Estimating the
number of beans in an opaque bag
Material : Kidney beans, opaque bag, specimen tube, marker pen
Method
1. About a
beakerful of kidney beans is put in an opaque bag. (The kidney beans represent
the population of animals.)
2. An empty
specimen tube is put into the bag filled with kidney beans and then taken
out. The kidney beans are placed in a
petri dish and counted. (This represents the first capture).
3. The
beans are marked with a marker pen and the returned to the bag.
4. The
marked and the unmarked beans are mixed thoroughly by hand.
5. An empty
specimen tube is put into the bag again, filled with beans and then taken
out. One must not look into the bag when
carrying out this step. (This represent
random recapturing).
6. The kidney
beans that have been scooped out are placed in a petri dish and counted. The
number of beans that are found marked is noted. (This represent the number of
animals found marked in the recapture).
Calculation
The number of
beans in the first scoop
|
X
|
The number of
beans in the second scoop
|
Y
|
The number of
beans found marked in the second scoop
|
Z
|
Therefore, an
estimation of the number of beans in the bag
|
X x Y
Z
|
Z
Note: The accuracy of the method can
be increased by
a) taking
out 2 scoops of beans each time instead of one
b) repeating
the experiment and finding the average of the results
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Discussion
1.
The material used to mark the organism is important. Why?
The substance used to mark the organism
should not expose the
organism to its predator, should not be
poisonous, should not affect the
activity of the organism, and yet not
easily removed
2.
What is the estimated population size of beans in the bag?
3.
What is the actual population size of the beans?
4.
Why is the estimated result different from the actual result?
Estimation only whereas the second method is
direct counting.
5.
Why is direct counting difficult in real life situation?
The organisms/animals move around, space/habitat
is too big, you
cannot capture them all, unlike the beans
which are all in the bag.
6.
How can the accuracy of the data be improved?
· Repeat
the experiment and find the average of the results.
· Mark a
greater number of individuals.
7.
State two assumptions made in this estimation method?
b. The
marked individuals can mix randomly in the population before the second sample
is taken
c. Marked
and unmarked organisms in second sample are caught randomly
d. The
substance used to mark the organism should not be poisonous or affect the
activity of the organism and yet not easily removed.
e. The
death rate and the birth rate are the same.
f. The
population to be estimated is stable/ the rate of the migration is equal to the
rate of emigration of the organism.
g. There
are no predators of the organism in the habitat to be studied.
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Question
1. A biologist
originally marked 40 butterflies in Taman Bukit Cerakah, Shah Alam using tags.
The marked butterflies were released back to where they were caught. After a
week, the butterfly traps caught 200 butterflies. Of those 200, 80 were found
to have tags. Based on this information, what is the estimated population size
of the butterflies in Taman Bukit Cerakah?
2.
A study of the garden snail population in a vegetable farm was carried
out. On the first trip, a total of 160
snail were caught, marked and released. A second trip was made after three
days. There were 40 marked snails out of a total of 180 caught. Estimate the
population of garden snails in the vegetable farm using the results obtained.
