Ecology of Population
For every ecosystem, the population depends on two factors viz. 1.Biotic potential 2.Environmental resistance. Biotic potential is nothing but an ability of particular species to reproduce at certain rate. Environmental resistance means the death of the species.
Characteristics of population growth:
To gain an understanding of interaction of biotic potential and environmental resistance we consider growth of yeast cells. If the data of time and number of individual present are plotted and the points are joined, the growth of population can be seen to be S shaped or sigmoidal. Growth rate of curve is representation of incremental increase in individuals with time. The two major components of growth curve are the period of increase and the period of equilibrium.
Carrying capacity of the environment is the maximum number of individuals that the environment can support. If the environment is shifted, different carrying capacity may result. Environmental shifts may be in the form of a chemical change such as removing toxic wastes, or a physical change such as an alteration of temperature.
Birth tends to push the growth curve up and mortality pushes it down. When they get balanced, equilibrium is attained.
In some cases, sudden deaths may bring the curve to J shape. E.g. insects.
Law of population growth:
The rate of change of population is directly proportional to environmental resistance. Mathematically,
ΔN /Δt œ (K – N)/ K
ΔN /Δt = (r N)[(K – N)] / K
Where,
N = Current population in the ecosystem.
K = Maximum population in the ecosystem.
rN = Biotic potential (proportionality constant).
(K – N)/ K = Environmental resistance.
r = Reproductive capacity of each individual
Biotic potential:
The increase in potential of population at any moment is dependent on both r, reproductive potential of each individual and N the number of individual present at the time and is expressed as ΔN/Δt = rN. in unlimited situations. i.e. the rate of change in the population is proportional to the number of individuals (e.g. females in sexually reproducing species)
Reproductive capacity is dependant on – Environmental conditions, age of individuals. If favorable conditions prevail i.e. environmental resistance is zero, the biotic potential will be higher. The population growth so observed is call as intrinsic rate of natural increase.
At equilibrium, birth rate = mortality rate i.e. ΔN / Δt = 0
The population remains constant through out the equilibrium period.
Mean life expectancy:
It is the point at which one half of the population is still surviving. It is much closer to the maximum duration of life.
Age structure:
Age structure is different for different species. For human beings, there are three age groups –
1. Pre- reproductive group (up to 15 years)
2. Reproductive group (15 to 45 years)
3. Post reproductive rate (45 years onwards)
In expanding population age structure will take a shape of pyramid. In stable population, pre reproductive and reproductive groups are more or less equal and post reproductive group remains at smallest. In diminishing population, pre- reproductive group is drastically reduced and it takes urn shape.
Population Dynamics:
Each population has the inherent power to grow. Growth or distribution of species in a particular area may be limited and depends on various factors for which organisms complete as space, light, CO2. Nutrients and water, competition may be due to characteristics of organisms or interactions with external factors as temperature, humidity, wind etc. When the environment is unlimited (space, food, other organisms not exerting a limiting effect), the specific growth rate (i.e. the population growth rate per individuals) becomes constant and maximum for the existing conditions.
‘S’ Shaped or Sigmoid growth form:
In this form the population increases slowly at first the acceleration phase, then more rapidly approaching a logarithmic phase but soon slow down gradually as the environmental resistance increases negative acceleration phase a more or less equilibrium level is reached and maintained the upper level beyond which no major increase can occur is the upper a asymptote of the sigmoid curve. It is also refract as the carrying capacity of a population which is maximum number of individuals that can be supported in a given hebetate. Once the size reaches the carrying capacity level its size fluctuates around it. In some cases however it may be followed by a population ‘crash’ before the size of populating with the environmental condition.
‘J’ Shaped curve:
Here the population density increases rapidly in exponential or compound interest fachion and then stops abruptly due to environmental resistance or another limit becoming effective more or less suddenly.
