106 CHAPTER 5 How Ecosystems Work
- What is the first law of thermodynamics?
the second? - Why is a balanced ecosystem unlikely to
contain only producers and consumers? only
consumers and decomposers? - How does energy move through a food web?
- How does gross primary productivity differ
from net primary productivity?
highest NPP, because of their abundant rainfall, warm
temperatures, and intense sunlight. Tundra, with its
harsh, cold winters, and deserts, with their lack of pre-
cipitation, are the least productive terrestrial ecosystems.
The most productive aquatic ecosystems are algal beds
and coral reefs. The lack of available nutrient minerals in
some regions of the open ocean makes them extremely
unproductive, equivalent to aquatic deserts. (Earth’s ma-
jor aquatic and terrestrial ecosystems are discussed in
Chapter 6.)
The Cycling of Matter in Ecosystems
LEARNING OBJECTIVE
- Diagram and explain the carbon, hydrologic,
nitrogen, sulfur, and phosphorus cycles.
I
n contrast to energy flow, matter, the mate-
rial of which organisms are composed, moves
in numerous cycles from one part of an eco-
system to another—from one organism to
another and from living organisms to the abiotic environ-
ment and back again. We call these cycles of matter bio-
geochemical cycles because they involve biological,
geological, and chemical interactions. Five different bio-
geochemical cycles of matter—carbon, hydrologic, nitro-
gen, sulfur, and phosphorus—are representative of all
biogeochemical cycles. These five cycles are particularly
important to organisms, for these materials make up the
chemical compounds of cells. Humans affect all of these
cycles on both local and global scales; we conclude the
chapter with an example of this human influence.
The Carbon Cycle
Proteins, carbohydrates, and other molecules that are
essential to living organisms contain carbon, so organ-
isms must have carbon available to them. Carbon makes
up approximately 0.04 percent of the atmosphere as a
gas, carbon dioxide (CO 2 ). It is present in the ocean
in several chemical forms, such as carbonate (CO 3 2–)
and bicarbonate (HCO 3 – ), and in sedimentary rocks
such as limestone, which consists primarily of calcium
carbonate (CaCO 3 ). The global movement of carbon
between organisms and the abiotic environment—
including the atmosphere, ocean, and sedimentary
rock—is known as the carbon cycle (Figure 5.9).
During photosynthesis, plants, algae, and certain
bacteria remove carbon (as CO 2 ) from the air and fix
(incorporate) it into chemical compounds such as
sugar. Plants use sugar to make other compounds. Thus,
photosynthesis incorporates carbon from the abiotic en-
vironment into the biological compounds of producers.
Those compounds are usually used as fuel for cellular
respiration by the producer that made them, by a con-
sumer that eats the producer, or by a decomposer that
breaks down the remains of the producer or consumer.
During respiration, sugar is broken down to carbon diox-
ide that is returned to the atmosphere. A similar carbon
cycle occurs in aquatic ecosystems, involving carbon
dioxide dissolved in the water.
Sometimes the carbon in biological molecules isn’t
recycled back to the abiotic environment for quite a
while. For example, a large amount of carbon is stored in
the wood of trees, where it may stay for several hundred
years or even longer. Coal, oil, and natural gas, called fos-
sil fuels because they formed from the remains of ancient
organisms, are vast deposits of carbon compounds—the
end products of photosynthesis that occurred millions
of years ago. In combustion, organic molecules in wood,
coal, oil, and natural gas are burned, with accompany-
ing releases of heat, light, and carbon dioxide. (The