1. LSFT0B2 – PRAC 3 2015
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LSFT0B2
PRACTICAL 3: DETERMING POPULATION SIZE
UNIT 3: POPULATION ECOLOGY
SURNAME:_____________________________ STUDENT NR. ___________________
Withinanecosystemthe organismsare rarelydistributedevenlyandtherefore itcanbe very
difficulttocounttheirnumbersaccurately.Scientiststherefore needtoadoptanappropriate
samplingtechnique toachieve arealisticestimate of populationnumbers.
One of the bestmethodstoestimate the populationsize of abundantspecieswhichdonotmove
arounda lotisthe quadratsamplingmethod.Thisisageneral methodusedtoestimate the
populationsize of plantspecies,several invertebrateslike,seashell organisms,somebeetlesfound
inopengrasslandsetc.
How thismethodworks:
 A few quadrats are constructed ina givenarea,usingline,washablepaint,wood,Perspexor
any constructive methodthatwill stayrigidthroughoutthe investigation. A quadratisa
samplingunitof knownareaand isusuallysquare frames.
 The size,surface areaand shape of the quadratsmustbe keptconstantthroughoutthe
investigation.A goodsize touse wheninvestigatingshell organismsinanintertidal zone is
30cm x 30cm quadrants(squares) coveringandareaof 1.5m x 1.5m. The size dependsvery
much onthe particularhabitatandtype of organismslikelytobe encountered.Plant
quadratsfor use on grasslandsare usually0.5mx 0.5m butlarger scale typesare employed
for largerecosystems.
 Simple randomsampling

2. LSFT0B2 – PRAC 3 2015
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 The area understudyis dividedupintoagridsystemof boxesandusingrandomnumbers
for the coordinatesanumberof boxescan be selectedrandomly,usually=/- 15% of the total
quadratsare chosene.g.if there are 100 quadratesthenchose 15 quadratsto workwithand
numberthem(canuse letterse.g.A-Oorthe coordinates1.5,1.8, 4.6 etc.)
A B
C
D
E
F
 Countthe amount of organismsof the specificspeciesyouhave chosen,andnote the results
on a table,suchas the one below.
Quadrat Number of specie A Number of specie B
A
B
C
D
E
F
TOTAL:
 Addthe total numberof eachspeciesseparatelyanddivide the answerbythe amountof
randomlychosenquadratstoobtainthe average numberof a speciesperquadrate.
Average numberof speciesA/quadrate = Total numberof specie A counted
Amountof random chosenquadrats
 Multiplythe average numberof speciesA bythe total amountof quadrats inthe area.
A B C D E F
1
2
3
4
5
6

3. LSFT0B2 – PRAC 3 2015
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PRACTICAL TASK
The diagram belowshowanareawith3 differentspeciesof flowers,indicatedby
Estimate the total numberof each speciesbyusingthe quadratsamplingmethod.
Constructa quadrate gridof 2cm x 2cm quadrats,a total area of 16cm x 16cm.