Features of Population are 1. Population size and density 2. Natality (Birth rate) 3. Mortality (Death rate) 4. Age Structure 5. Life tables 6. Biotic potential 7. Survivorship curves 8. Population growth curve 9. Population dispersion.
1. Population Density:
Population density is population size in relation to some unit of space and time. It can be measured in a number of ways:
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(a) Numerical density (Number of individuals per unit area or volume).
(b) Bio-mass density (bio-mass per unity area or volume) Density of population can refer to the total area (crude density) or the actual area of the habitat (ecological density)
It can be calculated by applying the formula: D = n/a/t Where ‘D’ is population density, ‘n’ is the number of individuals, ‘a’ is area and’t’ is time. Population density depends upon weather condition, availability of food, natality, mortality etc.
2. Natality (Birth Rate):
Production of new individuals of any organism is known as natality. In human population, natality means birth rate in unit time. But natality in ecology includes the new individuals born, hatched, germinated etc. Natality can be estimated by the following formula.
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Natatily =Total number of birth per unit time natality = average population
Natality may be (a) maximum natality; or (b) ecological natality
Maximum natality:
This is known as fecundity or absolute potential natality. It is the maximum production of new individuals under ideal conditions. There are no ecologic limiting factors except physiological factors. Maximum natality is a constant far given population.
Ecological natality:
This is known as realized natality. This relates to population increase under actual, existing and specific conditions. This is also known as fertility rate.
3. Mortality (Death Rate):
Mortality may be defined as the number of individuals those die in a population in a given period of time. It relates to the death of individuals.
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Mortality may be (a) minimum mortality, and (b) ecological or realized mortality.
Minimum mortality:
It is also known as specific or potential mortality. Under ideal or non-limiting conditions, the minimum loss is expressed as minimum mortality.
Ecological mortality:
Under given environmental conditions, the actual loss of the individuals is known as ecological mortality. Mortality is expressed as the number of individuals dying in a period:
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Mortality = Total number of death per unit time /Average population
4. Age Structure:
In a population, there are individuals of different age groups. The relative proportion of individuals pertaining to different age groups of population is called age structure of age distribution. Bodenheimer (1938) has identified three ecological age groups in a population which may be outlined below:
(i) Pre-productive (Juveniles): Up to age 14 years in case of human beings.
(ii) Reproductive: Up to age 15-55 yrs.
(iii) Post-reproductive: Above 55 yrs.
Considering in terms of population density, it is seen that the ^reproductive age group is expanding, the reproductive age 9roup is stationary and the post reproductive group is diminishing. The above facts are well marked in the form of age pyramids.
In all these pyramids, the pre reproductive are shown at the base, the reproductive at the middle and post reproductive at the top.
There are three types of age pyramids:
(a) Expanding age pyramid:
Since expanding population has more individuals of pre-reproductive age, the age pyramid is triangular having an expanded base. (Fig. 13.1a)
(b) Stationary age pyramid:
Since the relative proportion of individuals of different age groups is approximately the same, the stationary age pyramid is almost bell shaped. (Fig. 13.1b)
(c) Diminishing age pyramid:
Since post-productive are dominating, the diminishing age pyramid is urn-shaped. (Fig. 13.1c)
6. Biotic Potential:
Under all favourable conditions, the growth rate per individual in the population increase and reaches a maximum value which remains constant with time. The growth rate or reproductive capacity in a population of stable age distribution is known as intrinsic rate of natural increase.
The maximum value of this intrinsic rate of natural increase is called biotic potential. In other words, the constant percent growth rate of a population under optimum environmental condition is known as its biotic or reproductive potential.
Mathematically, biotic potential is the difference in between maximum natality and maximum mortality.
Biotic potential = Maximum natality-Maximum mortality.
Factors like non availability of food, living space, competition among organisms, diseases etc. are responsible for decrease in biotic potential. That is, these factors cause environmental resistance (ER). The maximum number of individuals of a population that can be supported in a given area at a particular time is known as the carrying capacity of that population. The carrying capacity undergoes a change according to changes in the ecosystem.
7. Survivorship Curves:
Date Presented in a graph from a life table by expressing age of the individuals on the X-axis and the number of survivors on the Y-axis, is known as survivorship curve. This curve assumes three shapes:
(i) Convex Curve:
When mortality rate is high in a population in the early stages of life and is also high near the end of life span (e-g. man).
(ii) Concave Curve:
When in a population the mortality rate is high during the young stage (e.g. oyster, fish).
(iii) Diagonal Curve:
When the mortality rate in a population is constant at all ages (hydra).
8. Population Growth Curve:
An increase of decrease in population numbers over a unit period of time is known as growth curve. Growth curves can be plotted by taking time in the X-axis and the number of organism on the Y-axis. There are two types of growth curves:
(a) S-shaped or sigmoid curve, (b) J-Shaped or growth curve (a) Sigmoid curve: The growth of population usually follows S shaped curve in which population increases slowly at first and grows rapidly in the next stage and lastly the growth rate slows down until an equilibrium in reached.
Human population and those of Drosophila or rabbit in laboratory conditions show S-shaped growth.
S-shaped curve can be divided into the following phases:
(i) Positive acceleration phase or lag phase:
This phase refers to initial slow growth phase.
(ii) Exponential or Logarithmic phase:
This phase refers to rapid growth phase which continues up to a particular point known as inflection point.
(iii) Negative acceleration phase:
After this point, there is a steady decrease in the growth rate due to environmental resistance. This stage is called negative acceleration phase. This phase may not be prominent in most of the cases.
(iv) Stabilisation phase:
This phase relates to maximum population limit (limiting size of the population)
9. Population Dispersion:
The movement of individuals in or out of the habitat is known as population dispersion.
Population dispersion may be of the following types:
(i) Emigration:
It may be defined as one way outward movement of individuals in order to lower overcrowding of the population of an area. It is regared as an adaptive behaviour that regulates the population in a particular habitat and avoids its exploitation. It also provides opportunities for inter breeding with other populations leading to genetic diversity and adaptability.
(ii) Immigration:
It may be defined as one way inward movement. It is opposite to emigration. It causes over population followed by high mortality among immigrants. It reduces reproductive capacity of the individuals.
(iii) Migration:
It is the periodic departure and return of the Population migratory movements and these are usually controlled by circadian, lunar and tidal rhythms as well as seasonal vibrations. Some possible reasons for migration are requirement for food, shelter or reproduction. It utilizes unexploited habitats and resources. It also protects the individuals from intraspecific competition. However it may cause mortality during dispersion.