Biology 12

518 MHR • Unit 5 Population Dynamics

this to a much larger population should give you

some insight into the difficulties involved in

determining carrying capacity, even when only

one variable is considered as a limiting factor.

Carrying Capacity:

A Challenging But Useful Concept

We have not yet come up with a foolproof method

of estimating the carrying capacity of Earth for

humans. This is partly because it depends not only

on environmental variables (in ways we do not yet

fully understand), but also on humans themselves

— technology we develop or accept, political

institutions, cultural values, and many other factors

(including individual and group choices about

how resources can and should be distributed). It

depends, for example, on how many of us eat meat,

eat grains, drive cars, ride bicycles, and a multitude

of other things.

Some people have suggested that rather than

trying to determine the global carrying capacity, we

should concentrate on estimating local, national, or

regional limits to growth. Although it may be easier

to define the extent of certain resources (such as

minerals or water) on a smaller scale, global events

or forces may still affect these resources. Human

carrying capacity cannot be defined for one country

alone if that country trades with other nations and

shares global resources such as the atmosphere,

oceans, and biodiversity.

Many of those who have attempted to determine

the human carrying capacity have provided an upper

and a lower estimate. Taking into consideration

only the upper estimates in these cases and all

single estimates developed by other researchers,

the median (middle value) of 65 separate estimates

of the maximum size of the human population made

before 1996 is roughly 12 billion people. If the lower

conservative values are considered along with all

single values, the median of these 65 estimates is

7.7 billion. The Population Division of the United

Nations predicts that the world’s population will

be roughly 7.8 billion — just above this lower

value — in 2025 and about nine billion in 2050.

Although calculating the median value of a wide

range of historical estimates does not give us the

actual carrying capacity, this analysis does provide

CONCEPT ORGANIZER Factors Influencing Earth’s Carrying Capacity

Interactions among

organisms

(Chapter 13,

sections 13.2, 13.3, 13.4)

Limits to population growth

(Chapter 14,

sections 14.3, 14.4)

Growth of the

human population

(Chapter 15, section 15.1)

Carrying capacity of

Earth for humans

(Chapter 15, section 15.2)

Figure 15.13Factors that influence the carrying capacity of Earth for humans.

The survival and reproduction of individuals depends on

interactions among species and between living things and

the abiotic components of their environment. All sorts of

interactions play a role in determining whether a population’s

size will grow, decline, or remain stable. These interactions

occur between organisms at different trophic levels (such

as between a predator and its prey), those at the same

trophic level (for example, competitive interactions or mating),

and those between organisms and their physical world. For

example, do climatic conditions promote survival? Are there

toxins present? As is true for all species, human populations

face both abiotic (storms, floods) and biotic (limited food

supplies, disease outbreaks) limits to their potential growth.

Determining the ultimate limit to the growth of the global

human population — the carrying capacity of Earth for

Homo sapiens— is difficult, if not impossible. This is true

for many reasons. One of the most important relates to the

fact that humans, perhaps more than any other species,

constantly change their environment in ways that both

increase (for example, improving food crops and the

efficiency of resource use) and decrease (polluting the

air and water) this carrying capacity.