Flow of Energy in Ecosystems
Energy cannot be created or destroyed. Energy can only be changed from one form to
another. This is such a fundamental law in nature that it has its own name: The Law of
Conservation of Energy. Plants do not create chemical energy from nothing. Instead,
they create chemical energy from abiotic factors that include sunlight. So they transform
solar energy into chemical energy. Organisms that use chemosynthesis start with chemical
energy to create usable chemical energy. After the producers create the food energy, it is
then passed on to consumers, scavengers, and decomposers.
Energy flows through an ecosystem in only one direction. Energy enters the ecosystem with
the producers. In nearly all ecosystems, sunlight is the original energy source. This energy is
passed from organisms at onetrophic levelor energy level, to organisms in the next trophic
level. Producers are always the first trophic level, herbivores the second, the carnivores that
eat herbivores the third, and so on.
An average of 90% of the energy that reaches a trophic level is used to power the organisms
at that trophic level. They need it for locomotion, heating themselves, and reproduction. So
animals at the second trophic level have only about 10% as much energy available to them
as do organisms at the first trophic level. They use about 90% of what they receive, and
so those at the third level have only 10% as much available to them as those at the second
level. This 10% rule continues up the trophic levels, so much less energy is available at the
next higher trophic level in an ecosystem.
The set of organisms that pass energy from one trophic level to the next is described as the
food chain(Figure18.9). In this simple depiction, all organisms eat at only one trophic
level. Animals at the 3rdtrophic level only eat from the 2ndtrophic level and those at the
2 ndeat only from the 1st. But many omnivores feed at more than one trophic level, with
plants and animals in their diets.
Since only 10% of the energy is passed up the food chain, each level can support fewer
organisms. A top predator, like a jaguar, must have a very large range in which to hunt so
that it can get enough energy to live. Top carnivores are quite rare relative to herbivores for
this reason. The result of this is that the number of organisms at each trophic level looks
like a pyramid. There are many more organisms at the base of the pyramid, at the lower
trophic levels than at the top of the pyramid, the higher trophic levels.
Food chains usually have only four or five trophic levels because there is not enough energy
to support organisms in a sixth trophic level. Food chains of ocean animals are longer than
those of land-based animals because ocean conditions are more stable. Organisms at higher
trophic levels also tend to be larger than those at lower levels. The reason for this is simple:
a whale must be able to eat a plankton, but the plankton does not have to be able to eat the
whale. Sometimes multiple smaller predators will act together to take down a larger prey,
so the organisms at the higher level are smaller than those at the lower level. This is true of
a pack of wolves, which acts together as one to hunt a moose.