Populations [2]: Sampling (A Level Biology)

Uploaded by freeeschool on 09.09.2012

A Level Biology: Populations 2 - Sampling
Hi! Welcome to my second video on the series about Populations. Today, we are going to
be looking at Sampling methods.
So when we sample a given area, what we do is we try to find out the size of the populations
of the organisms that live there; essentially, the types of organisms that live in a specific
habitat or ecosystem.
The first way to do that is to use a piece of equipment called a quadrant. Essentially,
a quadrant is essentially a large grid, usually made out of something with metal and it is
essentially is a technique or a piece of apparatus that divides up land into small squares, a
bit like this. So the first thing that you would have to do in order to sample an area
using a quadrant is you have to randomly generate some coordinates. Once you have gridded the
area, you use this coordinates in order to pick up specific areas that you are going
to sample. Rather than counting all the organisms that live in a given area, so if you are looking
at sampling an area of 100 square meters, it would be very difficult to count all of
the species living in that area. What is better is to sample a specific section and then draw
conclusions from that.
Of course, what you would do if you were sampling an area is you wouldn’t just do it once.
You do numerous numbers and you do several repeats. There are a couple of ways in which
you can determine how enough is enough. Probably the well-used way is until you’ve covered
1% of the area. In some are cases, you will see that isn’t going to be possible because
even 1% of a given area can be quite a large amount. What you might decide is until the
results are very similar in which you are getting results which are consistent, that
might be when you decide that you have gotten enough data.
Once you have gathered this information, you can use it in order to do a statistical test
to determine the populations and to gather things like percentage cover.
Quadrats are particularly useful if you want to do a sample of an area where you are trying
to count a specific number of specific organisms. Let’s say you were sampling a grass field
and you were counting the number of grass plants in that field.
If you were looking at sampling an area where changes are quite markedly, then a transect
is a better piece of equipment. A transect is essentially a piece of strip of rope or
a strip of line that has regular marking indicated on it. What you would do is you would place
this along an area and then you would sample the regular intervals and then see what organisms
occurred there.
Now, it is very good to measure change of ecosystems, particularly things like coastlines
because coastlines have different areas. For instance, you have the kind of beachy, sandy
area that you then have a small sharp area and then you start to go towards large trees,
the further away that you go away from the beach or from the shorefront.
Here, you have a set of people actually doing a transect and so this is line and it has
several markers on it. These people would do down the several markers and then mark
down what organisms are there at that point.
Now, those types of things can measure organisms that don’t move around too much, things
like plants. If you were trying to estimate the amount of a living thing that does move
around like an animal, you’d use this method called Mark, release and recapture.
What Mark, release and recapture actually is where you take a first sample, so you capture
a number of organisms. Let’s say you were sampling the number of frogs. You would take
a number in the first sample and so you would capture the frogs. Then you would multiply
that by the number in the second sample and so what you would do is you leave enough time
in between doing the samples and then take a second capture and you divide that number
by the number of marked in the second sample.
When you took the first sample, you’d put a mark around the organism and then release
them and then come back, maybe in a few days and then do another sample and hence, the
number divided by the marked in the second sample.
There are a number of considerations that you need to make when you do a Mark, release
and recapture. One is you leave enough time between the first and second samples. You
can’t do a sample and then an hour later, do a second, obviously because there would
not be enough time for the captured ones to reintegrate into the ecosystem.
You’d also have to not mark the organisms so that they stand out. So it’s a selection
bias against them. Obviously, if you are capturing a frog and some frogs are with a camouflage,
you wouldn’t mark them with a huge, bright color or for instance, you wouldn’t put
a tag on them that impairs their movement. So they are more likely to get captured or
eaten by a predator.
So in summary, what we have gone through today is we’ve looked at the way we do samples.
We’ve looked at the different types of sampling methods. Things like using quadrats and then
transects and what circumstances you would use those pieces of equipment.
We have also gone through another method of sampling called Mark, release and recapture
and the pitfalls that can be involved with that.
If you will join me next time, we would be looking at predation.
[end of audio – 05:37] A Level Biology: Populations 2 - Sampling