Investigating an ecosystem
Ecological sampling
Key points
Sampling is a type of scientific surveying that can save time and money and give information on the
population of animals and plants.
There are many different types of sampling; the most useful type depends on the habitat and what is
being sampled.
Why sampling is important
They could all be counted individually to find out how many polar bears there are in the Arctic. However, it would
take a long time and the chances of actually finding them all would be quite small. To make this process easier
scientists look at a smaller part of the population or its habitats to conclude the whole area. This type of scientific
surveying is called sampling. Obtaining an estimate by sampling is faster, less expensive, and, in most cases, gives
us all the necessary information.
There are lots of different methods of sampling, but they all involve investigating living organisms in one or more
habitats. This information is extremely important. It is the first step in working out which species are nearing
extinction. Because of this, sampling is essential for conservation.
It is essential that any sampling does not affect the living things or their habitat.
Quadrats
A quadrat is a square frame that is used in sampling. It is placed in a location and the plants or slow-moving
animals inside it is counted.
The following things can be counted within a quadrat:
1. The number of a single species.
2. The number of different species - a measure of biodiversity.
3. The percentage of the quadrat taken up by one species - it is much easier to do this than count all the
individual blades of grass, for example.
Most sampling using quadrats is random. This means it has the same chance of being placed in any position within
the habitat. You cannot close your eyes, turn around a few times and throw your quadrat. This might seem random
but it can’t ever land at your feet or beyond your throw, so it isn’t. Sampling also shouldn't harm any species - it is
important to leave everything as you found it.
Sampling must not be biased- for instance, choosing to sample where there are lots of plants. Here is an example of
a way sampling could be undertaken without bias.
1. When you have chosen a sampling area, first divide it up into a grid, for instance, having 10 × 10 divisions.
2. Use a suitable method to generate a pair of random coordinates on your grid. For example, you could draw
numbers out of a hat.
3. Place the first quadrat on your grid using these coordinates.
4. Count and record the number of each species within this quadrat.
5. Repeat steps 1-4 so that you have a total of at least 10 counts. This will give you more data, which will
make the results more accurate.
Field investigations
Ecologists need to estimate the number of organisms in a population to:
better understanding of the relationships in a community
help to monitor the impact of changing environmental conditions, e.g. climate change or conservation
projects that aim to conserve endangered species or habitats.
It is almost always impossible to count all of the organisms in a population. So, a small section of a population can
be looked at to estimate the distribution or numbers in the whole population.
This process requires sampling and the area or part of the population looked at is called a sample.
Sampling
When sampling a population:
the numbers of organisms are counted within a sample site, and then the results multiplied to estimate
the total number in the entire habitat
large animals and plants can often simply be counted, sometimes from an aerial photograph
smaller animals such as insects and smaller mammals need to be trapped first so they can be counted and
then released afterward - pitfall traps are small traps dug into the ground, which often have food inside to
attract small mammals and the sides of these traps are smooth to stop the mammals escaping
