Thus, even under the best conditions individuals of a population would die of old age determined by their physiological longevity. So it is constant for a population.
(ii) Ecological or Realized Mortality:
It refers to the death of individuals of a population under existing environmental conditions. Since it varies with environmental conditions, it is never constant. The maximum mortality occurs at the egg, larval, seedling and old age. Mortality is affected by a number of factors, such as, density, competition, disease, predation and environment. Death rates vary among the species and are correlated with birth rates. When the rate of natality is equal to the rate of mortality the population is stationary.
A birth death ratio (Births/death x 100) is called vital index. For a population, the survival of individuals is more important than the death. The number of births in relation to the carrying capacity of the habitat is a fundamental factor influencing the mortality rate. When more young’s are born than the habitat can support, the surplus must either die or leave the area. Because the number of survivors is more important than the number of dying individuals, mortality is better expressed as survival or as life expectancy. The life expectancy refers to the average number of years the members of a population have left to live.
Life Tables and Survivorship Curve:
The species differ in respect of birth rates, average life span and mortality rate. When sufficient information’s about a species are available, life- table can be formulated which provides vital statistics of mortality and life expectancy for the individuals of different age-groups in the population. In such tables age is usually represented by the subscript index x which is some convenient fraction of species life span, such as, years or stage of development.
The life table is set up on the basis of an initial cohort or group of 100, 1000, 10,000 10, 00, 00 individuals and the number of living in the beginning of each successive age interval is symbolized as Ix. Plotting these data gives a survivorship curve for a species. The number of dying individuals within each age group is denoted as dx. The rate of mortality during each age interval (qx) is commonly expressed as the percentage of the number at the beginning of the interval.
qx = dx/lx x 100
Survival rate is the difference between the mortality rate and 100 per cent (i.e., 100 — qx) and is denoted by 5x. Life expectancy (ex), thus, is the mean time between any specified age and the time of death of all individuals in the age group.
Types of Survivorship Curve:
If it could be assumed that all members of an original population have the same capacity for survival (environmental effects for the moment are ignored), plotting the number of surviving individuals against time would produce a survivorship curve in the form of a right angle. There are three general types of survivorship curves which represent different natures of survivors in different types of population (Fig. 4.1).
(i) First Type or Highly Convex Curve:
Curve A in the Fig. 4.1 is the characteristic of the species in which the population mortality rate is low until near the end of life span under ideal environmental conditions. Thus, all the members born at the same time live out the full physiological life span characteristic of the species and all die at about the same time. Many species of animals as deer, mountain sheep, and modem man show such curves.
(ii) Second Type or Diagonal Curve:
Survivorship curve B in the figure 4.1 is characteristic of organisms in which rate of mortality is fairly constant at all age levels, a more or less uniform percentage decrease in the number that survives.
(iii) Third Type or Highly Concave Curve:
Survivorship Curve C in the Fig. 4.1 is characteristic of such species in which mortality rate is high during the early stage and constant in all other age-groups. Oyster, some birds, oak trees, etc. show this type of curves.
4. Population Growth:
The growth is one of the dynamic features of species population. Population size increases in a characteristic way. When the number of individuals of population is plotted on the y-axis and the times on the x-axis, a curve is obtained that indicates the trend in the growth of population size in a given time. This curve is called population growth curve.
There are two types of growth curves:
(i) Sigmoid Curve:
When a few organisms are introduced in an area, the population increase is very slow in the beginning (positive acceleration phase or lag phase), in the middle phase, the population increase becomes very rapid (logarithmic phase) and finally in the last phase population increase is slowed down (negative acceleration phase) until an equilibrium is attained Lund which the population size fluctuates according to variability of environment.
The level beyond which no major increase can occur is referred to as saturation level or carrying capacity. In the last phase the new organisms are almost equal to the number of dying individuals and thus there is no increase in population size. In this way, one gets sigmoid or 5-shaped growth curve (Fig. 4.2).
(ii) J-Shaped Curve:
The second type of growth curve is J-shaped. Here in the first phase there is no increase in population size because it needs some time for adjustment in the new environment. Soon after the population is established in the new environment, it starts multiplying rapidly. This increase in population is continued till large amount of food materials exist in the habitat. After some time, due to increase in population size, food supply in the habitat becomes limited which ultimately results in decrease in population size. This will result in J-shaped growth curve rather than S-shaped (Fig. 4.2).
5. Age Distribution:
Age distribution is another important characteristic of population which influences natality and mortality. Mortality, usually varies with age, as chances of death are more in early and later periods of life span. Similarly, natality is restricted to certain age groups, as for example, in middle age-groups in higher animals. According to Bodenheimer (1958), the individuals of a population can be divided into pre-reproductive, reproductive and post-reproductive groups. The individuals of pre-reproductive group are young, those of reproductive group are mature and those in post-reproductive group are old.
The distribution of ages may be constant or variable. It is directly related to the growth rate of the population. Depending upon the proportion of the three age-groups, populations can be said to be growing, mature or stable, and diminishing In other words, the ratio of various age groups in a population determines the reproductive status of the population. Rapidly increasing population contains a large proportion of young individuals, a stable population shows even distribution of individuals in reproductive age-group and a declining population contains a large proportion of old individuals.
Age pyramid is a model in which the numbers or proportions of individuals in various age groups at any given time are geometrically presented. In an age pyramid, the number of pre-reproductive individuals is shown at the base that of reproductive age group in the middle and the number of post-reproductive individuals at the top.
The shape of age-pyramid changes with the change in the population age distribution over a period of time (Fig. 4.3). The age pyramid indicates whether a population is expanding or stable or diminishing and accordingly three hypothetical age pyramids have been suggested.
These are as follows:
(i) Pyramid with broad base:
This pyramid shows a high percentage of young individuals and an exponential growth of population due to high birth rate, as for example in yeast, housefly, Paramecium (Fig. 4.4A).
(ii) Bell-shaped pyramid:
This type of age pyramid shows a stationary or stable population having, more or less equal number of young and middle-aged individuals and post-reproductive individuals being the smallest in number (Fig. 4.4 B).
(iii) Pyramid with narrow base:
This is an um-shaped pyramid which shows increased numbers of middle aged and old organisms as compared to young ones in the population. It is indicative of contracting or diminishing population (Fig. 4.4 C).
6. Population Fluctuations:
The size and density of natural population show a changing pattern over a period of time. This is called population fluctuation.
There are three types of variations in the pattern of population change:
When the population remains static over the years, it is said to be non-fluctuating.
The cyclic variations may be (i) seasonal, and (ii) annual. Sometimes seasonal changes occur in the population and there are additions to the population at the time of maximum reproduction and losses under adverse climatic conditions. Common examples of seasonal variations are met in mosquitoes and houseflies which are abundant in particular season and so also the weeds in the field during the rainy season. When the population of a species shows regular ups and downs over the years, it is called annual cyclic variation. It appears in the form of a sigmoid curve with regular drops in population after peaks.
When the change in population density does not occur at regular intervals or in response to any obvious environmental factor, it is said to be irruptive fluctuation. In this there is a sudden exponential or logarithmic increase in population density in short time followed by equally quick drop in population density due to deaths, and final return to normal level or even below that level.