1. Investigating Populations
21/06/12
Miss Radford
Investigating Populations
WAL:
about the techniques used to investigate different populations.
All
Most
Some
How can the mark-release-recapture method be used to measure the abundance of mobile species?
How is the abundance of difference species measured?
What factors should be considered when using a quadrat?

2. Today we are covering from the specification:

3. Key words & definitions
Habitat
The place where an organism normally lives, which is characterised by physical conditions and the species of other organisms present.
Random sampling
Random sampling is used to avoid any bias in collecting data. Avoiding bias ensures that the data obtained is valid.
Systematic sampling
Samples are taken at regular intervals within a set sampling area. Time-consuming... But more reliable?
Abundance
Counting the number of individuals of a species in a given space.
Frequency
Frequency is the likelihood of a particular species occurring in a quadrat. It gives a quick idea of the species present and their general distribution within an area. However it does not provide information on the density and detailed distribution of species.
Percentage cover
Is an estimate of the area within a quadrat that a particular plant species covers. Advantage – data be collected rapidly & individual plants do not need be counted Disadvantage – less useful organisms occur in several overlapping layers.
Mark-release-recapture
Known number animals caught, marked and released. Some time later given number of individuals are collected randomly and mark recorded.

4. Studying Habitats
When studying a habitat, ecologists will first estimate the populations of the species living there.
The abundance of each species can never be known exactly, but sampling can give reasonably accurate estimates.
Why would it be wrong to try to count every individual of a population?
So small samples are studied at random locations, and then scaled up to fit the entire habitat.
There are of course, a range of ecological techniques at an ecologist’s disposal.

5. Introducing Sampling Techniques
The approach to sampling can be in one of two ways:
1. Random
2. Systematic
Random Sampling:
This is usually employed when trying to eliminate bias. Two numbered axis can be laid out over the sample area. Generation of random numbers provides co-ordinates for areas to study.
Systematic Sampling:
A similar grid is laid over the entire area, but samples are taken at regular intervals. Time-consuming... But more reliable?
These two approaches are realised using the following apparatus.
Random grid
Island

6. Quadrats Frame Quadrat Point Quadrat
A quadrat is just a fancy square used by biologists/ecologists.
There’s two types of quadrat:
Frame Quadrat
Point Quadrat

7. More About Quadratting
Three things to consider when using quadrats:
The size of the quadrat:
Depends on the size of species being investigated, or what kind of groups or colonies the species live in.
The number of samples being taken in the area:
The more samples you take in the habitat, the more reliable the results will be... Depends if time is an issue.
The position of each quadrat:
Producing unbiased results within a small time-frame is the best idea. Random sampling would work well.
A problem that arises during quadrat sampling is the clumping of plants. To get around this... We measure the ‘mean density’ or ‘percentage cover’ instead.

8. Mean Density
You count all the individuals of a single species in a quadrat. Do this for a few quadrats (as painstaking as it may be).
The quadrat must be of a known size.
Plug the numbers into the following formula:
Estimated mean = Total number of induviduals counted
density Number of quadrats x Area of quadrat

9. Transects
Transect sampling is more of a systematic technique, but can be adapted to a random technique if required.

10. What about animals?
What we’ve seen up until now is fine for sampling plant populations, but studying animals and insects is trickier.
A set of animals are caught and then marked in some way.
They’re then released back into the community.
After a specified length of time, the community is revisited and the same number or individuals is caught again.
The number of marked individuals is counted.
The population size is calculated:
Estimated pop. = Total number of individuals in the first sample x
size Total number of individuals in the seconds sample
number of marked individuals recaptured

11. Learning check
20 min
What is the difference between random and systematic sampling?
What types of animal can be sampled using a frame quadrat?
In the context of quadrat sampling, what is meant by species frequency?
If there are 2100 daisies, evenly distributed in a 10 m by 10 m area, what would you expect the species density of the daisies to be?
What is meant by percentage cover?
If you obtained three ‘hits’ with a needle on a particular species of plant, while using a ten-needle point quadrat, what percentage cover would you record for that plant species?
In what kind of environment would you collect samples along a transect line?
What precaution must be taken when marking mobile animals as part of the mark–release–recapture technique?
When using the mark–release–recapture technique, why must a reasonable length of time be left between release and recapture?
If 20 rabbits are marked and released and then, of 15 recaptured a week later, 5 are found to be marked, what would you estimate the size of the rabbit population to be in the area sampled?

12. Learning check
What is the difference between random and systematic sampling?
In random sampling, sampling points are chosen randomly. In systematic sampling, sample points are taken at regular intervals or in some other fixed pattern.
What types of animal can be sampled using a frame quadrat?
Sessile, or non-moving, animals and sedentary, or slow-moving, animals.
In the context of quadrat sampling, what is meant by species frequency?
The number of quadrats in which a particular species is found.
If there are 2100 daisies, evenly distributed in a 10 m by 10 m area, what would you expect the species density of the daisies to be?
21 per m2.
What is meant by percentage cover?
The proportion of an area covered by a plant or sessile or sedentary animal.

13. Learning check
If you obtained three ‘hits’ with a needle on a particular species of plant, while using a ten-needle point quadrat, what percentage cover would you record for that plant species?
30%
In what kind of environment would you collect samples along a transect line?
One in which there is a transition between communities along an environmental gradient, e.g. a beach.
What precaution must be taken when marking mobile animals as part of the mark–release–recapture technique?
The marking must not affect the animals’ mobility or survival chance.
When using the mark–release–recapture technique, why must a reasonable length of time be left between release and recapture?
To allow marked animals to mix in with the unmarked population.
If 20 rabbits are marked and released and then, of 15 recaptured a week later, 5 are found to be marked, what would you estimate the size of the rabbit population to be in the area sampled? 60