4.2.3 Practical: Investigating Biodiversity Using Sampling

Question 1

Method for choosing sample sites

Answer 1

• Mark area being studied on a map
• Form a grid with coordinates
• Use a random number generator to choose a set of coordinates
• This avoids sample bias, sample better represents the area

Question 2

What information can you gather from counting organisms within a randomly thrown quadrat?

Answer 2

• Population density
• Percentage cover
• Species frequency

Question 3

When do we estimate the percentage cover?

Answer 3

• When number of individuals within a quadrat is difficult to count
• So if organisms are present in 89/100 squares in the quadrat, then the % cover is 89%

Question 4

Limitations of using quadrats

Answer 4

• Only for small plants or slow moving species
• Some species too small or hard to count
• High mean % cover but low frequency indicates species lives in groups in preferred areas of habitat, answers questions such as, does species prefer light, shade, wet or dr areas, etc

Question 5

What is the mark-release-capture method?

Answer 5

• For mobile animals
• First large sample taken
• Organisms marked in a way that does not affect their survival
• Returned to habitat
• Allowed to mix with population
• After given amount of time, another large sample is taken
• Number of marked and unmarked individuals within sample are counted
• Proportion of marked : unmarked is used to estimate the population size

Assumptions
- Marked individuals given enough time to disperse and mix fully with the main population
- Marking doesn’t affect survival rates
- Marking is visible and doesn’t rub off
- Population stays the same size (no reproduction and hopefully little death)