Environmental Engineering Reference
In-Depth Information
Figure 3-15 Natural capital: two species found in tropical forests are part of the earth's biodiversity. On the left
is an endangered white ukari in a Brazilian tropical forest. On the right is the world's largest flower, the flesh
flower (Rafflesia), growing in a tropical rainforest of West Sumatra, Indonesia. The flower of this leafless plant
can be as large as 1 meter (4.3 feet) in diameter and weigh 7 kilograms (15 pounds). The plant gives off a
smell like rotting meat, presumably to attract flies and beetles that pollinate the flower. After blossoming once a
year for a few weeks, the flower dissolves into a slimy black mass.
Figure 3-15 shows two tropical forest species that
make up part of the earth's species diversity.
The earth's biodiversity is the biological wealth or
capital that helps keep us alive. It supplies us with
food, wood, fibers, energy, raw materials, industrial
chemicals, and medicines—all of which pour hun-
dreds of billions of dollars into the world economy
each year. It also helps preserve the quality of the air
and water, maintain the fertility of soils, dispose of
wastes, and control populations of pests that attack
crops and forests.
Biodiversity is a renewable resource as long we
live off the biological income it provides instead of
nibbling away at the natural capital that supplies this
income. Understanding, protecting, and sustaining bio-
diversity is a major goal of ecology and of this topic.
All organisms, whether dead or alive, are potential
sources of food for other organisms. A caterpillar eats a
leaf, a robin eats the caterpillar, and a hawk eats the
robin. Decomposers consume the leaf, caterpillar, robin,
and hawk after they die. As a result, there is little waste in
natural ecosystems.
A sequence of organisms, each of which serves as
a source of food for the next, is called a food chain. It
determines how energy and nutrients move from one
organism to another through the ecosystem, as shown
in Figure 3-16.
Ecologists assign each organism in an ecosystem
to a feeding level, or trophic level (from the Greek
word trophos, meaning “nourishment”), depending
on whether it is a producer or a consumer and on
what it eats or decomposes. Producers belong to the
first trophic level, primary consumers to the second
trophic level, secondary consumers to the third, and
so on. Detritivores (detritus feeders and decom-
posers) process detritus from all trophic levels.
Of course, real ecosystems are more complex.
Most consumers feed on more than one type of organ-
ism, and most organisms are eaten by more than one
type of consumer. Because most species participate in
several different food chains, the organisms in most
3-4
ENERGY FLOW IN ECOSYSTEMS
Food Chains and Food Webs
Food chains and webs show how eaters, the eaten,
and the decomposed are connected to one another in
an ecosystem.
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