This pages gives outline details of the content of the topic together with essential questions and student skills and applications. Helpful for revision.

4.1 Species and communities

• Species are "groups of organisms that can potentially interbreed to produce fertile offspring".
• Members of a species may be reproductively isolated in separate populations.
• Species can be autotrophic or heterotrophic in nutrition (a few species are both).
• Consumers are heterotrophs that feed on living organisms by ingestion.
• Detritivores are heterotrophs that obtain organic nutrients from detritus by internal digestion.
• Saprotrophs are heterotrophs that obtain organic nutrients from dead organisms by external digestion.
• A community is "populations of different species living together and interacting with each other."
• An ecosystem is "a community and its interactions with the abiotic environment."
• Autotrophs obtain inorganic nutrients from the abiotic environment.
• The supply of inorganic nutrients is maintained by nutrient cycling.
• Ecosystems have the potential to be sustainable over long periods of time

4.2 Energy flow

• Ecosystems get a continuous supply of energy from sunlight.
• Photosynthesis converts light energy to chemical energy in carbon compounds, eg glucose.
• Chemical energy in carbon compounds flows through food chains by means of feeding.
• Energy released from carbon compounds by respiration is used in living organisms and is eventually converted to heat which is lost from ecosystems.
• Living organisms cannot convert heat to other forms of energy.
• The length of food chains is restricted by energy losses as is the biomass of higher trophic levels.
• Biomass in terrestrial ecosystems diminishes with energy along food chains due to loss of carbon dioxide, water and other waste products, such as urea.
• Inorganic nutrients are cycled within ecosystems - and their supply is finite - this is in contrast to energy.

4.3 Carbon Cycle

• Carbon dioxide is converted (by photosynthesis) into carbohydrates and other carbon compounds in autotrophs.
• Carbon is present as dissolved carbon dioxide and hydrogen carbonate ions (HCO₃^-) in aquatic ecosystems.
• Carbon dioxide moves from the atmosphere or water into autotrophs by diffusion.
• Carbon dioxide is produced by respiration and diffuses out of organisms into water or the atmosphere.
• Methane (CH₄) is produced from decaying organic matter in anaerobic conditions by bacteria
• Methane is oxidized to carbon dioxide and water in the atmosphere.
• Peat forms when organic matter is not fully decomposed in waterlogged soils.Organic matter was converted into coal, oil and gas that accumulate in porous rocks.
• Carbon dioxide is produced by the combustion of biomass and fossil fuels.
• Animals such as reef-building corals and mollusca have hard parts that are composed of calcium carbonate (CaCO₃)and can become fossilized in limestone
• Students need an awareness that carbon fluxes are estimates
• Experience of the analysis of data from air monitoring stations.
• The skill to draw a diagram of the carbon cycle.

4.4 Climate change

• Details of the carbon cycle are assumed (from 4.3)
• In aquatic ecosystems carbon is present as dissolved carbon dioxide and hydrogen carbonate ions.
• Details of the Greenhouse effect are assumed.
• Carbon dioxide and water vapour are the most significant greenhouse gases.
• Other gases including methane and nitrogen oxides have less impact.
• The impact of a gas depends on its ability to absorb long wave radiation and its concentration in the atmosphere.
• Global temperatures and climate patterns are influenced by concentrations of greenhouse gases.
• There is a correlation between rising atmospheric concentrations of carbon dioxide since the start of the industrial revolution 200 years ago and average global temperatures.
• Recent increases in atmospheric carbon dioxide are largely due to increases in the combustion of fossilized organic matter.