The original source of almost all
energy in an ecosystem is the Sun. All of the energy the
sun releases does not reach Earth. One one-billionth of
the Sun's total energy output actually reaches the Earth.
Of all the energy that does reach Earth, slightly less
than 34 percent is reflected back to space by clouds.
The Earth itself reflects another 66 percent back to space.
Less than one percent of the total energy that reaches
Earth is used by plants for photosynthesis. Plants are
often called producers because of their ability
to make their own food from the sun's energy.
When scientists discuss energy, they
often refer to the Laws of Thermodynamics. The First Law
of Thermodynamics states that energy can neither be created
or destroyed. The Second Law states that energy is constantly
converted from high quality to low quality. High quality
energy is capable of performing a large amount of work,
while low quality energy is capable of performing less
work. Scientists know that energy always changes from
high to low quality when work is performed. During the
change, some energy is lost in the form of heat, which
cannot do work. The amount of energy
lost as heat is often as high as 90 percent of the total
Putting all this together in an example,
if 1,000,000 units of solar energy were to reach Earth,
one percent or 10,000 units, would be available for plants
to use. Of these 10,000 units, plants would lose 90 percent,
or 9,000 units, as heat.
If an animal then ate the plants,
it would only receive 1,000 units of energy. These animals
are called primary consumers because they cannot
produce their own food. Cows and sheep are examples of
primary consumers. If another animal eats the cow or sheep,
it would only receive 100 units of energy, since the cow
or sheep would lose 900 units as heat. Animals that eat
other animals are called secondary consumers. Scientists
believe that four or five of these energy transformations
are the most possible before the amount of energy transferred
is too small to support life.
Written by By Dr. Nicholas Smith-Sebasto