8/7/2019 FFB-311-L3,4 & 6(Popl.GCurve)

1/16

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

8/7/2019 FFB-311-L3,4 & 6(Popl.GCurve)

2/16

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

8/7/2019 FFB-311-L3,4 & 6(Popl.GCurve)

3/16

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

8/7/2019 FFB-311-L3,4 & 6(Popl.GCurve)

4/16

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.

8/7/2019 FFB-311-L3,4 & 6(Popl.GCurve)

5/16

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.

8/7/2019 FFB-311-L3,4 & 6(Popl.GCurve)

6/16

8/7/2019 FFB-311-L3,4 & 6(Popl.GCurve)

7/16

8/7/2019 FFB-311-L3,4 & 6(Popl.GCurve)

8/16

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.

8/7/2019 FFB-311-L3,4 & 6(Popl.GCurve)

9/16

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.

8/7/2019 FFB-311-L3,4 & 6(Popl.GCurve)

10/16

Role of DDFs & DIFs on Population Growth

8/7/2019 FFB-311-L3,4 & 6(Popl.GCurve)

11/16

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

8/7/2019 FFB-311-L3,4 & 6(Popl.GCurve)

12/16

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.

8/7/2019 FFB-311-L3,4 & 6(Popl.GCurve)

13/16

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.

8/7/2019 FFB-311-L3,4 & 6(Popl.GCurve)

14/16

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.

8/7/2019 FFB-311-L3,4 & 6(Popl.GCurve)

15/16

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.

8/7/2019 FFB-311-L3,4 & 6(Popl.GCurve)

16/16

Carrying Capacity

Migration

(emigration / immigration)

Density Dependent

Mortality

Density DependentNatility

Density Dependent

Population Growth

Ecological Conditions

Density Independent

Exploitation

Density Independent

Sustainability of Stock