Abiotic factors are the non-living factors of the environment. They affect which organisms can live in a community and how many there are. The abiotic factors you need to know are: light intensity, temperature, moisture levels, soil pH and mineral content, wind intensity and direction, carbon dioxide levels for plants, and oxygen levels for aquatic animals.
A change in one abiotic factor can change the whole community. For example, less light intensity slows photosynthesis, so plants grow less and make less food. Fewer plants then means less food for the animals that eat them. A drop in oxygen levels in a pond, for instance after pollution, means fewer fish and other aquatic animals can survive there, so their numbers fall.
When you explain a change, link the abiotic factor to the organism, then to the effect on numbers. Say how the factor affects survival, growth or reproduction, and then how that changes the population. You may also be asked to read charts, graphs and tables showing how an abiotic factor affects organisms.