HUMAN BIOLOGY

474 Chapter 25

Why is there an upper limit to the number of

humans the earth’s resources can support?

• Earth’s resources are not unlimited, so over the long term
  given areas can only support a finite number of individuals of
  any species, including humans.


• Disease organisms and effects of pollution also limit
  population growth.

taKe-Home messaGe

carrying capacity The
number of individuals of a
species that can be sus‑
tained indefinitely by the
resources in a given area.

limiting factor Any factor
that limits the growth of a
population, such as the sup‑
ply of food or the availability
of living space.

Once such factors take their toll on a population and its

density decreases, the pressures ease and the population

may grow once more.

A classic example is the bubonic plague that killed

sixty million Asians and Europeans—about one­third of

the population—during the Middle Ages. Yersinia pestis,

the bacterium responsible, normally lives in wild rodents,

and fleas transmit it to new hosts. It spread like wildfire

through the cities of medieval Europe because dwellings

were crowded together, sanitation was poor, and rats were

everywhere. In 1994, bubonic plague and a related disease,

pneumonic plague, raced through rat­infested cities in

India where garbage and animal carcasses had piled up for

months in the streets. Only crash efforts by public health

officials averted a public health crisis.

Density-independent controls can also operate. These are

events such as floods, earthquakes, or other natural disas­

ters that cause deaths regardless of whether the members

of a population are crowded or not.

n The human population can’t continue to grow faster and

faster. Even when conditions are ideal, there is a maximum

rate at which any population can grow.

Human populations can grow at a maximum rate of 2 to

5 percent per year. The rate is determined by how soon

people begin to reproduce, how often they reproduce, and

how many offspring are born each time.

Even when a population isn’t reproducing at its full

potential, it can grow exponentially (in doubling increments

from 2 to 4, then 8, 16, and so on). For instance, it’s biologi­

cally possible for human females to bear twenty children

or more, but few do so. Yet since the mid­1700s, our popu­

lation has been growing exponentially. This rapid growth

can’t continue forever, however, because “Mother Nature”

doesn’t work that way.

there is a limit on how many people

earth can support

Environmental factors prevent most populations of organ­

isms from reaching their full biotic potential. For instance,

when a basic resource such as food or water is in short sup­

ply, it becomes a limiting factor on growth. Other kinds

of limiting factors include predation (as by pathogens) and

competition for living space.

The concept of limiting factors is important because it

defines the carrying capacity—how many individuals of

a species that the resources in a given area can sustain on an

ongoing basis. Some experts believe that Earth’s resources

can support from seven to twelve billion humans, with a

reasonable standard of living for many. Others believe that

the current human population of seven billion is already

exceeding Earth’s carrying capacity. Both viewpoints share

the premise that overpopulation is the root of many, if not

most, of today’s environmental problems.

A low­density population grows slowly at first, then

grows more rapidly. Growth levels off once the carrying

capacity is reached. This pattern, called logistic growth,

gives us an S­shaped curve (Figure 25.4). This curve is a

simple approximation of what goes on in nature.

some natural population

controls are related to

population density

When a growing population’s den­

sity increases, the high density and

overcrowding result in competition

for resources. They also put individ­

uals at increased risk of being killed

by infectious diseases and parasites,

which are more easily spread in crowded living conditions.

These are density-dependent controls on population growth.

Number of individuals

TIMEA BCD E

new carrying capacity

initial carrying capacity

25.2 Nature’s Controls on Population Growth

Figure 25.4 The S-shaped curve of

logistic growth. The curve flattens out

as the carrying capacity is reached.

Changed environmental conditions

can reduce the carrying capacity. This

happened to the human population in

Ireland in the late 19th century, when a

fungus wiped out potatoes, which were

the mainstay of the diet for all but the

wealthy. An estimated one million

people starved to death and half a

million more emigrated to the United

States and elsewhere in search

of a better life. Right: Sculpture

commemorating Irish immigration

in the 19th century. (Keith Murphy/

Shutterstock.com)

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