Environmental Engineering Reference
In-Depth Information
Active Figure 6-10
Natural capital:
natural restoration of
disturbed land.
Secondary ecological succession
of plant
communities on an abandoned farm field in North
Carolina. It took 150-200 years after the farmland was
abandoned for the area to become covered with a
mature oak and hickory forest. A new disturbance
such as deforestation or fire would create condi-
tions favoring pioneer species. In the absence
of new disturbances, secondary succession
would recur over time, but not necessarily in
the same sequence shown here.
See an ani-
mation based on this figure and take a short
quiz on the concept.
Mature oak-hickory forest
Young pine forest
with developing
understory of oak
and hickory trees
Shrubs
and pine
seedlings
Perennial
weeds and
grasses
Annual
weeds
contrast, the size of a population that is dominated by
individuals past their reproductive stage is likely to
decrease. The size of a population with a fairly even
distribution between the three reproductive stages will
likely remain stable because the reproduction by
younger individuals will be roughly balanced by the
deaths of older individuals.
6-5 POPULATION DYNAMICS AND
CARRYING CAPACITY
Changes in Population Size: Entrances
and Exits
Populations increase through births and immigration
and decrease through deaths and emigration.
Four variables—
births, deaths, immigration,
and
emigra-
tion
—govern changes in population size. A population
increases by birth and immigration and decreases by
death and emigration:
Limits on Population Growth
No population can increase its size indefinitely
because resources such as light, water, and nutrients
are limited and competitors or predators are usually
plentiful.
Populations vary in their capacity for growth, also
known as the
biotic potential
of a population. The
in-
trinsic rate of increase (
r
)
is the rate at which a popula-
tion would grow if it had unlimited resources. Most
populations grow at a rate slower than this maximum.
Some species have an astounding biotic potential.
Without any controls on their population growth, the
descendants of a single female housefly could total
about 5.6 trillion flies within about 13 months. If this
rapid exponential growth continued, within a few
years flies would cover the earth's entire surface!
Fortunately, this is not a realistic scenario because
no population can increase its size indefinitely.
In the real
Population change
(Births
Immigration)
(Deaths
Emigration)
A population's
age structure
can have a strong ef-
fect on how rapidly its size increases or decreases. Age
structures are usually described in terms of organisms
that are not mature enough to reproduce (the
pre-
reproductive age
), those that are capable of reproduc-
tion (the
reproductive stage
), and those that are too old
to reproduce (the
post-reproductive stage
).
The size of a population that includes a large pro-
portion of young organisms in their reproductive stage
or that will soon enter this stage is likely to increase. In
