7C - Investigating populations

Question 1

What methods can be used for investigating populations?

Answer 1

Random sampling through the use of quadrats and transects and mark-release-recapture.

Question 2

Why is taking samples a method used instead of trying to find species distribution for an entire area?

Answer 2

Most of the time it would be too time-consuming to measure the number of individuals in a species (population size) and the distribution of that species (i.e where it’s found) in the entire area.

Question 3

How do you carry out random sampling?

Answer 3

1) Choose an are to sample - a small area within the area being investigated.
2) Samples should be random to avoid bias, e.g. if you were investigating a field you could pick random sample sites by dividing the field into a grid and using a random number generator to select coordinates.
3) Use an appropriate technique to take a sample of the population.
4) Repeat the process, taking as many sample as possible. This will reduce the likelihood that your results are down to chance.
5) The number of individuals for the whole area can then be estimated by taking the mean of the data collected in each sample and multiplying it by the size of the whole area.

Question 4

Why should samples be random?

Answer 4

To avoid bias.

Question 5

Why should the process of random sampling be repeated?

Answer 5

To reduce the likelihood that your results are down to chance.

Question 6

What are quadrats and transects used to investigate?

Answer 6

Non-motile organisms and also slow-moving organisms like limpets.

Question 7

What are non-motile organisms?

Answer 7

Ones that don’t move about - like plants.

Question 8

What is a quadrat?

Answer 8

A square frame, usually divided into a grid of 100 smaller squares by strings attached across the frame.

Question 9

How are quadrats used to investigate populations?

Answer 9

1) A square frame, usually divided into a grid of 100 smaller squares by strings attached across the frame.
2) Quadrats are placed on the ground at different points within the area you’re investigating.
3) The species frequency (how often a species is found) or the number of individuals of each species is recorded in each quadrat.
4) The percentage cover of a species can also be measured by counting how much of the quadrat is covered by the species - you can count a square if it’s more than half covered. Percentage cover is a quick way to investigate populations and you don’t have to count all the individual organisms.

Question 10

Species frequency

Answer 10

How often a species is found.

Question 11

Why is percentage cover a good way to investigate populations?

Answer 11

Because it is quick and you don’t have to count all the individual organisms.

Question 12

What are transects used for?

Answer 12

To help find out how plants are distributed across an area.

Question 13

How are belt transects used?

Answer 13

Quadrats are placed next to each other along the transect to work out species frequency and percentage cover along the transect.

Question 14

What is interrupted belt transect?

Answer 14

When quadrats are placed at intervals along the transect/line to cover a larger distance.

Question 15

What is interrupted belt transect used for instead of a normal belt transect?

Answer 15

To cover larger distances.

Question 16

What is the mark-release-recapture method used for?

Answer 16

To measure the abundance of more motile species.

Question 17

Explain the mark-release-recapture method

Answer 17

1) Capture a sample of a species using an appropriate technique, e.g. you could use pitfall traps (a steep sided container sunk into the ground) to capture insects, and count them.
2) Mark them in a harmless way, e.g. by putting a spot of paint in them, or by removing a tuft of fur.
3) Release them back into their habitat.
4) Wait a week, then take a second sample from the same population.
5) Count how many of the second sample are marked. You can then use the equation below to estimate the total population size:
Total pop. size = (no. caught in 1st sample x no. caught in 2nd sample) / no. marked in 2nd sample.

Question 18

What is the equation for working out total population size from mark-release-recapture?

Answer 18

Total pop. size = (no. caught in 1st sample x no. caught in 2nd sample) / no. marked in 2nd sample.

Question 19

What assumptions do you need to make when using the mark-release-recapture method?

Answer 19

1) The marked sample has had enough time and opportunity to mix back in with the population.
2) The marking hasn’t affected the individual’s’ chances of survival (e.g. by making them more visible to predators), and the marking itself is still visible (e.g. it hasn’t rubbed off).
3) There are no changes in population size due to births, deaths and migration during the period of the study.

Question 20

How can you investigate the effect of an environmental factor on the distribution of a species? Use soil pH on marram grass in a coastal ecosystem as an example.

Answer 20

1) Place a tape measure in a straight line from the shore, heading inland. This will be your transect.
2) Take a 1m^2 quadrat divided into 100 squares - 10 by 10.
3) Starting from the shore, place the quadrat next to the tape measure.
4) Count the squares containing marram grass and record the result in a table as percentage cover.
5) At each sample point, you should also measure the pH and record the results in the table.
6) Repeat the observations every 10m along the transect.

Question 21

What factors need to be considered when investigating populations?

Answer 21

Safety issues.

Ethical issues.

Question 22

Why do/what safety issues need to be considered when investigating populations?

Answer 22

You need to think about what risks you’ll be exposed to during fieldwork, so you can plan ways to reduce the chance of them happening. For example, you need to:

• Use tide timetables, so you know what the local tide times are when you’re working on a beach. Low tide is the best time to work.
• Wear suitable clothing and footwear for the weather and terrain, e.g. a sun hat if it is hot and sturdy shoes to stop you slipping.
• Wash your hands before eating, especially after handling soil.

Question 23

Why do/what ethical issues need to be considered when investigating populations?

Answer 23

All fieldwork affects the environment where its carried out, e.g. lots of people walking around may cause soil erosion and marram grass can be killed by people trampling all over it. Investigations should be planned to have the smallest impact possible, e.g. people should restrict where they walk to the area being studied and try to avoid treading on the plants themselves.