I MSc II Semester - Characteristics of a population.ppt

MSc Ecology - II Semester

1. Characteristics
of a population

2. Themostimportantcharacteristicsof
populationare,
1.Populationdensity
2.Natality
3.Mortality
4.Populationgrowth
5.Agedistributionofpopulation
6.Populationfluctuations.

3. 1.PopulationDensity
•Populationdensityreferstothesizeofanypopulationin
relationtosomeunitofspace.
•Itisexpressedintermsofthenumberofindividualsor
biomassperunitareaorvolume(500teaktreesperhectare;
40lionsper100km
2
,5milliondiatomspercubicmeterof
water).
•Populationdensityisseldomstaticanditchangeswith
timeandspace.

4. Populationsizecanbemeasuredby,
(i)Abundance
Absolutenumberofindividualsinpopulation.
(ii)NumericalDensity
Numberofindividualsperunitareaorvolume.Itisexpressedwhenthe
sizeofindividualsinthepopulationisrelativelyuniform,asinmammals,
insectsandbirds.
(iii)BiomassDensity
Biomassdensityisexpressedintermsofwetweight,dryweight,volume,
andcarbonandnitrogenweightperunitareaorvolume.

5. Populationdensitycanbeexpressedas,
(i)CrudePopulationDensity
Whenthedensityisexpressedwithreferencetototalareaata
particulartime.
(ii)EcologicalDensity
Whenthedensityisexpressedwithreferencetototalareaof
habitatavailabletothespecies.

6. Population density can be calculated by,
D = n/a
t
Where, Dispopulationdensity
nisthenumberofindividuals
aisareaandtisunittime.
Densityofhumanpopulationcanbeobtainedbydividingthetotal
numberofpersonsintheareabythetotallandareaoftheregion.

7. Density of population of a country can be obtained by
dividing the total number of persons living in the given
region by total land area of that region.

8. 2.Natality
Natalityreferstotherateofreproductionorbirthper
unittime.
Itisanexpressionoftheproductionofnewindividuals
inthepopulationbybirth,hatching,germinationor
fission.

9. BirthrateorNatality(B)=Numberofbirthsperunit
time/Averagepopulation.
Themaximum numberofbirthsproducedper
individualunderidealconditionsofenvironmentis
calledpotentialnatality.
Itisalsocalledreproductiveorbioticpotential,
absolutenatalityormaximumnatality.

10. 3.Mortality
Mortalityreferstothenumberofdeathsinpopulation
perunittime.
Mortalityrate=D/twhereDisthenumberofdeathsin
thetimet.

11. Mortalitycanbeexpressedas,
(i)MinimumorSpecificorPotentialMortality
Itrepresentstheminimumoftheoreticallossofindividualsunderidealor
non-limitingcondition.
(ii)EcologicalorRealizedMortality
Itreferstothedeathofindividualsofapopulationunderexisting
environmentalconditions.
Themaximummortalityoccursattheegg,larval,seedlingandoldage.
Mortalityisaffectedbyanumberoffactors,suchas,density,competition,
disease,predationandenvironment.

12. Abirthdeathratio(Births/deathx100)iscalledvital
index.
Forapopulation,thesurvivalofindividualsismore
importantthanthedeath.
Thenumberofbirthsinrelationtothecarrying
capacityofthehabitatisafundamentalfactor
influencingthemortalityrate.

13. 4.PopulationGrowth
Thegrowthisoneofthedynamicfeaturesofspeciespopulation.
Populationsizeincreasesinacharacteristicway.Whenthe
numberofindividualsofpopulationisplottedonthey-axisand
thetimesonthex-axis,acurveisobtainedthatindicatesthetrend
inthegrowthofpopulationsizeinagiventime.Thiscurveis
calledpopulationgrowthcurve.

14. There are two types of growth curves:
(i) Sigmoid Curve:
When a few organisms are introduced in an area, the
population increase is very slow in the beginning
(positive acceleration phase or lag phase), in the middle
phase, the population increase becomes very rapid
(logarithmic phase) and finally in the last phase
population increase is slowed down (negative
acceleration phase) until an equilibrium is attained

15. The level beyond which no major increase can occur is
referred to as saturation level or carrying capacity. In
the last phase the new organisms are almost equal to
the number of dying individuals and thus there is no
increase in population size.

16. (ii) J-Shaped Curve:
The second type of growth curve is J-shaped. Here in
the first phase there is no increase in population size
because it needs some time for adjustment in the new
environment. Soon after the population is established in
the new environment, it starts multiplying rapidly. This
increase in population is continued till large amount of
food materials exist in the habitat. After some time, due
to increase in population size, food supply in the habitat
becomes limited which ultimately results in decrease in
population size. This will result in J-shaped growth
curve

18. 5. Age Distribution:
Age distribution is another important characteristic of
population which influences natality and mortality.
Mortality, usually varies with age, as chances of death
are more in early and later periods of life span.
Similarly, natality is restricted to certain age groups.
Age Pyramids:
Age pyramid is a model in which the numbers or
proportions of individuals in various age groups at any
given time are geometrically presented.

19. The age pyramid indicates whether a population is
expanding or stable or diminishing and accordingly
three hypothetical age pyramids have been suggested.
(i) Pyramid with broad base:
This pyramid shows a high percentage of young
individuals and an exponential growth of population due
to high birth rate.
Eg: yeast, housefly, Paramecium

20. (ii) Bell-shaped pyramid:
This type of age pyramid shows a stationary or stable
population having, more or less equal number of young
and middle-aged individuals and post-reproductive
individuals being the smallest in number

21. (iii) Pyramid with narrow base:
This is an um-shaped pyramid which shows increased
numbers of middle aged and old organisms as
compared to young ones in the population. It is
indicative of contracting or diminishing population

22. 6. Population Fluctuations:
The size and density of natural population show a
changing pattern over a period of time. This is called
population fluctuation.
There are three types of variations
(i) Non-fluctuating:
When the population remains static over the years, it is
said to be non-fluctuating.

23. (ii) Cyclic:
The cyclic variations may be (i) seasonal, and (ii)
annual. Sometimes seasonal changes occur in the
population and there are additions to the population at
the time of maximum reproduction and losses under
adverse climatic conditions.
(iii) Irruptive:
When the change in population density does not occur
at regular intervals or in response to any obvious
environmental factor, it is said to be irruptive fluctuation.