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a given species that can be supported
indefinitely by the local environment.
K is defined by limiting factors—limited
amounts of space, limited
amounts of food and water, competition
from other species and other
threats.
You may remember the ominous
graphic representation of carrying
capacity from your introductory biology
course (see illustration). The
number of individuals erupts as the
species exploits an area's supply of
food and space, reproducing unchecked.
As resources run out, the
population first plateaus, then plummets
sickeningly as individuals die,
eventually stabilizing at a number
just below the almighty K.
Where does Pimentel put us on
the graph of Earth's human population?
"Right now, while we still have
relatively abundant fossil fuel energy,
I would say that we're getting near
the inflection—the point where the
sharp rise in population begins to
flatten off."
Sharp rise, indeed. Almost 5.6
billion people now live on the planet.
The population is projected to reach
nearly 8.4 billion by the year 2025
and 15 billion by 2100. Every day the
total rises by a quarter-million. The
relative abundance of young people
in their reproductive prime gives the
world population explosion surprising
momentum. "Even if we adopt a
zero population growth strategy tomorrow—a
little over two children
per couple—the world population
will nearly double by 2060," explains
Pimentel. "It wouldn't stop growing
for about 60 years."
According to the Cornell team,
the primary limiting factors that define
the Earth's carrying capacity for
humans are fertile land, fresh water,
fossil fuel energy and a diversity of
helpful natural organisms. All are
essential in the production of food,
and worldwide stocks of all four are
being consumed faster than they can
be replaced.
More than 25 million acres of
arable and pasture land, the source
of 98 percent of the world's food, are
critically degraded and abandoned
each year. An additional 12.5 million
acres of new fertile land must be put
into production to feed the 92 million
new mouths added each year to
RESEARCH
the world population. One and a quarter
acres of good crop land are needed
to provide a diverse, nutritious diet
of plant and animal products for one
person. The current world average
of crop land per capita is just over
two-thirds of an acre.
Fresh water supplies are being
overdrawn from surface and groundwater
sources. The primary consumer:
agriculture, which absorbs,
without possibility of recovery, 87
percent of the fresh water used each
year in the world. "It takes 1,400
pounds of water to produce one
pound of food," says Pimentel.
Fossil energy from oil, gas and
coal may be the first limiting resource
to disappear. Essential for industrial
production, fuel, construction, heating
and cooling, packaging, delivery
of clean water and fertilizer manufacture—about
319 quads (that's
10,000,000,000,000,000 BTUs in airconditionerese)
of fossil fuel are used
worldwide each year (one-fourth of
which is burned in the United States).
The Cornell team cites research
suggesting that world supplies of oil
and gas should last about 35 years at
current pumping rates, coal and uranium
stocks about 100 years.
The easiest natural resource to
overlook is provided by the living
organisms that share the planet with
us. "Humans have no technologies
that can substitute for the services
provided by wild biota," says
Pimentel. "There are about half a
million species of animals, plants, and
microbes that provide essential functions
for humans in the United
States." Large-scale food production
would be impossible without pollinators,
decomposers, scavengers and
waste recyclers. Biodiversity also
ensures a future gene bank for
tomorrow's forestry and agriculture.
Researchers estimate that 150 species
are lost daily due to human activity.
(See "Biodiversity: What's in it
for Us?" November 1993, Cornell
Magazine.,)
In order to arrive at their optimum
population figures, Pimentel
and his colleagues make the optimistic
assumption that humanity will
make maximum use of Earth's finite
resources. Even if humans make a
transition to renewable energy
sources, stop polluting and degrad-
CORNELL MAGAZINE
18
ing their environment and accept a
standard of living equal to one-half of
that enjoyed by Americans today, the
numbers stay the same.
Getting there will be painful,
Pimentel admits, but achievable.
Population reduction will require a
growing proportion of elderly to be
supported by a shrinking number of
younger, more productive humans.
"It will create economic and social
stress when we have to make these
changes," he acknowledges. "But
our alternative is far worse economic
and social stress." If global human
fertility can be reduced from the
current rate of 3.3 children per female
to 1 to 1.5 children per female,
Pimentel estimates that the population
can be reduced to carrying capacity
in 100 to 150 years.
Population control presents ethical
problems. "You have to take your
choice," explains Pimentel. "People
say that they should have the freedom
to reproduce. I'm sympathetic
to that view. But you're going to either
lose some of that freedom, or
your children and your grandchildren
will lose some of their freedoms—
freedom from starvation, freedom
from disease."
Pimentel wonders about critics
(rumored to include high-voltage media
commentator Rush Limbaugh)
who claim that technology will allow
humanity to keep pace with a steep
population curve. "Look at fishery
production. We built bigger ships,
larger nets, and now the fish populations
of the oceans, lakes and rivers
are lower than they've been since
1970. Look at the Colorado River. As
it flows south, California, Arizona,
and Colorado take a big piece out of
it to support their populations. By the
time that river reaches Mexico, it's
dry. What technology do we have
available, short of manipulating the
climate, that can double the flow of
the Colorado River?"
And how does Pimentel respond
to those who say that the endless,
painful cycle of population booms and
busts is nature's way? "I think that
we're too intelligent to let nature
control our numbers," he says, ultimately
the optimist. "I think that we
can do a more effective job of limiting
our numbers than nature can."
—HillelJ. Hoffmann '85