A population is all the organisms of one species in a habitat. Populations of different species form communities. These communities are found in a particular habitat and are based on dynamic feeding relationships. The relationship of pyramids of number, biomass and energy to their corresponding food chains and webs

Photosynthesis is the major route by which energy enters an ecosystem. Energy is transferred through the trophic levels in food chains and food webs and is dissipated. Quantitative consideration of the efficiency of energy transfer between trophic levels.

Complex organic molecules are broken down in an ecosystem by microorganisms. Carbon dioxide and inorganic ions are thus made available for re-use.

• The role of microorganisms in the carbon cycle
• The role of microorganisms in the nitrogen cycle in sufficient detail to illustrate the processes of saprophytic nutrition, deamination, nitrification, nitrogen fixation and denitrification. (Names of individual species not required.)

An ecosystem supports a certain size of population of any one species. This population size may vary as a result of

• the effect of abiotic factors
• interactions between organisms
• inter-and intra-specific competition
• predation.

Within a habitat a species occupies a niche governed by adaptation to food and/or prevailing abiotic forces.

In natural and suitable conditions land will gradually become colonised by a range of herbaceous plants, then by shrubs and finally by trees as a climax community. There is change in the communities with time, because of the interaction between species and their environment. At each stage certain species can be recognised which change the environment so that it becomes more suitable for other species. Candidates should be able to describe one example of succession.

There is a balance of food production and conservation.

• The impact of monoculture and the removal of hedgerows on the environment.
• The effects of organic effluent, nitrates and phosphates on aquatic ecosystems, including eutrophication and effects on biochemical oxygen demand.
• Biodegradable and non-biodegradable pesticides. The bioaccumulation of pesticides in food webs.
• Farms may be managed in ways that help to ensure sustainability and reduce the impact on wildlife, such as the use of organic fertilisers, prevention of erosion, control of pesticide use and maintenance of habitat variety.

Evaluate evidence and make balanced judgements between the need to meet the demands for increased food production by agriculture and the need to conserve the environment.

Studied an ecosystem in the field and be familiar with the uses, roles and limitations of

• frame quadrats
• line transects
• measurement of abiotic factors such as pH, light and temperature.

Candidates should understand the principles involved in the use of standard deviation and the chi-squared test in reporting the results of ecological studies.