Biology 12

Chapter 15 Human Ecology • MHR 533

however, believe that humans have played a
significant role in causing the disappearance of
other species during the past 50 000 years. They
point to the fact that during this geologically recent
time period it has been mainly conspicuous species
of mammals, reptiles, and flightless birds that have
disappeared. This suggests that human hunters
have played a critical role in their demise. Other
circumstantial evidence also points to the
involvement of humans. For example, in places
where the time of human arrival can be
determined, most extinctions occurred shortly
thereafter and among species humans would likely
have hunted, as Figure 15.23 illustrates.
The list of endangered species (those at risk of
extinction) is long and growing. The International
Council for the Preservation of Birds, for example,

Figure 15.23In many locations, the extinction rate of many
species increased dramatically after the arrival of humans.
Why do you think the rate of extinction was more or less
constant in Africa over this same time period?

estimates that almost 2000 species of birds have

become extinct over the past 2000 years, most as a

result of human activity. Comparable numbers for

smaller, less conspicuous organisms are not

available; experts predict that many species of

insects went (and are going) extinct even before

we discovered and named them, particularly in

tropical rain forests.

Besides the ethical and moral reasons to conserve

species, we constantly discover new drugs and

other beneficial chemicals produced by species

previously unknown to us. In many parts of the

world, scientists are racing to learn all they can

about species before they disappear. More

importantly, the health of most ecosystems depends

on the existence of a diversity of species, each

contributing something to the myriad of

interactions that sustain the biosphere.

The Problem: Is It Population

Growth or Consumption?

The global human population has increased

dramatically during the last 100 years. So too has

the disparity in wealth among different segments of

the population. In industrialized countries, per capita

consumption of all types of goods and resources (as

calculated by United Nations staff) has increased

steadily at a rate of about 2.3 percent annually.

While the increase has occurred more slowly in

less developed nations, per capita consumption has

actually declined in some: the average household

in some parts of Africa, for example, consumes

20 percent less today than it did 25 years ago. Nearly

60 percent of the 4.4 billion people in developing

countries lack basic sanitation and about 30 percent

do not have access to clean water. The inequalities

in consumption among different parts of the world

(and even within countries or regions) are greater

than the differences in fertility rates. The richest

20 percent of the human population lives in the

highest-income countries and consumes 45 percent

of all meat and fish, 58 percent of the world’s total

energy supply (see Figure 15.24 on page 534), and

84 percent of all paper, and owns 87 percent of the

world’s cars. By comparison, the poorest 20 percent

of the global population eats five percent of all

meat and fish, uses less than four percent of the

total energy, consumes 1.1 percent of all paper, and

owns less than one percent of all cars in the world.

100 000 10 000 1000 100

50

0

100

50

0

100

50

0

100

Percent of species

Years ago

50

0

100

Madagascar and

New Zealand

North America

Australia

Sharp declines coinciding with arrivals of humans

Gradual declines with humans

present for whole period

Africa