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?

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

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

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.

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

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

Example 2: Pitfall traps

Example 3: For aquatic animals… … netting

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…

3. Release the animals back into their environment

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

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

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

Number marked in second sample 8 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

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)

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!