Topic 7—C:Populations in ecosystems- 3. Investigating populations Flashcards
What do you have to look at when investigating populations of organisms?
Involves looking at:
- abundance
- distribution of species in a particular area
Definition of abundance
- the number of individuals of one species in a particular area
How can abundance be estimated ?
- by simply counting the number of individuals in samples taken
What are the other measures of abundance that can be used too?
Frequency
Percentage cover
Definition of frequency
- the number of samples a species is recorded in
E.g. 70% of samples
Definition of percentage cover
How much of the area your investigating is covered by a species
Definition of distribution
Where a particular species is within the area your investigating
Why do you take samples?
- 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 your investigating
Random sampling
- choose an area to sample- a small area within the area being investigated
- samples should be random to avoid bias. You can use a random number generator to ensure your samples are random
- use an appropriate technique to take a sample of the population
- repeat the process taking as many samples as possible. This will reduce the likelihood that your results are down to chance
- 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
Random number generators
- if you were investigating populations in a field, you could pick random sample sites by dividing the field into a grid and using a random number generator and a random letter generator to select coordinates
- this will give you coordinates at random
- then you just take your samples from these coordinates
Running means
- its important that you take enough samples to give a good estimate
- one way of doing this is to take a running mean (this is where you work out the mean for all the data each time you collect a new sample)
- once the mean no longer changes by a large amount you should have data that gives a realistic estimate for the whole area
When choosing what methods for studying populations of organisms what does it depend on?
- type of organism
- habitat
What can quadrants and transects be used for?
- studying non-motile organisms e.g. plants and corals or slow moving organisms like limpets
If your studying more motile organisms like insects what would be more appropriate to use?
Nets and traps
Quadrants
- square frame
- usually divided into a grid of 100 smaller squares by strings attached across the frame
- quadrats are placed on the ground at different points within the area youre investigating
- The species frequency (how often a species is found) 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 can 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
Transects
You can use lines called transects to help find out how organisms are distributed across an area e.g. how a plat species change from a hedge towards the middle of a filed
Belt transects
- quadrats are placed next to eachother along the transect to work out species frequency and percentage cover along the transect
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 to easier to cover a large distance
Capturing mobile organisms
- you might need to use equipment to capture them
- the best method of capturing organisms will depend on what your studying
Flying insects
- you would use a sweep net
Ground insects
- you would use a pitfall trap (a steep-sided container that’s sunk into the ground)
Aquatic animals
You would use a net
Mark release recapture
- its a method used to measure the abundance of more mobile species
Here’s how it is done:
1. Capture a sample of a species using an appropriate technique and count them
2. Mark them in a harmless way e.g. 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 from the second sample are marked
6. You can use this equation to estimate the total population size
Total population size equation
Number caught in 1st sample x number caught in second sample/ number marked in second sample
What are the assumptions that the accuracy of the mark-release-recapture method depend on?
- the marked sample has had enough time and opportunity to mix back in with the population
- the marking hasn’t affected the individuals chance pod survival (e.g. by making them more visible to predators) and the marking itself is more visible (e.g. hasn’t rubbed off)
- there are no changes in population size due to births, deaths and migration during the period of the study
Ethics of mark-release-recapture method
- some people think that the capturing of animals for study is unethical as it might cause them unnecessary stress
- also if animals are put under too much stress during capture it could reduce their chances of survival after release, or influence them to avoid trap in the future
- these would interfere with the accuracy of any estimates of population size made using the mark-release-recapture method
- to minimise stress investigations should be planned so that organisms are treated carefully and are kept and handled as little as possible
