Trophic level and organic productivity
Organisms in an ecosystem may be grouped by function according to their trophic level
– the position in the food supply chain or web at which they gain nourishment. Each
successive trophic level’s organisms depend upon those of the next lowest for their
energy requirements (food). The first trophic level, primary producers (or autotrophs),
in all but a few cases convert solar radiation (sunlight) into chemical energy. The exceptions
which do not depend on sunlight include hydrothermal-vent communities and
some micro-organisms deep below ground level. Seldom are there more than four or
five trophic levels because organisms expend energy living, moving, and in some cases
generating body heat – and transfer of energy from one trophic level to the next is
unlikely to be better than 10 per cent efficient. Given these losses in energy transfer, it
is possible to feed more people if they eat at a low rather than high trophic level. Put
crudely, a diet of grain supports a bigger population than would be possible if it were
used to feed animals for meat, eggs or milk (it has been calculated that only about one
part in 100,000 of solar energy makes it through to a carnivore).