Because only a small fraction of energy passes from one trophic level to the next, food chains are short. They rarely have more than four or five trophic levels.Worked example: if producers fix 100 000 kJ m−2 year−1 and about 10% is transferred at each step:producers 100 000 → primary 10 000 → secondary 1000 → tertiary 100 → quaternary 10 After only four transfers, just 0.01% of the original energy is left. There is not enough energy to support a population at a further level.Consequences:
- Top predators are few in number and need to hunt over large territories to find enough food. This makes them vulnerable to habitat loss and to persistent pollutants that build up along the chain.
- A long food chain needs a very large amount of energy fixed by producers at its base.Implications for humans: more people can be fed from a given area of land if they eat crops directly, rather than feeding the crops to livestock and then eating the meat. Each extra step loses around 90% of the energy.Farmers can make livestock production more efficient by reducing energy losses, for example by keeping animals indoors in warm conditions and limiting their movement, so less energy is used in respiration. This raises welfare concerns.Aquatic food chains are often longer than those on land, partly because many aquatic consumers are ectotherms, which use less energy keeping warm and so pass on a larger proportion.
- Top predators are few in number and need to hunt over large territories to find enough food. This makes them vulnerable to habitat loss and to persistent pollutants that build up along the chain.
- A long food chain needs a very large amount of energy fixed by producers at its base.Implications for humans: more people can be fed from a given area of land if they eat crops directly, rather than feeding the crops to livestock and then eating the meat. Each extra step loses around 90% of the energy.Farmers can make livestock production more efficient by reducing energy losses, for example by keeping animals indoors in warm conditions and limiting their movement, so less energy is used in respiration. This raises welfare concerns.Aquatic food chains are often longer than those on land, partly because many aquatic consumers are ectotherms, which use less energy keeping warm and so pass on a larger proportion.