Energy flows and nutrient cycles in ecosystems - Fundamentals of the Physical Environment - page 662

and webs . Each 'step' in the flow of energy is a trophic level , and organisms can be
classified according to their functional trophic level. ' Producers ' (or autotrophs ) have
the ability to fix carbon through photosynthesis via green chloroplasts in their leaves.
Herbivores are the primary consumers of organic molecules fixed by the producers.
Carnivores are secondary consumers, living off the organic molecules of the herbivores.
There may be several levels of carnivores in any one ecosystem; in such cases the
ultimate level will be occupied by the ' top carnivore '. The final group of organisms in
an ecosystem are ' decomposers ' or detrivores , small animals, bacteria and fungi which
can break down the complex organic chemicals of dead material and waste products. It
should be recognized, however, that a food chain model is a simplified attempt to
structure energy flow; in reality a particular species can occupy a position at several
trophic levels. Thus some species of algae and bacteria can act both as photosynthesizers
(autotrophs) and as grazers (heterotrophs). Foxes also can obtain part of their feed from
eating the fruits and leaves of plants (herbivore) and part by eating herbivores such as
rabbits, mice and voles (carni-
Plate 22.1 Massive western red cedar ( Thuja plicata) and
saplings of western hemlock ( Tsuga heterophylla) in the
Pacific rain forest of Canada. This climax ecosystem has
evolved to regulate the flow of energy to maximum efficiency.
Photo: Ken Atkinson.