C3. Investigating Populations

Question 1

Abundance and distribution
Abundance is the ______________________________________________________ (i.e. population size). Abundance can be estimated by simply counting the number of individuals in samples taken. There are other measures of abundance that can be used too:
- _____________- the number of samples a species is recorded in, e.g. 70% of samples.
- ______________ ______ (non-motile or slow-moving species only) - how much of the area you’re investigating is covered by a species (see next page).

Distribution is where a particular species is within the area you’re investigating.

Answer 1

Abundance is the number of individuals of one species in a particular area (i.e. population size). Abundance can be estimated by simply counting the number of individuals in samples taken. There are other measures of abundance that can be used too:
- Frequency - the number of samples a species is recorded in, e.g. 70% of samples.
- Percentage cover (non-motile or slow-moving species only) - how much of the area you’re investigating is covered by a species (see next page).

Distribution is where a particular species is within the area you’re investigating.

Question 2

Random sampling
Most of the time it would be too time-consuming to measure the abundance (population size) and the distribution of each species present in the entire area you’re investigating, so instead you take samples:(5 steps)

Answer 2

1. Choose an area to sample-a small area within the area being
   investigated.
2. Samples should be random to avoid bias. You can use a random number generator to ensure your samples are random (see below).
3. Use an appropriate technique to take a sample of the population
4. Repeat the process, taking as many samples 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. The percentage cover for the whole area can be estimated by taking the mean of all the samples.

Question 3

Quadrats
Quadrats are placed on the ground at different points within the area you’re investigating. The species frequency (how often a species is found) or the number of individuals of each species is recorded in each quadrat.
The percentage cover of a species can also be measured by counting how much of the quadrat is covered by the species- you count a square if it’s more than ____-covered (see Figure 5). Percentage cover is a ________way to investigate populations and you don’t have to count all the individual organisms.

Quadrats are useful for quickly investigating areas with plant species that fit within a ______quadrat-areas with larger plants and trees need very large quadrats.

Answer 3

Quadrats are placed on the ground at different points within the area you’re investigating. The species frequency (how often a species is found) or the number of individuals of each species is recorded in each quadrat.
The percentage cover of a species can also be measured by counting how much of the quadrat is covered by the species- you count a square if it’s more than half-covered (see Figure 5). Percentage cover is a quick way to investigate populations and you don’t have to count all the individual organisms.

Quadrats are useful for quickly investigating areas with plant species that fit within a small quadrat-areas with larger plants and trees need very large quadrats.

Question 4

Transects
You can use lines called transects to help find out how organisms are distributed across an area, e.g. how plant species change from a hedge towards the middle of a field. You need to know about:
1. Belt transects-quadrats are placed next to each other along the transect
to work out species frequency and percentage cover along the transect.
2. Interrupted belt transects- instead of investigating the whole transect you can take measurements using a quadrat placed at regular __________, e.g. every 2 metres. This can make it easier to cover a large distance.

Answer 4

You can use lines called transects to help find out how organisms are distributed across an area, e.g. how plant species change from a hedge towards the middle of a field. You need to know about:
1. Belt transects-quadrats are placed next to each other along the transect
to work out species frequency and percentage cover along the transect.
2. Interrupted belt transects- instead of investigating the whole transect you can take measurements using a quadrat placed at regular intervals, e.g. every 2 metres. This can make it easier to cover a large distance.

Question 5

Capturing motile organisms
If you’re investigating motile organisms, you might need to use equipment to capture them. The best method of capturing organisms will depend on what you’re studying.
-Examples
For flying insects, you’d use a sweep ___(a ___on a pole).
For aquatic animals you’d use a ___.
For ground insects
you’d use a ________trap (a steep-sided container that’s sunk into the ground,

Answer 5

If you’re investigating motile organisms, you might need to use equipment to capture them. The best method of capturing organisms will depend on what you’re studying.
-Examples
For flying insects, you’d use a sweep net (a net on a pole).
For aquatic animals you’d use a net.
For ground insects
you’d use a pitfall trap (a steep-sided container that’s sunk into the ground,

Question 6

Mark-release-recapture
Mark-release-recapture is a method used to measure the abundance of more mobile species. Here’s how it’s done: 6 Steps

Answer 6

1. Capture a sample of a species using an appropriate technique and count them.
2. Mark them in a harmless way, eg, by putting a spot of paint on them or by using an identification tag
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.
6. You can then use this equation to estimate the total population size:

Question 7

Mark-release-recapture assumptions - 3 things

Answer 7

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

Question 8

Investigating the effect of an environmental factor on the distribution of a species - RP12

Answer 8