1. Population: A group of individuals… … of the same species…
… living in a certain defined area.
Question:
How do you measure the size of a population?

2. Plants and sedentary animals are easy to count…1 3 4 2 2 1 3 4

3. Large animals which are present in small numbers are also easy to count…1

4. Small animals which move about a lot are more difficult to count…
… especially if there are large numbers of them!
Scientists have to find a way of estimating the size of the population.

5. The Lincoln Index
Since it is impossible to count every single individual in a large population, a method of marking the animals is used.
We call this a Mark-Release-Recapture method:
Animals are captured, marked and released, then…
A second sample of animals is captured, and the marked animals recorded.
We can estimate the size of the population from the number of recaptured (marked) animals

6. 1. Capture a large number of animals
The Practicalities
1. Capture a large number of animals
The method of capture should not cause the animals any undue stress which might make them more vulnerable…
Example 1:
Shoot them…
… with a tranquilliser dart

7. Example 2:
Pitfall traps

8. Example 3:
For aquatic animals…
… netting

9. 2. Mark the animals
…or make it more obvious to predators
e.g. with non-toxic paint
It is important that the manner of the marking does not affect the behaviour of the animal…

10. 3. Release the animals back into their environment

11. Animals go about their daily business…
Avoiding
Ecologists
…and generally mix with other animals

12. Avoiding
Ecologists
4. A couple of days later…
… capture a new sample

13. 5. Estimate the population size using the Lincoln index
Avoiding
Ecologists
Lincoln
Index =
Number marked in second sample
Total caught in second sample
Total marked in whole population
Size of whole population
Remember: Total marked in whole population is the same as the size of the first sample!
Working out population size, x x =
Size of First sample
Size of Second sample ×
Number marked in second sample

14. Number marked in second sample8
For our example…
Size of first sample =
Avoiding
Ecologists 9
Size of second sample =
Number marked in second sample = 3
Size of Second sample x =
Size of First sample ×
Number marked in second sample
8 × 9 3
= 24

15. How accurate is our estimate?
Avoiding
Ecologists
How accurate is our estimate?
Estimates are more accurate if:
1. there is enough time between samples to allow animals to mix with rest
2. no animals migrate into or out of the population
PERU
3. the procedure is repeated several times
(For each repeat, animals should be marked with a different colour or on different parts of their bodies)

16. IB students need to do the Lincoln index as a practical
Small furry animals are protected by law
The law is less fussy about experimenting on small children!
If you are a year 7 pupil watching this presentation, you are part of the first sample…
… and you are about to be tagged.
At some point during the day, you may be recaptured!