Biology Now, 2e

330 ■ CHAPTER 18 General Principles of Ecology

ECOLOGY

Fire and Water

During her first burn in 2004, Balch was

surprised to see that the plants in the two exper-

imental plots were fairly resistant to fire. The

Amazon’s tall, dense tree canopy creates humid

air below it, protecting the forest from most fires,

which tend to burn slowly under such condi-

tions. For this reason, Balch’s experimental plots

initially suffered little damage.

That wasn’t true in the following years.

Remember that the Amazon saw a major

drought in 2005, and droughts cause tree death,

allowing more sunlight to reach the forest floor

and dry the leaves and shrubs there, priming

the forest for fire (Figure 18.10). During the

planned burns after the drought in 2005, the two

experimental plots experienced major damage.

The majority of the trees on the plots died, and

grasses took their place—particularly at the

forest edge, where it was hottest and driest.

In both the 1-year and the 3-year burned plots,

grasses invaded from adjacent pasturelands.

This experimental result suggests that wild-

fires do indeed push the Amazon toward a drier,

The red circles are direct

measurements at the Mauna

Loa Observatory in Hawaii, at

11,135 feet above sea level.

The green circles indicate CO 2

levels measured from bubbles

of air trapped in ice that formed

many hundreds of years ago.

200

250

300

350

400

450

0

Year

Atmospheric CO

concentration (ppm) 2

100011001200130014001500160017001800190020002100

2016: 404.21 ppm

Figure 18.8

Atmospheric CO 2 levels are rising rapidly

Atmospheric CO 2 levels (measured in parts per million, or ppm) have

increased greatly in the past 200 years.

Q1: What measurements do the green circles represent?

Q2: What measurements do the red circles represent?

Q3: For approximately how many years has the Mauna Loa

Observatory been recording CO 2 levels?

Average global temperature 1880–2016

Temperature (

°F)

1880 1900 1920 1940 1960 1980 2000 2020

56.5

0

57.0

57.5

58.0

58.5

59.0

Figure 18.9

Global temperatures are on the rise

Average global temperature has increased greatly over the 140 years since it

has been recorded.

Q1: In what years were global temperatures the lowest?

Q2: In what years were global temperatures the highest?

Q3: What trend is apparent in this graph of actual global

temperatures?

Figure 18.10

Before and after the Amazon drought

of 2005

The drought devastated the forest to this extent

over much of the forest’s range.