Biology 12

Chapter 15 Human Ecology • MHR 513

Figure 15.7How many different occupations or activities
can you recognize in this representation of an ancient clay
tablet?

The same events seem to have repeated
themselves many times throughout history. In
A.D. 100, for example, Rome was a city of roughly
one million people and the centre of a great empire.
The Romans did not develop agricultural systems
sufficient to support their city. As their empire
expanded, they supported their growing population
by importing food from farther and farther afield.
Eventually, challengers to the empire were able to
cut off these supply lines and Rome collapsed. This
was due, at least in part, many historians think, to
poor environmental practices (including loss of soil
fertility in a wide area around the city).
This is one of the main ecological problems
posed by cities. Typically, cities disrupt the
biogeochemical cycles that sustain ecosystems. Large
numbers of people concentrated in urban areas
require vast amounts of food, but have little space
in which to grow it. Therefore, they must import
food, often from long distances, thus requiring the
input of extra energy (usually from fossil fuels) to
transport it. The nutrients in this food are usually
not returned to the soil in which it was grown.
They are commonly dumped in landfills and/or
pumped through sewage lines into oceans or rivers.
Unconnected to the land and knowing little about
what is involved in getting food to the store where
they buy it, most city residents are unaware of the
true ecological cost of this lifestyle.
Cities also demand many trees for fuel, paper,
and building materials, but rarely involve

themselves in reforestation activities. They require

large amounts of water, but do not typically return

the same amount of usable water to the environment.

In other words, most cities have what can be

described as a linear metabolism, in which no

attempt is made to balance inputs and outputs.

Most cities do not return nutrients to the soil, plant

new forests, or maintain the hydrologic cycle, as

shown in Figure 15.8(A).

Figure 15.8 (A)Is linear metabolism sustainable?

What is needed is a rethinking of how cities

work so that they have a more circular metabolism,

in which every output can also be used as an input.

Figure 15.8(B) shows a circular metabolism. This

pattern reduces the demand for “new” resources,

thus affecting a much smaller area.

Figure 15.8 (B)Does the city you live in or are closest to

have linear metabolism or circular metabolism?

Historically, many Chinese cities demonstrated

circular metabolism. They took great care to

replenish the soil from which they got their food.

Both animal and human wastes were returned to

the belt of farmland that surrounded the city.

Forests were replanted and water was used

carefully. This is true of many Chinese cities even

today (for example, Beijing, Shanghai, Wuhan).

B

input output

Circular metabolism

food and water

clean energy

technology

and maximum

efficiency

processed

goods

timber and

pulp from

sustainable

forests

efficient water use

and treatment plant

nutrient recycling

sulfur and nitrate

interception

recycled goods (with

minimum pollution)

reforestation

(uptake of )CO 2

A Linear metabolism

input

food and water

fuels and energy

processed goods

timber and pulp

building materials

output

sewage

exhaust gases

household and

factory wastes

(liquid and solid)

unnecessary

disposal