Essentials of Ecology

S6 SUPPLEMENT 2

on the Hubbard Brook experiment in which sci-

entists measured changes over time in the pres-

ence of soil nutrients in a forest (the dependent

variable) in response to removal of trees from

the forest (the independent variable).

On a line graph, the range of values for

the independent variable is usually placed

on the x-axis, while range of values for the

independent variable usually appears on the

y-axis (for example, see Figure 17-14, p. 455).

However, another way to represent changes

Finally, but no less important, a common

scientifi c use of the line graph is to show experi-

mental results. Usually, such graphs represent

variables, which are factors or values that can

change. Experimenters measure changes to a

dependent variable—a variable that changes in re-

sponse to changes in another variable called the

independent variable. The latter may be manipu-

lated by experimenters in order to cause changes

in the dependent variable. For example, in the

Core Case Study of Chapter 2 (p. 28), we report

representing the same data can look quite differ-
ent. For example, in Figure 6, the upper graph
shows the growth of the human population
from 8000 B.C. to the present, while the lower
graph shows human population growth between
1950 and 2010. The latter would be only a small
segment on the right end of the curve in the
upper graph.

Questions

1. What would be your overall impression
   of human population growth if you saw
   only the upper graph? What would be your
   overall impression if you saw only the lower
   graph?


2. On the upper graph, mark what you would
   estimate to be the left and right ends of
   the segment of the curve that fall between
   1950 and 2007. Why do you think the slope
   (steepness) of this segment varies so much
   from the slope of the curve in the lower
   graph? Describe the differences between the
   two graphs that might explain this difference
   in slopes.


3. You can see from these graphs that by ad-
   justing a graph’s time span and the height of
   its y-axis, you can change the slope of the
   curve. This can give the reader a different
   first impression of the data. Does this make
   the changed graph inaccurate or somehow
   wrong? Explain. What does this tell you
   about what you need to look for when read-
   ing a graph?
   It is also important to consider what aspect of
   a data set is being displayed on a graph. The cre-
   ator of a graph can take two different aspects of
   one data set and create two very different look-
   ing graphs that would give two different impres-
   sions of the same phenomenon. For example,
   we must be careful when talking about any type
   of growth to distinguish the question of whether
   something is growing from the question of how
   fast it is growing. While a quantity can keep
   growing continuously, its rate of growth can go
   up and down.
   One of many important examples of growth
   used in this book is human population growth.
   Look again at Figure 6. The two graphs in this
   fi gure give you the impression that human
   population growth has been continuous and
   uninterrupted, for the most part. However con-
   sider Figure 7, which plots the rate of growth of
   the human population since 1950. Note that all
   of the numbers on the y-axis, even the smallest
   ones, represent growth. The lower end of the
   scale represents slower growth, the higher end,
   faster growth.

Questions

1. If this graph were presented to you as
   a picture of human population growth,
   what would be your first impression? Do
   you think that reaching a growth rate of
   0.5% would relieve those who are con-
   cerned about overpopulation? Why or why
   not?


2. As the curve in Figure 7 proceeds to the right
   and downward, what do you think will hap-
   pen to the curve in the lower graph in Fig-
   ure 6? Explain.

A

0

1

2

3

4

5

6

7

8

9

10

11

12

13

2–5 million

years

8000 6000 4000 2000

A. D.

2000 2100

Time

Billions of people

Black Death—the Plague

Industrial revolution

B. C.

Hunting and

gathering

Agricultural revolution Industrial

revolution

?

Billions of people

B

0

1

2

3

4

5

6

7

8

1950 1960 1970 1980 1990 2000 2010

Figure 6 Two graphs show-

ing human population growth.

The upper graph spans the time

between 8000 B.C. and 2100

A.D. The lower graph runs from

1950 A.D. to 2010.

0.0

0.5

1.0

1.5

Average annual global growth rate (percent)

2.0

2.5

1950 1970 1990

Year

2010 2030 2050

Figure 7 Annual growth rate in

world population, 1950–2007,

with projections to 2050. (Data

from U.N. Population Division and

U.S. Census Bureau)