Population
Ecology
Some significant attributes that populations possess are – birth
rate / natality, population density, death rate/ mortality, sex ratio, age
distribution.
Birth rate or natality refers to the average number of young
ones produced by birth, hatching or germination per unit time (usually per
year). In the case of humans, it is commonly expressed as the number of births
per 1000 individuals in the population per year.
The maximum birth rate that a species
can achieve under ideal environmental conditions called potential natality. However,
the actual birth rate under the existing conditions is much less. It is termed
as realized natality.
Death rate or mortality is the average number of individuals
that die per unit time (usually per year). In humans, it is commonly expressed
as the number of deaths per 1000 persons in a population per year. Lowest death
rate for a given species in most favourable conditions is called potential
mortality, while the actual death rate being observed in existing
conditions is called realized mortality.
The death rate of a population can be easily represented by
survivorship curve. In this curve, time is plotted against the number of
survivors.
There are three kinds of survivorship
curves–
Diagonal curve :If the death rate of different age groups of organisms are equal,
then the curve is represented or plotted as a straight line.
e.g., Hydra, mice and many adult birds.
Convex curve : When organisms have completed their potential life span and died
in old age then the curve is convex, the curves goes horizontal till potential
life span and then declines rapidly. e.g., Man, rabbit and many mammals.
Concave curve :This kind of curve is mostly found in such organisms who die
before their potential life span. e.g., Fish, Oysters and Invertebrates.
Sex ratio is the ratio of males to females in a population.
Age distribution : Various age groups in a population determine its reproductive
status. The three ages referred to as ecological ages in a population are – pre-reproductive,
reproductive and post-reproductive. Population with more young
members grow rapidly, while the declining populations have a large proportion
of older individuals.
Representation of age pyramids for human population
Population density is the number of individuals present per unit area or volume at a
given time. For instance, the number of animals per square kilometer, number of
trees per area in a forest, or number of planktonic organisms per cubic meter
of water.
The population density is determined by four basic processes-
1. Natality
2. Mortality
3. Immigration
4.
Emigration.
The percentage ratio of natality over mortality is known as vital
index i.e.,
Natality / mortality × 100. It determines the growth of a population.
Immigration is the number of individuals of the
same species that have come into the habitat from elsewhere during the time
period under consideration.
Emigration is the number of
individuals of the population who left the habitat and gone elsewhere during
the time period under consideration.
Change in population size during time interval
= (Birth + Immigration during
time interval) – (Death + Emigration during time interval)
The above expression in words may be
represented in a simple way by a mathematical model.
Suppose,
N = population size and
t = time.
The Greek letter delta, (Δ), indicates
change.
We can now represent change in
population as ΔN, and time interval as Δt.
The verbal equation can be written as
ΔN/ Δt = (B + I) – (
D + E )
in which,
B = absolute number of births in the
population during the time interval,
D = the absolute number of deaths
during that interval;
I = immigrants
E = emigrants.
I and E , being insignificant, may be
ignored. Then the equation simplifies to ΔN/ Δt =B – D.
MALTHUS
THEORY OF HUMAN POPULATION GROWTH
Thomas Malthus, a British political economist, put forward a
theory of human population growth in 1778. Malthus in his "Essay on
the principle of population" pointed out that population tends to increase
in geometric progression while food supply increases only in arithmetic
progression. Faster growth of population than of its requirements causes an
imbalance between the two. When this imbalance reaches a certain limit,
environmental factors like famine, epidemic of a disease, earthquake, flood,
war etc. reduce the population to a size, the available resources can support.
The factors that control the population size were called positive checks by
Malthus.
Growth of a population is controlled by an interaction
between three factors :biotic potential, environmental resistance
and carrying capacity of the environment.
Biotic or reproductive potential is the natural capacity of a population to increase at its
maximum rate under ideal environmental conditions and stable age and sex
ratios. Biotic potential in the human female is estimated to be about 12 per
female during its reproductive period between puberty and the menopause period.
Environmental resistance :In nature, full biotic potential of an organism or
population is never realized, since conditions are rarely ideal. Various
harmful environmental (abiotic) factors like non-availability of food and
shelter, natural calamities like drought, cloud bursts, floods, fires,
temperature fluctuations, accidents, etc. and certain biotic factors like
pathogens, parasites, predators etc. check the biotic potential from being
realized. The sum of all these inhibitory factors is called environmental
resistance.
Carrying capacity:is defined as the "feeding
capacity of an environment of an ecosystem for a population of a species under
provided set of conditions". When a population reaches the carrying
capacity of its environment, the population has zero growth rate. So the
population generally stabilizes around the carrying capacity. The carrying
capacity of the earth for human population is considered to be about 8 to15
billions.
The theoretical relationships between biotic potential, environmental resistance and carrying capacity
The populations are not stable and do change due to a number
of extrinsic as well as intrinsic factors.
GROWTH CURVES
Sigmoid or S-shaped growth curve
It is shown by yeast cells and most of
the organisms.
It is formed of five phases :
v Lag phase: In this phase, individuals adapt themselves to the new
environment, so there is no or very little increase in population.
v Positive acceleration phase: It is the period of slow increase in population in the
beginning.
v Logarithmic or exponential phase :It is the period of rapid rise in population due to the
availability of food and the requirements of life in plenty and no competition.
v Negative acceleration phase :In this again, there is a slow rise in population as the
environmental resistance increases.
v Stationary (Plateau) phase : Finally, growth rate becomes stable because mortality and
natality rates become equal to each other. So, there is zero growth rate.
A stable population is said to be in equilibrium, or at saturation level. This
limit in population is a constant (K) and is imposed by the carrying capacity
of the environment.
S-shaped curve is also called logistic curve. Sigmoid growth curve
was described by Verhulst, (1839).
Population exponential growth equation is given below :Nt = N0 e–rt
where,
Nt = Population density
after time t.
N0 = Population density at
time zero.
r = intrinsic rate of natural increase.
e = the base of natural logarithms
(2.71828).
The equation describes the exponential
or geometric growth pattern of a population and results in a J-shaped curve.
The J-shaped curve of exponential growth is characteristic of some population
that are introduced into a new or unfilled environment or whose numbers have
been drastically reduced by a catastrophic event and are rebounding.
J-shaped Growth curve
It is shown by small population of
Reindeer experimentally reared in a natural environment with plenty of food but
no predators. This type of population growth is also called Verhulst Pearl
Logistic growth.
It has only two phases :-
Lag phase : It is period of adaptation of animals to new environment so and
thus, is characterized by slow or no growth in population.
Logarithmic or Exponential phase :It is characterized by rapid growth in a population which continues till enough food is available. But with the increase in reindeer population, there is a corresponding decrease in the availability of food and space which finally becomes exhausted leading to mass starvation and mortality. This sudden increase in mortality is called population crash. Lemming of tundra, some insect, algal blooms and annual plants also show J-shaped curves. The population growth curve is S-shaped in most of the organisms. Human population also shows S-shaped curve.
Verhulst-Pearl Logistic Growth equation is given below–
Where, N = Population density at
time t.
r = Intrinsic rate of natural increase.
K = Carrying capacity
Fig. : The J- shaped growth curve of
Reindeer
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