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Population Density (Abundance)The success of a population is reflected in its
density i.e. number of individuals per unit areaor volume.
The density of a population may be studied-
i. In a given geographic region and termed as the average or
regional density. A region, however, includes unfavorablehabitats from which the species may be absent
ii. In a suitable niche in which it is populous and called aseconomic or niche density. It is always than its regionaldensity.
The density of a population may-5Increase (recruitment rate>mortality rate)
5Decrease (recruitment rate
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Population Growth
(Logistic & Exponential )Population growth is the capacity for increase in
individual numbers. At first there is a slow in number ofindividuals, which is successively followed by anincrease rapid rate, a gradual slowing down, and finally,a stabilization of population size takes place.
Logistic Growth: When a population is allowed to grow in alimited environment it shows logistic growth and accordinglythe growth curve SigmoidorS-shaped.
Exponential (Geometric) Growth: When a population is
allowed to grow in an unlimited environment, it showsgeometric orexponential growth.In 1st case growth curve differs from 2nd in 2-ways-
i. It has an upper asymptote (i.e. curve does not exceed a certain level) and
ii. It approaches this asymptote smoothly, not abruptly
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Sigmoid Population Growth Curve
It consists of 4-phases:
1st phase having slow rate of growth and being considered as lag periodand is followed by accelerating stage of multiplication.
2nd phase shows extremely rapid population increase at an exponential or
logarithmic rate. During this phase number of individuals increase in
geometric manner [2,4,8,16,32] instead of an arithmetic manner
[2,4,6,8,10].3rd phase shows the population increase begins to slow down.
4th phase is the phase of stability
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Why does a population not maintain itslogarithmic phase of its growth for a
long period of time?The reason is the environmental resistance. In other
words, the various physical, chemical and biotic factors
of the environment and biological characteristics of
species itself are incapable to support a maximum rate
of reproduction for over a long period of time [hydro-
biological conditions, competition, parasitism,
predation].
Thus the equilibrium phase is the limit at which theenvironment can support to certain number of individuals to
grow & reproduce optimally and accordingly this limit is
referred as carrying capacity of the environment. It is
represented by K.
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J-shaped Population Growth Curve
It happens under un-
stabilized environmental
conditions.
In such cases a populationcontinues increasing at
any accelerating rate and
instead of leveling off,
precipitously decreasesits rate to zero.
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Age Structure of a Population
3-major age groups exist in a population-
A. In rapidly growing/expanding population, birth rate is high and poplngrowth my be exponential & thus each successive will be more
numerous than the preceding one and as a result a pyramid structure
develop.
B. In a stabilizing popln pre-reproductive and reproductive age groups
become equal and post-reproductive group remains as the smallest,
thus, give a bell shape .C. If the birth rate is drastically reduced, the rproductive and post-
reproductive groups would increase in proportion to the pre-
reproductive, resulting in an urn-shaped age structure which gives the
reflecton of diminishing popln growth.
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Regulation of Population Size
By 2-category of factors-
i. Density-Dependent Regulatory factors
ii. Density-independent Regulatory factors
i. Density-Dependent Regulatory factors: Their intensity increases/decreases in
relation to popln density. They are biotic and depend on intra-specific/ inter-
specific conditions. This concept of population density dependent factors
indicates that all populations are regulated automatically.
Accordingly-
As the population density increases, the birth-rate (b) falls under the
influence of a DDFs, while it remains constant with a DIFs.
Similarly, mortality (d) rises under the influence of DDFs, while it remains
constant with a DIFs.
A stable can be achieved only at the point where a rising mortality (d)
intersects a falling natality (b) provided DDFs are operating. Thus where b &d intersect is the population size (N) at the carrying capacity (K) under the
specified conditions.
If the mortality and natality are controlled by DDFs, and the birth rate
exceeds the death rate, the population must grow exponentially under
favorable conditiond.
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Role of DDFs & DIFs on Population Growth
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Critical Operating DDFs-
i. Competition [ecological, spatial,biological resulting predation &
dispersal and low reproductive
performance].ii. Reproductivity
iii. Territorial behavior
iv. Dispersion [i.e. emigration &immigration]
v. Physiological stress & diseases
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Critical Operating DIFs-The level at which populations become stabilized is
determined by such factors as space, prevailing
environmental conditions, food supply, pollution,
all are abiotic factors.
5Accordingly, DIFs set the ultimate limit or carrying capacity
of habitat of species, whereas, DDFs stabilize populations
at or some what below that ultimate level.5Accordingly, both, the biotic (DDFs) and abiotic/climatic
(DIFs) are valid but differently in different habitats-
i. Under favorable environmental conditions to the
species, number will change because of DDFs. This
happens in permanent habitats.ii. Under fluctuating (unfavorable) environmental
conditions to the species number will change because of
DIFs. This happens at the edge of species distribution or
in a temporary habitats.
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Theories of Population Regulation-The question as what stops population growth and what determines
average abundance have been answered by Krebs (1972) on the
basis of following theries:i. Biotic School: According to Nikolson (1933) the only factor that control
population is some kind of competition [competition for food, space or
copetition of either predators or parasites].
ii. Climate School: Accordingly to Bodenheimer (1928) suggested that
even abiotic factors being DIFs may change in response of
population density and thus work as ultimate factor of populationregulation.
iii. Comprehensive School: According to Thompson (129) a network of
interactions of biotic & abiotic factors termed as Grandocoen is
responsible to regute a population size.
iv. Self-regulatory School: According to Chittay (1960) all species arecapable to regulate their population size without destroying
renewable resources of their habitat by action of intrinsic factors.
Under this concept, population size is kept under control because of
intra-specific differences in behavioral and physiological responses of
different species co-existing all together.
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Indicator of fluctuating populations-
Various criteria, such as changes in age composition in the catches,shifts in areas of fishing, changes in catches etc. may indicatenature and extent of fluctuation in abundance. The following act
as good indicators:1) Sustained decrease in age and size of the individuals indicate that the
population is regressing.
2) Sustained stunting of the individuals is a sign of expanding population.
3) Sustained changes in the species composition of the catches is regardedas indication of fluctuating populations. The relative numbers of thedifferent species in the catches, by the change in proportion. Wouldindicate which population is regressing.
4) Sustained decline in the catch per unit effort is a reliable indication ofregressing population.
5) The necessity to shift the fishing grounds, in view of the falling numbersin catches, is an indicator of the declining abundance in thepopulation.
6) For schooling pelagic species, contraction of the area in which the
7) For schooling pelagic species, contraction of the area in which thebuoyant eggs and larvae are collected, i.e. shrinkage of the spawningareas is an indication of the declining adult abundance.
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Apparent Indicator of fluctuating populations-Fluctuations in the Catches:
Serious economic handicaps are felt in the fishing industry
owing to large flocculation in the catches particularly inthe event of severe decline in tile catch front the previousyear(s). These fluctuations may arise from very differentand unrelated factors operating.
Three main kinds of these are generally accepted-
The first is the fluctuation in the size of the population, as discussed above.The second one is the variation in the availability of fish to the fishing effort
on account of the movement of the fish showing changes at varioustimes of the day or night, or during different seasons of a year orunder the influence of the migratory urge. If the direction or the pathof migration undergoes deviation, catches decline considerably,unless this was detected by modern devices like use of echosounders aero places or electronic transmitters for following the routeof migration.
The third factor is tile difference in the intensity of fishing.
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Carrying Capacity
Migration
(emigration / immigration)
Density Dependent
Mortality
Density DependentNatility
Density Dependent
Population Growth
Ecological Conditions
Density Independent
Exploitation
Density Independent
Sustainability of Stock