Understanding:
• Sampling techniques are used to estimate population size
A population is all the individuals of a given species living in the same area at the same time
- Populations are fluid and are subject to continual change in numbers through natality, mortality, immigration and emigration
- Populations may be large and impractical to count unless the species is large and the target area is small
Population sampling involves identifying individual numbers in small areas and then extrapolating to estimate population totals
- Sampled areas must be chosen randomly to avoid selection bias causing a misrepresentation of the population size
- The more samples that are taken (and the larger the sampling area), the more accurate population estimates are likely to be
Different sampling techniques are used to estimate population sizes for non-motile (sessile) and motile species
- Non-motile species can be sampled using quadrats (measurements can include direct counts, percentage cover or frequency)
- Motile species can be sampled using the capture-mark-release-recapture method (with estimates based on the Lincoln index)
Application:
• Use of the capture-mark-release-recapture method to estimate the population size of an animal species
Capture-Mark-Release-Recapture
The capture-mark-release recapture method is a means of estimating the population size of a motile species
- An area is defined and marked off, then a selection of individuals are captured, counted, marked and released (n1)
- Marking must not be easily removable or adversely affect the animal’s survival prospects
- After sufficient time has passed to allow marked individuals to reintegrate in the population, a second capture is made (n2)
- In this second capture, both unmarked individuals and marked individuals (n3) are counted
- Based on the three values generated (n1 ; n2 ; n3), an estimated population size is derived using the Lincoln Index
The Capture-Mark-Release-Recapture Method
Lincoln Index
The Lincoln index is used to estimate population size based on the capture-mark-release-recapture method
- Lincoln Index: Estimated Population = (n1 × n2) ÷ n3
The Lincoln index requires that the following assumptions are true:
- That all individuals in a given area have an equal chance of being captured (sampling must be random)
- That marked individuals will be randomly distributed after release (n1 cannot be allowed to influence n3)
- That marking individuals will not affect the mortality or natality of the population
The accuracy of the Lincoln index can be improved by a number of means:
- Increasing the size of the capture samples (larger samples will be more representative but also more difficult to collect)
- Taking repeated samples in order to determine a statistical average
Lincoln Index Equation:
Practice Question
The following data was obtained when monitoring mountain gorilla numbers in a nature reserve: