Environmental Engineering Reference
In-Depth Information
Lower limit
of tolerance
Upper limit
of tolerance
No
organisms
No
organisms
Few
organisms
Abundance of organisms
Few
organisms
Zone of
intolerance
Zone of
physiological stress
Optimum range
Zone of
physiological stress
Zone of
intolerance
Low
Temperature
High
Figure 3-11
Range of tolerance
for a population of organisms, such as fish, to an abiotic environmental
factor—in this case, temperature.
various inorganic minerals or salts dissolved in a
given volume of water.
Factors That Limit Population Growth
Availability of matter and energy resources can limit
the number of organisms in a population.
A variety of factors can affect the number of organisms
in a population. Sometimes one factor, known as a
lim-
iting factor,
is more important in regulating population
growth than other factors. This ecological principle, re-
lated to the law of tolerance, is called the
limiting fac-
tor principle:
To o much or too little of any abiotic factor can
limit or prevent growth of a population, even if all other fac-
tors are at or near the optimal range of tolerance.
On land, precipitation often is the limiting factor.
Lack of water in a desert limits plant growth. Soil nu-
trients also can act as a limiting factor on land. Suppose
a farmer plants corn in phosphorus-poor soil. Even if
water, nitrogen, potassium, and other nutrients are at
optimal levels, the corn will stop growing when it uses
up the available phosphorus.
Similarly, too much of an abiotic factor can be lim-
iting. For example, providing excess water or fertilizer
can kill plants—both common mistakes made by
many beginning gardeners.
Important limiting factors for aquatic life zones in-
clude temperature, sunlight, nutrient availability, and
dissolved oxygen (DO) content
—the amount of oxy-
gen gas dissolved in a given volume of water at a par-
ticular temperature and pressure. Another limiting
factor in aquatic life zones is
salinity
—the amounts of
Biological Components of Ecosystems:
Producers, Consumers, and Decomposers
Some organisms in ecosystems produce food, while
others consume food.
The earth's organisms either produce or consume
food.
Producers,
sometimes called
autotrophs
(self-
feeders), make their own food from compounds ob-
tained from their environment.
On land, most producers are green plants. In
freshwater and marine life zones, algae and plants are
the major producers near shorelines. In open water,
the dominant producers are
phytoplankton
—mostly mi-
croscopic organisms that float or drift in the water.
Most producers capture sunlight to make complex
compounds (such as glucose, C
6
H
12
O
6
) by
photosyn-
thesis.
Although hundreds of chemical changes take
place during photosynthesis, the overall reaction can
be summarized as follows:
carbon dioxide
water
solar energy
glucose
oxygen
6 CO
2
6 H
2
O
solar energy
C
6
H
12
O
6
6 O
2
(See Science Supplement 4 at the end of this topic for
information on how to balance chemical equations
such as this one.)
