Why energy transfer is inefficient

Why energy transfer is inefficientSpec 19.2.17, 19.2.18, 19.2.19

Only a small fraction of the energy at one trophic level is transferred to the next. The transfer of energy from one trophic level to another is often not efficient, because energy is lost in these ways:

• Not all of an organism is eaten. Some parts, such as bones, roots and fur, are left uneaten.
• Some of what is eaten is not digested and is lost as faeces (egestion).
• Energy is lost in excretion, for example in urine.
• Energy is released by respiration to power life processes such as movement and, in mammals and birds, keeping warm. Much of this is lost as heat to the environment.

Food chains are short

Energy is lost at each trophic level, so less energy is available at each step. By the fourth or fifth trophic level there is too little energy left to support another population. This is why food chains usually have fewer than five trophic levels.

Eating plants or eating livestock

Humans can eat crop plants directly, or feed the crops to livestock and eat the livestock. It is more energy efficient to eat crop plants, because:

• there are fewer trophic levels in the food chain, so less energy is lost to the environment before it reaches humans;
• much of the energy in the crop fed to livestock is lost, for example in respiration, movement and keeping warm, and in faeces and urine, and is not stored in the meat or milk;
• so more people can be fed from the same area of land growing crops than from the same area used to grow food for livestock.