Thomas
Park, an ecologist, states that statistical functions define group
characteristics of a population which are not applicable on unitary unit called
organism.
The
group characteristics of population such as growth forms, age distribution,
density, dispersion, dispersal, mortality and biotic potentials are explained
below.
1.
Population
Density
In
a given population at a particular time the total number of organisms per unit
area represents the Population Density. The unit of population density differs
a sin case of number of organism per unit area (in Kilometres) = N/Km2,
and in case of number of organisms per unit volume (in Litres) = N/L
Population
density has defined limits maintained by certain homeostasis mechanisms instead
of which the characteristic varies according to the climate change, season and
even by food supply. The size and tropic levels highly influence the limit of
population density. The smaller the organism, the higher the density of
organism in population is expected per unit area.
It also depends on the
productivity of the population, for example the higher the productivity or
fecundity rate of population the higher the expected density. Energy flow
utilization and resource availability influence the population density, higher
the resources available i.e. more resources to sustain the demands of
population, higher will be the density.
Dispersal
and physiological stress also have higher impact on the population density. The
higher the physiological stress the lower population density expected. Other
important factors are emigration, and mortality leading to decrease in
population density while immigration and natality leads to increase in
population density.
The
density of population is influenced by several factors such as energy flow,
dispersal, resource availability, stress etc. and categorized into- Specific/
Ecological density and Crude density.
i.
Ecological
density
Ecological
density is defined as the number of individuals per unit of available habitat
area. The individuals will colonize only if the prevailing conditions are
favourable on the land or area. So, out of the geographical area of population
only that part of the land is utilized by the individuals where the resource
availability is high along with the other optimum conditions and it is denned
as ecological or specific density.
ii.
Crude
density
The
total number of individuals per unit of total land area or volume. For example
total number of tigers in Jim Corbett National Park. The crude density vary due
to non uniform distribution of individuals on the total land area as the
habitat conditions are not favourable over the geographical range of
population. Only at the places with favourable conditions the density is high.
In
1964, Karl revealed that the two densities crude and ecological vary according
to the number of fishes, season and depth of water in the pond. He observed
that as the summer season reaches the depth of water steep down leading to
decrease in the total geographical area hence decrease in the crude density (as
it depends on geographical area) while at the same time the ecological density
increases as by now the fishes are more crowded in pond due to low depth of
water, thus the density of fishes is high in that suitable area leading to high
specific/ecological density. At the time of winters, the depth of water is
high, thus, the total geographical areas increase and hence, the crude density
also increases but the ecological density decreases as by now the fishes are
more randomly distributed in the total volume of pond.
Thus, the two densities
are inversely proportional to each other depending on the climatic/seasonal
variations.
Methods
of measuring Population density
There
are three different techniques of measuring population density:
a. Arithmetic density: or
real density. It’s the number of individuals divided by total land.
b. Physiological density: is
the number of individual in its suitable area.
c. Agricultural density: the
number of people involved in agriculture (agriculturist) divided by arable
area.
d. Residential density: the
number of individuals divided by the residential area
e. Urban density: number
of individuals living in a urban area divided by total urban area.
f. Ecological optimum: the
number of individuals supported by the available natural resources.
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