324 ■ CHAPTER 18 General Principles of Ecology
ECOLOGY
F
lames rise from the forest floor, licking
at Jennifer Balch’s heels. Balch walks
carefully ahead of the heat, stoking it,
encouraging it. She tips a large metal container,
dripping flaming kerosene onto another pile of
dead branches on the ground (Figure 18.1). The
forest burns behind her.
Balch steps back to look at her work—the
destruction of a small patch of the Amazon rain-
forest. Balch, an ecologist at the University of
Colorado Boulder, is in the southern part of the
Amazon basin in Brazil, in the state called Mato
Grosso. There, on a small square kilometer and a
half of land belonging to a soybean farmer, Balch
is experimenting to see what happens when the
Amazon burns.
The Amazon is the largest rainforest in the
world and a critical part of our planet’s biosphere.
The biosphere consists of all of Earth’s organ-
isms, plus the physical space we all inhabit. It
includes inorganic chemicals like water, our
nitrogen-rich atmosphere, every living organ-
ism, and more (Figure 18.2). Put more simply,
the biosphere is the integration of all the envi-
ronments on Earth. It is crucial to our survival
and well-being because humans depend on the
biosphere for food and raw materials.
Within the biosphere, the Amazon is home to
more than half of the world’s millions of species
of plants and animals. It also contains one-fifth
of the world’s freshwater. Yet the Amazon is
under threat: since 1960, the human population
of the Brazilian Amazon region has increased
from 6 million to 25 million people, leading to
a dramatic expansion of agriculture, including
major deforestation as humans cut down trees
to make room for roads, cattle, and fields. The
forest cover in the Amazon has declined by
20 percent during this period, and the grass-
lands and pastures that have taken its place—
including soybean fields like the one Balch works
at—are threatening the rainforest that is left.It’s traditional for farmers in Brazil to clear
land for agriculture by setting fires to burn away
trees and brush. In addition, fire is a useful tool
for clearing out old grasses to encourage the
growth of new grasses for cattle to feed on. Yet
when these fires are not kept under control, they
can spread to nearby rainforest and destroy it.
Historically, this has not been a major concern,
because the Amazon is wet and humid, so forest
fires burned slowly, extinguishing themselves
before spreading too far. Yet a dramatic shift in
the planet’s climate is making the Amazon drier
and more vulnerable to wildfires.
Our planet is warming. As a result of global
climate change, the temperature in Earth’s
atmosphere is increasing, causing less rainfall
and more drought in the Amazon, leaving the
forests primed for fire. And they’ve begun to
burn. Even worse, that loss of trees adds to the
warming, resulting in a vicious cycle in which the
Amazon becomes hotter and drier, burns again,
and contributes to further climate change.JENNIFER BALCH
Jennifer Balch is an ecologist at the University
of Colorado Boulder who studies how fire
disturbance affects ecosystems. In 2004, she
initiated a one-of-a-kind experimental burn
study in the Amazon to see how wildfires affect
the tropical rainforest.Figure 18.1
Jennifer Balch ignites a controlled burn
in an experimental plot in the rainforest