A Quick Primer on How Nature
Really Works
Everything alive needs food. Everything alive is
potential food. Food consists of chemicals, molecules
structured so as to contain potential energy, or what
is called “calorie content” on packaged foods. We eat
food and thus we gain some of what’s in it, proteins,
carbohydrates, fats, and minerals such as potassium,
phosphorus, and calcium. And we also gain calories,
some of the potential energy contained in the food.
Pretty simple, right? Well, not so much. Aside from the
mighty complex biochemistry and physiology we have
merely alluded to, where does the food come from?
In all but a few cases (mostly in the deep oceans),
ultimately it is green plants, those with the unique
pigment chlorophyll, abundant in terrestrial as well
as in various aquatic species. Plants incorporate a tiny
amount of solar radiation (sunlight) into low- energy
molecules, thus converting them to high- energy
molecules. To do this plants need three raw materials:
sunlight, water, and various atoms. This biochemical
process, which is the key to life on Earth, is called
photosynthesis. Without it, life as we know it would
not be possible. Earth’s biodiversity would not exist.
By now you know that there is an abundance of light,
water, and various atoms in the tropics. So the tropics
are areas with high rates of photosynthesis, or what
ecologists call primary productivity.
Primary Productivity: An
Introduction
Notice how green the rain forest shown in plate 5- 1 is?
That is because of a pigment molecule called chlorophyll.
Here’s how it works. Primary productivity is the total
amount of solar radiation (sunlight) converted by plants
into high- energy molecules such as carbohydrates.
Photosynthesis is the complex biochemical process
by which this energy transformation is accomplished.
Plants capture red and blue wavelengths of sunlight,
what is called the photosynthetically active radiation.
Plants use some of the energy from sunlight to split
water molecules into their component atoms, hydrogen
and oxygen. To accomplish this, plants utilize the
green pigment chlorophyll. The reason a tropical forest
(or any forest, lawn, or grassland) is green is because
chlorophyll reflects light at green wavelengths, while
absorbing light in the blue and red portions of the
spectrum. The essence of photosynthesis is that energy-
enriched hydrogen taken from water is combined with
the simple, low- energy compound carbon dioxide (an
atmospheric gas) to form high- energy carbohydrates
(such as glucose) and related compounds. This is an
evolutionarily ancient and fundamental process rooted
in deep time. Oxygen from water is released as a by-
product. Photosynthesis, occurring over the past 3
billion years, has been responsible for changing Earth’s
atmosphere from one of virtually no free oxygen to
its present 21% oxygen. Take a deep breath and recall
an old bumper sticker message: “Have you thanked a
green plant today?”
Of all natural terrestrial ecosystems on Earth, none
accomplishes more photosynthesis than tropical rain
forest. On an annual basis, a hectare (10,000 m^2 , or
about 2.5 acres) of rain forest is more than twice as
productive as a hectare of northern coniferous forest,
half again as productive as a temperate forest, and
between three and five times as productive as savanna
and grassland (plate 5- 2).
Ecologists distinguish between gross primary
productivity (GPP) and net primary productivity
(NPP). The former refers to the total amount of
photosynthesis accomplished, while the latter refers to
the amount of carbon fixed (which means captured in
compounds used in growth) in excess of the respiratory
(metabolic) needs of the plant. In other words, NPP
is the amount of carbon (as plant tissue) added to
the plant for growth and reproduction. The simple
equation GPP = NPP + R describes the relationship
(R stands for respiration). Most published data on
productivity is expressed as NPP.
By way of example, if you measure the growth in
biomass of a field of corn from seed to harvest, you are
computing net primary productivity. You do not know
how much energy the corn has used to maintain itself
during its growing season. Such respiratory energy,
essential to the metabolism of the plants, has been
radiated back to the atmosphere as heat energy (along
with a certain amount of carbon dioxide, oxygen, and
water vapor from transpiration). A growing cornfield
photographed from above with an infrared camera
would reveal a deep red image, indicating the heat that is
continually emitted from the corn. This is the energy ofChapter 5. Sun Plus Rain Equals Rain Forest
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