Teaching Equipment and Methods
Here are some ideas about making your own equipment and suggetions on how to use it.
1. Sampling vegetation using quadrats
A quadrat is a square frame used for sampling an area.
Quadrats can be used to sample an area either systematically or randomly. The sampling strategy chosen will depend on the hypothesis being investigated. Random sampling is particularly useful if two habitats are being compared. For random sampling, every point in the sample area has an equal chance of being sampled. Systematic sampling is used to investigate changes in a habitat caused by one environmental factor. The best way to measure these types of changes is to use a transect. Random sampling means that every part of the sample area has an equal chance of being sampled. If enough samples are taken, the results should be representative of the whole sample area.
(b) Sampling within a quadrat
The simplest way to process the results of quadrat surveys is to count the number of plants that they find in each quadrat. Where each plant is a discrete individual, such as dandelions and daisies, this is acceptable. When all the results have been collected, calculate the mean number of plants per quadrat for each of the sample areas. For a greater challenge, transform the figure for mean number per quadrat into mean number of plants per square metre.
Many grasses and other grassland plants, such as white clover, speedwell and creeping buttercup, cannot easily be counted in this way, because the edges of each plant are not immediately obvious. Instead it is easier to calculate the local frequency of the plant in each quadrat. This is more challenging.
Frequency is easier to measure using a gridded quadrat. In the example shown to the left, the quadrat has been divided into 100 squares. Pupils count the number of squares in which the plant (or, in this case, red paper) is found. Since the red paper is found in 56 squares, the local frequency of red paper is 56%. Once the frequency of plants has been calculated for each quadrat, the mean frequency can then be calculated in the usual way for the whole sample area.
Assuming a class of 8 groups of 3 or 4 pupils, if each group takes 10 random samples, the total number of samples taken by the class will be 80. If you choose a 20m x 20m area to sample, and pupils take samples using 0.5m 2 sized quadrats, this means that approximately 4%, or 20 square metres of the total sample area of 900 square metres is sampled.
(d) Teaching using quadrats
The instructions for using quadrats with classes are notoriously variable. Throwing a quadrat is not truly random - pupils may aim for conspicuous patches of plants, and they are unlikely to sample ground close to their feet and areas immediately behind trees. You may also be conscious of the dangers of pupils throwing quadrats at each other. A useful strategy that has been successfully used is outlined below.
- Prepare two sets of numbered pieces of paper from 1-30. Put these into a bucket, or similar.
- Put the pupils into groups of 3 or 4.
- Lay two 20m measuring tapes perpendicular to each other in the study area.
- Invite a pupil from one group to take a piece of paper out of the bucket. Once the paper has been replaced, he/she should walk along the tape until reaching the distance, e.g. if 14 is chosen, walk to 14m.
- Invite a second pupil from the same group to do the same for the other tape.
- Then they turn into the plot at a right angle to the tape and walk into the plot until they meet. They should place the quadrat here.
- Instruct the pupils to wait in a queue for random numbers, and to return to the back of the queue when they have finished sampling.
(e) Make your own quadrat
An appropriate size for the grassy areas of parks is 50cm x 50cm. Although ready-made quadrats can be purchased from educational suppliers, they can easily be constructed from ropes, canes or plastic tubes. Make sure that any sharp corners are removed before use.
2. Measuring soil hardness
Soil hardness (or soil compaction) can be measured in the field by using a metal stake or pin. These can easily be made by using knitting needles. Hold the stake out at arm's length above the centre of the quadrat and let it fall through the fingers. Measure the depth of entry into the soil. This may seem unscientific, but it really does work if pupils take care to let the stake fall from the same height above the quadrat each time.
Alternatively, after a dry period when the soil is hard, ask pupils to apply uniform pressure to the stake at each measuring site and measure the depth it reaches into the soil. Clean the stake of soil before it is used for the next measurement.
(b) Make your own soil pin
A knitted needle is long enough to measure most soil depths which will be found. It may be useful to use indelible ink to mark depths on the side of the needle.
3. Infiltration rate
Infiltration rate is the speed at which water soaks down from the soil surface into the soil below. It can be measured by using an infiltration tube, which is easily constructed from simple materials
(b) Make your own equipment
Use a 30cm length of 10cm diameter plastic pipe. You will also need a 30cm ruler, a jug of water, a mallet and a piece of wood.
