From Carrying Capacity to Footprint, and Back Again
Michael Cain
Michael Cain is an adjunct professor of ecology and evolutionary
biology at Bowdoin College. His research specialty is the ecology
and evolution of plants. He has published over 30 professional
articles and is the co-author of Discover Biology (2nd ed, 2002),
a college textbook for non-science majors. He will also be a coauthor in future editions of my books on environmental science.
For the populations of most species, the traditional idea of a carrying capacity makes
sense: We can think of a population’s carrying capacity as the maximum number of
individuals a given area can support. But this way of thinking does not work well for
people.
To see why, imagine what a city dweller might do on a Saturday morning. On
waking, the person might take a shower, have a cup of coffee, listen to music on the
radio, and eat breakfast. These activities depend on resources located outside of the
city. The shower water may be piped-in from far away. The coffee was most likely
grown in another country, then transported to the city using fossil fuels from yet
another country. The radio was made using materials derived from other regions of
the world. So too with other aspects of life on a Saturday morning: Virtually
everything that is used—energy, food, clothing, and household goods—originates
from outside the city.
As you can see, cities depend on distant lands and waters for their very
existence. As a result, the ecological location of a city is not the same as its
geographic location. To account for the energy and resources that we obtain from
places other than where we live, ecologists William Rees and Mathis Wackernagel
turned the carrying capacity concept on its head: Instead of asking how many people
a given area can support, they asked how much area is required to support a given
population. They called this area the “ecological footprint” of a population.
As defined by Rees and Wackernagel, an ecological footprint estimates the area
of land and water ecosystems that are required to produce the resources consumed
and to assimilate the wastes generated by a population, no matter where those lands
or waters are located. Ecological footprints can be estimated for any human
population, from a village to a nation to the entire planet. Ecological footprints are
Copyright ©2005 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning
is a trademark used herein under license.
based on statistics on agricultural output, production of goods, energy use, resource
use (e.g., timber, fish), rates of resource regeneration (e.g., timber growth),
pollution, and population size. Once these statistics are obtained, the land or water
required to support a population’s activities can be estimated.
To do this, a list is made of major items that are consumed, such as grain,
timber, energy, and other key resources. Next, the footprint required for each item is
calculated as:
A = C/Y,
where “A” is the ecological footprint of the item (in hectares), C is the total
consumption of the item (in kilograms), and Y is the yield of the item (kilograms
produced per hectare of land). For example, the domestic consumption of grain in
Canada in 1993 was 25,486,700,000 kg. The average yield was 2,744,000 kg/ha,
resulting in an ecological footprint of 9,288,156 ha of arable land. Since there were
28,817,000 Canadians in 1993, the per person ecological footprint resulting from
grain consumption was 0.32 ha. Similar estimates can be made for other items that
people use. Finally, all such estimates are summed to yield the total ecological
footprint of the population.
Estimates of our ecological footprint are sobering: People appear to be using
resources more rapidly than they can be regenerated. For example, in 2000, our per
capita ecological footprint (2.2 ha/person) was greater than the per capita amount of
biologically productive land available (1.9 ha/person). The 1.9 ha/person value is for
an “average” person—the per capita ecological footprint is much higher in some
countries (9.6 ha/person in the USA) and much lower in others (0.5 ha/person in
Bangladesh). On a global basis, rich countries (with high ecological footprints) use
large amounts of resources obtained from poor countries (with low ecological
footprints); this leaves people in poor countries with less than their fair share of
resources and more than their fair share of environmental damage.
In 2000, our total global footprint of 13.3 billion hectares was greater than the
11.5 billion hectares of productive land available. This sounds impossible, but it can
happen for short periods of time by using resources more rapidly than they are
regenerated. We cannot do that for long (resources will run out), so ultimately our
footprint cannot exceed the amount of land available. We can use this fact to
estimate how many people Earth can support. For example, given the amount of
productive land available in 2000 (a total of 11.5 billion ha), our per capita ecological
footprint (2.3 ha/person) suggests that the carrying capacity of the world’s human
Copyright ©2005 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning
is a trademark used herein under license.
population at that time was 5 billion people (11.5 billion ha divided by 2.3
ha/person)—greater than the world’s population of 6 billion people in 2000.
Of course, estimates of the carrying capacity of the human population depend
on patterns of resource use. If each person on Earth had consumed resources at the
level of an average person in the USA, the carrying capacity of the world’s human
population in 2000 would have been 1.2 billion people—only 20% of the world’s
actual population (6 billion)! On the other hand, had everyone used resources as in
Bangladesh, the carrying capacity would have been nearly 23 billion people.
By providing a snapshot of whether current human activities are sustainable,
footprint analyses make it clear that the choices we make as individuals and as
societies matter. With respect to individual choices, at the website www.earthday.net
you can explore how the choices you make affect your personal ecological footprint.
You may find that you could reduce your footprint by 50% or more if you were to
make such choices as driving less, living in a smaller home, buying food grown
locally, and eating less meat.
In terms of social issues, the good news is that we can reduce our total and per
capita ecological footprints and still continue to raise the standard of living for the
world’s many poor people. This may sound like a pipe dream but it can happen if (1)
our global population size stabilizes, then slowly decreases; (2) people in wealthy
nations engage in “smart consumption” (consume less, buy energy-efficient
products, buy few disposable products, buy local products when possible); (3)
societies invest heavily in sustainable forms of energy and in technologies that
increase the efficiency of resource use; and (4) the earth’s lands and waters are
protected so that their ability to supply resources and assimilate wastes is not
reduced. Reaching these goals will be a monumental and exciting challenge.
Critical Thinking
Do you believe that we can reduce our total and per capita ecological footprints using
the four strategies outlined by the author by 2050? List three major consequences of
not achieving these goals. Do you plan to reduce your own ecological footprint?
How?
Copyright ©2005 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning
is a trademark used herein under license.