2.5a Investigating Ecosystems

Question 1

What is the definition of “sampling”?

Answer 1

“A process used in statistical analysis in which a predetermined number of observations will be taken from a larger population.”

Question 2

Why are larger samples usually better?

Answer 2

are likely to be more representative (and accurate) of the whole population

Question 3

What are the 3 sampling strategies?

Answer 3

• Random Sampling
• Systematic Sampling
• Stratified Sampling

Question 4

What is “Random Sampling”?

Answer 4

Every item has an equal chance of being selected through random numbers. This strategy does not involve any subjectivity and is thus the least biased.

Method:

• Make a map with a numbered grid
• With a random number generator choose two random numbers
• -> This makes up a coordinate, sample at that coordinate

Question 5

What is “Systematic Sampling”?

Answer 5

This is where samples are chosen in a regular way.
This technique is often referred to at the nth method

Method:
- EG. you measure every 5th person that passes by

Question 6

What is “Stratified Sampling”?

Answer 6

This technique is used when the population is known to contain subsets.
It is important to know the size of the subsets in the whole population so that your sample reflects the same proportions.

Method:

• Stratified sampling does not stand-alone but is combined with random or systematic sampling.
• Once the sub sets and their size are identified an appropriate sampling method can be used to select sample points, lines of areas.

Question 7

What are some advantages and disadvantages of “Random Sampling”?

Answer 7

Advantages:

• Unbiased
• Suitable for large populations

Disadvantages:

• You may not have access to some of the sample points
• In large study areas the sample points may miss some places

Question 8

What are some advantages and disadvantages of “Systematic Sampling”?

Answer 8

Advantages:

• Easier to apply than random sampling as there is no need for a grid
• Coverage of the whole study area can be achieved

Disadvantages:
- May be biased because places have a different chance of selection

Question 9

What are some advantages and disadvantages of “Stratified Sampling”?

Answer 9

Advantages:

• Representative of the population so long as the proportions of the whole population are known
• Flexible – can be used in many situations as it combines with random and systematic
• Good for comparing sub-sets

Disadvantages:
- You must know the size of the subsets to get an accurate picture.

Question 10

What’s a “quadrat”?

Answer 10

A quadrat is an appropriately shaped plot used to identify the area you wish to study.
The size depends on the organism that is being studied.

Question 11

What calculations do you use a quadrat for?

Answer 11

• Number of individuals
• Percentage frequency
• Population density
• Percentage cover

Question 12

How do you measure “The number of individuals”?

Answer 12

Add up all the individuals within the quadrat
–> This is sometimes difficult with for example grass (do you count all the grass blades?)

Question 13

How do you measure “Percentage frequency”?

Answer 13

How often a particular species appears in an area
–> Use a gridded quadrat, so its easy to count how many squares the species appear in

Question 14

How do you measure “Population density”?

Answer 14

Once you know the number of individuals in an area this is a simple calculation
number of individuals in quadrat (quadrat area)
–> eg. 30m^2 (30 per meter squared)
–> if the quadrat is not 1m squared watch out with maths!

Question 15

How do you measure “Percentage cover”?

Answer 15

If the quadrat is subdivided into 100 smaller squares – each square represents 1% coverage.

Question 16

What’s a “line transect”?

Answer 16

A line is placed according to a sampling strategy and then the vegetation that touches the line can be recorded at intervals, usually every 1 meter.

Question 17

Does the vegetation have to touch the line transect to be recorded?

Answer 17

Yes, only the vegetation touching the line is recorded (at intervals)

Question 18

How do you use random sampling with a line transect?

Answer 18

Generate two random points and draw a line (use this to sample)

Question 19

How do you use stratified sampling with a line transect?

Answer 19

Where the number of lines in each area reflect the proportions seen in the whole area
–> requires KNOWN subsets

Question 20

How do you use systematic sampling with a line transect?

Answer 20

if the study area is an environmental gradient then transect line must be placed along that gradient

Question 21

What’s a “belt transect”?

Answer 21

These use a quadrat to create a belt of sampling.
The placement of the belt can be achieved just the same as for a line transect.
Once the line of the transect is determined a quadrat is used to create a belt of sampling

Question 22

What’s the difference between a line and a belt transect?

Answer 22

With a line transect the vegetation touching the line is recorded, using a belt transect only the vegetation in the quadrat next to the transect are recorded