- Population Density :
Population density is the number of individuals per unit area and often expressed as biomass/area or biomass/volume.
e.g. 100 people/square k.m. or 2000 euglena/m3 of water
The population density may vary according to food supply, environmental conditions and climatic variation but the upper limit and lower limit has definite number. In general the upper limit of density is influenced by size of the organisms and it’s trophic level, for example the upper limit density of specific crustaceans is more than that of deer population as the crustaceans size is more than that of deer,therefore forest ecosystems favour the increase of crustaceans than deer. Incase of lower limit however it is not well defined but a homeostatic mechanism helps to maintain the definite limit.
Some factors are there which influence or put impacts on population density are ; natality, mortality, immigration and emigration.
i) Natality :
Natality is defined as the birth rate of a population. In general we can say natality is the production of new individuals through hatching, germination or fission.
Types of natality :
a) Absolute / Physiological natality :
It is defined as the physiological ability of an organism to produce maximum numbers of individuals at a given optimum conditions. It is always considered for a given population. It is very different to achieved the absolute natality at wild population, because of the presence of unsuitable environmental conditions and when major limiting factors are temporarily absent which may usually hinders the growth or multiplication.
e.g. If a small number of paramecium placed in the culture medium under favourable condition ; they achieved maximum natality at maximum reproductive rate.
b) Ecological/ realised natality :
The name itself suggest that realised natality means the increase in population size under ecological or real environmental condition. Through various limiting factors acts upon population, therefore natality is never constant for a population but varies with age factor, composition, size and physical environmental condition.
Natality : ∆ Nn / ∆ t = Absolute or crude natality or birth rate.
Specific natality / Birth rate = ∆ Nn / N ∆t
∴ The number of new individuals per unit time per unit population.
Whereas ; N = Total population or the reproductive part of population.
Nn = New individuals added into the population.
∆t = time lapse or time taken during change in population.
Birth rate or Crude Natality = No. of birth / 1000 Individuals of population
Birth rate is inversely related to parental care.
ii) Mortality :
It is the rate of death of individuals in a population.
Types of Mortality :
There are two types of mortality can be recognised;
a) Ecological or realised mortality :
It is the number of death of an individuals under a given environmental condition. It is constant for a population because the environmental condition is ever changing according to time and presence of disease, predator etc.
b) Theoretical or Minimum mortality :
It is the number of death of an individuals under ideal condition or non- limiting condition, as the external environmental pressure is absent. It may be constant for a population. Even if the ideal condition is prevail still there is a loss of individuals because of old age i.e. related to physiological longevity. Minimum mortality is always greater than longevity.
Mortality is the number of death of indivisuals per thousand in a given period of time, but generally mortality is expressed interms of probability of dying or death rate. So the death rate can be calculated by no. of death during a given interval of time divided by average population.
e.g. Suppose earlier 500 individuals are there and at a given interval of time 400 indivisuals remaining and 100 death occurs then the death rate would be ;
No. of death = 100
Average of population = Initial Value + Final Value / 2
= (500+400) / 2 = 900/2
= 450
Death Rate = 100/450 = 0.22
Then probability of death also be calculated by no. of death divided by no. of individual at the beginning of period i.e.
Probability of dying ; = 100/500 = 0.2
The complimentary of probability of dying is probability of surviving, therefore we can also express probability of surviving as ;
Probability of surviving = No. of indivisual survive / No. of indivisual alive at the beginning
= 400/500 = 0.8
However, we may express mortality interms of the above mentioned way and also by life expectancy ( it is the average number of years to be lived in future by the members of a given age in the population ).
iii) Immigration :
Immigration is the no. of indivisuals of the same species that have came into the habitat from other habitat for a specific time period what ever reason may be.
iv) Emigration :
Emigration is the number of indivisuals of a population left the habitat and gone to another place for a given period of time for various reason and purposes.