It is important to maintain the same 'head' of pressure when pouring water into the infiltration tube. Although there are several ways in which infiltration tubes can be used with students, one method that is active for students but still scientifically valid, is as follows.
Press the plastic tube down into the soil until it is buried by at least 10cm. You may need to use a mallet and a piece of wood to hammer it into the soil. Pour enough water into the plastic tube to reach the 10cm mark. Start the stopwatch. Stop the stopwatch when the water level has dropped to 9cm.
4. Identifying plants and animals
The accurate identification of plants and animals can be an obstacle to fieldwork. It is useful to restrict the number of choices to a small number. The Field Studies Council fold-out charts Freshwater name Trail, Woodland Name Trail, Bugs on Bushes and Playing Field Plants have been designed specifically for fieldwork with school-aged groups.
For sampling plants, sometimes it is useful with classes to spend a few minutes creating a 'species board'. Choose the 5 commonest grassland plants (not grasses) growing at the time of sampling, and ask each group of pupils to attach with sticky tape a labelled sample of each plant (including flowers and leaves) to a wooden board. They can use this board for reference while collecting data from quadrats.
5. Investigating air pollution
It is often difficult to measure air pollution in the field, as often sophisticated equipment and long-term monitoring are needed to obtain worthwhile data. One way to overcome this problem is to choose an aspect of air pollution which can easily be measured and combine it with secondary data available on the Internet.
Particulate pollution (i.e. soot) adhering to tree bark can rapidly be measured using nothing more complicated than sticky tape. Immediately this introduces a number of variables which could be investigated, such as the direction in which the bark is facing, its height off the ground and its distance from a point source of pollution.
Press the sticky side a 2cm length of tape firmly onto the bark of the tree, leave for 10 seconds, and then remove it. Soot and other particles from the air will have adhered to the tape, along with debris such as loose bark and moss from the tree. Take two samples of particles at 1 metre above the base of the tree.
Stick the samples onto the individual results sheet for the correct site.
(c) Analysing results
The sticky tape on the slides can be examined under the microscope. Make mini-quadrats by photocopying graph paper onto acetates. Lay an acetate grid over the pollution sample (sticky tape). Use random co-ordinates to locate a quadrat. Estimate and record the percentage frequency of black particulates in the chosen quadrat.
Repeat this estimation of particulates between 15-20 times for different quadrats, and calculate an average percentage cover of particulates for the sample site. Only soot particles should be recorded; ignore bark and moss. A hand lens may be useful.
6. Measuring light levels on land
If, for example, students are comparing shaded or unshaded woodland, or carrying out a transect from grassland to woodland, it is useful to be able to measure light levels. Ideally students need a measurement of long-term variations in the amount of sunlight reaching the ground surface. Unfortunately, short-term variations in light levels during sampling (such as the clouds moving in front of the sun) can easily obscure the underlying pattern.
An alternative surrogate measure of long-term light level is to consider the proportion of the sky which is visible from the ground surface. Completely open ground with no trees will have a measurement of 100% visible sky, whereas the amount of visible sky will be almost 0% at ground level in a dense conifer plantation.
Use a gridded 0.5 square metre quadrat. Hold the quadrat above the head, then count the number of squares which are mostly occupied by sky. Alternatively, make a 'light tube' by taping a gridded piece of acetate to the end of a tube (a cardboard toilet roll tube will do, as long as it is not raining) and look through it to measure the % visible sky.
7. Measuring light levels in water
Turbidity is the percentage of suspended sediment in water, and it is used to assess the amount of light which passes through a body of water to the bottom. The more turbid ('murky') the water, the more suspended sediments there are, and the less light will pass through to the bottom.
Turbidity can be measured in the field by lowering a float into the water and measuring the depth that it reaches before it is no longer visible to the observer. A home-made Secchi disc will
(b) Make your own Secchi disc
Use a circular disc of about 20cm in diameter. A stiff plastic or thin metal lid will do. Paint opposite quarters of the disc black and white. Using a drill fix a bolt, washers and a hook to the centre of the disc. This prevents the disc from floating. Tie the string or nylon twine to the hook.
Lower the Secchi disc into the water until you lose sight of it, then slowly let it go a little deeper. Pull it slowly back out, watching closely for when the disc re-appears. Measure the length of the string or nylon twine to from the water surface to the disc, and use this as a measure of turbidity. The greater the length of string, the lower the turbidity - i.e. the deeper the disc is visible, the less 'murky' the water is.