Environmental Engineering Reference
In-Depth Information
Fourth Trophic
Level
Second Trophic
Level
Third Trophic
Level
First Trophic
Level
Producers
(plants)
Primary
consumers
(herbivores)
Secondary
consumers
(carnivores)
Te r tiary
consumers
(top carnivores)
Heat
Heat
Heat
Solar
energy
Heat
Heat
Heat
Heat
Heat
Detritivores
(decomposers and detritus feeders)
Active Figure 3-16
Natural capital:
a
food chain.
The arrows show how chemical energy in food flows
through various
trophic levels
in energy transfers; most of the energy is degraded to heat, in accordance
with the second law of thermodynamics.
Critical thinking: food chains rarely have more than four trophic lev-
els. Can you figure out why? See an animation based on this figure and take a short quiz on the concept.
ecosystems form a complex network of interconnected
food chains called a
food web,
as shown in Figure 3-17
(p. 48). Trophic levels can be assigned in food webs just
as in food chains.
only about 1,000 units of energy will be available to
support herbivores and only about 100 units to sup-
port carnivores.
The more trophic levels or steps in a food chain or
web, the greater the cumulative loss of usable energy
as energy flows through the various trophic levels. The
pyramid of energy flow
in Figure 3-18 (p. 49) illus-
trates this energy loss for a simple food chain, assum-
ing a 90% energy loss with each transfer. How does
this diagram help explain why there are so few tigers
in the world?
Energy flow pyramids explain why the earth can
support more people if they eat at lower trophic levels
by consuming grains, vegetables, and fruits directly
rather than passing such crops through another
trophic level and eating grain eaters such as cattle.
The large loss in energy between successive trophic
levels also explains why food chains and webs rarely
have more than four or five trophic levels. In most
cases, too little energy is left after four or five transfers
to support organisms feeding at these high trophic lev-
els. As a result, there are relatively few top carnivores
such as eagles, hawks, tigers, and white sharks. This
Energy Flow in an Ecosystem
There is a decrease in the amount of energy
available to each succeeding organism in a food chain
or web.
Each trophic level in a food chain or web contains a cer-
tain amount of
biomass,
the dry weight of all organic
matter contained in its organisms. The chemical energy
stored in biomass is transferred from one trophic level
to another.
The percentage of usable energy transferred as
biomass from one trophic level to the next is called
ecological efficiency.
It ranges from 2% to 40% (that is,
a loss of 60-98%) depending on the types of species
and the ecosystem involved, but 10% is typical.
Assuming 10% ecological efficiency (90% loss) at
each trophic transfer, if green plants in an area manage
to capture 10,000 units of energy from the sun, then
