Transects & RP 9 - Quadrat Sampling & RP 10 - Decay of Milk

What is Required Practical 9? (Quadratic Sampling)

Measure the population size of a common species in a habitat and use sampling techniques to investigate the effect of a factor on its distribution.

What is a quadrat?

A square frame of known area used to sample organisms within a habitat.

When are quadrats useful?

For organisms that do not move or move very little, such as plants.

How can you estimate population size using quadrats?

Place quadrats randomly in the sampling area. -- Count the target organisms in each quadrat. -- Use a large number of quadrats/repeats. -- Calculate the mean number per quadrat. -- Use the mean and the areas to estimate the total population.

Why should quadrats be placed randomly?

To avoid sampling bias and make the sample more representative of the whole area.

How could you choose random quadrat positions?

Lay out two tape measures at right angles, generate random coordinates, and place the quadrat at those coordinates.

Why should many quadrats be sampled?

A larger sample is more representative, reduces the effect of anomalous/local variation and allows a more reliable mean to be calculated.

How do you calculate mean abundance?

Mean= Total number counted / number of quadrats.

How can you estimate population size?

Estimated population= mean number per quadrat x (total habitat area / quadrat area)

What is a transect used for?

To investigate how the distribution or abundance of organisms changes along an environmental gradient.

Describe a belt transect method.

Lay a tape measure along the environmental gradient and place quadrats at regular intervals along it. Record the abundance/distribution of the target species and measure the relevant abiotic factor.

Why shouldn't quadrats be randomly positioned along a transect investigation?

The purpose is to investigate change with distance along a gradient, so measurements should be made at defined/regular distances.

Give an environmental factor that could be investigated using a transect.

Light intensity, soil moisture, soil pH or distance from a feature such as a path.

What is Required Practical 10? (RP - Decay of milk)

Investigate the effect of temperature on the rate of decay of fresh milk by measuring pH change.

Why does the pH of milk decrease as it decays?

Microorganisms break down substances in the milk and produce acidic products, decreasing pH.

What is the independent variable? (RP - Decay of milk)

Temperature.

What is the dependent variable? (RP - Decay of milk)

Rate of decay, determined from the change in pH/time taken for a particular pH change.

Give suitable control variables. (RP - Decay of milk)

Volume/type of milk, starting pH, volume/concentration of indicator or other reagents, and microorganism conditions should be kept constant where appropriate.

Why use a water bath? (RP - Decay of milk)

To maintain the milk at a constant controlled temperature.

How could rate of decay be calculated?

Rate = 1 / time OR Rate = change in pH / time