Sampling

Question 1

Why might scientists use random sampling?

Answer 1

reduce/avoid bias
allows the use of statistical tests

Question 2

Describe how the population of non-motile organisms can be estimated

Answer 2

using quadrats

Question 3

How can the population size of motile organisms be estimated?

Answer 3

mark-release-recapture

Question 4

Outline the steps involved in mark-release-recapture

Answer 4

• capture and mark sample
• release back into habitat and leave time for redistribution
• recapture a sample
• count the number of marked individuals

Question 5

Give the equation used in the mark-release-recapture method

Answer 5

estimated pop. size = (initial capture * no. in 2nd sample) / no. of 2nd sample that were marked

Question 6

What assumptions are made in the mark-release-recapture method?

Answer 6

• enough time for even distribution
• no immigration/emigration
• birth rate = death rate
• no marks rub off/are lost
• the mark is not toxic/does not limit survival
• proportion in 2nd sample is equal to proportion in whole population

Question 7

Why is sampling used?

Answer 7

• time efficient
• can be more accurate if done correctly

Question 8

How can sampling be made more accurate?

Answer 8

• random used in uniform distribution
• line transects to examine change over distance
• large number of samples (30+)

Question 9

What are the 2 types of quadrats?

Answer 9

open or gridded

Question 10

What is the first step in random sampling?

Answer 10

• lie 2 tape measures at a right angle to create a gridded areas

Question 11

Describe how you would use random sampling to investigate population size

Answer 11

KEY POINTS
1) 2 tape measures, right angle, gridded area
2) Random number generator, co-ordinates
3) place quadrat, collect data (local freq, pop. density or % cover)
4) repeat (30+ times), calculate mean

Question 12

What are the 2 types of transect?

Answer 12

belt or interrupted belt

Question 13

Describe how you would use systematic sampling to investigate change over distance

Answer 13

KEY POINTS
- place tape measure, transect line
- place quadrat every X metres or every position
- collect data (pop. density, % cover, frequency)
- repeat (30+)

Question 14

What are the advantages and disadvantages of local frequency?

Answer 14

+VES: quick, useful if too many to count
-VES: poor accuracy as doesn’t consider size or overlap

Question 15

What are the advantages and disadvantages of population density?

Answer 15

+VES: more accurate if plant is distinguishable and not too many, can be used to estimate species richness
-VES: more time consuming

Question 16

What are the advantages and disadvantages of percentage cover?

Answer 16

+VES: quicker than density, useful if indistinguishable or too many
-VES: subjective (limits accuracy), doesn’t consider size or overlap

Question 17

Why might mark-release recapture be unreliable in larger sample areas?

Answer 17

• unlikely species will distribute evenly
• less likely to recapture fish