B2.4 Organisms and their environment

Living organisms form communities, and we need to understand the relationships within and between these communities. These relationships are affected by external influences.

Physical factors that may affect organisms are:

• temperature
• availability of nutrients
• amount of light
• availability of water
• availability of oxygen and carbon dioxide.

Quantitative data on the distribution of organisms can be obtained by:

• random sampling with quadrats
• sampling along a transect.

A sampling quadrat is usually a 1m x 1m (1m²) square frame of wood or plastic,
So if you measure the number of organisms in a quadrat you get the density in organisms per square metre. You can average the random individual quadrat results to get the mean value for a particular organism/m². To work out the total population of an organism in the area you have been randomly sampling, you multiply the mean by the total area. The more samples you take, the more reliable is your data, and so any deductions made will be more reliable, but the data is only statistical, never completely precise.