Light energy is trapped by producers (plants) in photosynthesis and is the external energy source for most ecosystems. Only a very small percentage of the total light energy is utilised by producers.
Energy is passed through food chains, which are linked with other chains as food webs.
Energy is passed from producers to primary consumers (herbivores), to secondary consumers (carnivores), to tertiary and sometimes quaternary consumers. Each level in the food chain is called a trophic level.
Decomposers (mostly fungi and bacteria) break down the dead organic matter from producers and consumers, and play an important role in nutrient recycling.
Three types of ecological pyramid:
- pyramids of numbers
- pyramids of biomass
- pyramids of energy
Pyramids of energy are the most useful and show that approximately 90% of the energy transferred to the next trophic level is lost as waste matter or heat.
Transfer of energy to carnivores is more efficient than to herbivores. Endotherms (mammals and birds) have a very low efficiency of energy transfer.
Productivity is a measure of how efficient organisms are at locking up energy in organic molecules in their body cells.
Gross primary productivity (GPP) is all the biomass produced (or energy 'trapped') by plants per square metre per year. Since some energy is used in respiration, this leaves the plants with a net primary productivity (NPP).
NPP = GPP - R
Secondary productivity is the rate of production of biomass (or gain in energy 'trapped') by animals in an ecosystem.
NP = C - (F + U + R)
The carbon cycle
Carbon is found in all organic molecules.
In the carbon cycle:
- CO2 is removed from the air and fixed into organic molecules in photosynthesis
- CO2 is replaced in the air when any organism respires or when fossil fuels are burned
- Organic molecules are passed from plants to animals by feeding and assimilation
- Organic molecules are passed from plants and animals to decomposers, which feed on the dead remains of organisms
The nitrogen cycle
Nitrogen is found in amino acids (proteins), nucleotides (nucleic acids) and ATP.
In the nitrogen cycle:
- Nitrate ions are absorbed from the soil by plants and used to make amino acids, proteins, etc
- Animals eat plants, digest the plant protein and use their amino acids to make animal protein
- When the plants and animals die, decomposers release ammonioum ions from nitrogen-containing molecules
- Nitrifying bacteria oxidise the ammonium ions to nitrates
- Nitrogen-fixing bacteria (free in soil or in root nodules of legumes) reduce nitrogen gas to ammonium ions and hence to amino acids
- Denitrifying bacteria use nitrates and release nitrogen gas into atmosphere. Mostly found in waterlogged or oxygen-deficient soils