SUPPLEMENT 2 S7
- What is the most productive of aquatic eco-
systems shown here? What is the least pro-
ductive?
An important application of the bar graph
used in this book is the age structure diagram (Fig-
ure 10, p. S8), which describes a population by
showing the numbers of males and females in
certain age groups (see pp. 130–132). Environ-
mental scientists are concerned about human
population growth, and one of the key factors
determining a particular population’s growth
rate is the relative numbers of people in various
age categories.
In particular, the number of women of child-
bearing age and younger gives an important clue
to whether the population might grow rapidly.
If most of the women fall into that category, the
population will grow much more quickly than
it would if most of the women are beyond their
child-bearing years. Likewise, a population with
most of its women beyond childbearing age will
likely shrink in future years.
net primary productivity (or NPP, a measure of
chemical energy produced by plants in an eco-
system) for different ecosystems, as represented
in Figure 9.
In most bar graphs, the categories to be
compared are laid out on the x-axis, while the
range of measurements for the variable under
consideration lies along the y-axis. In our ex-
ample in Figure 9, the categories (ecosystems)
are on the y-axis, and the variable range (NPP)
lies on the x-axis. In either case, reading the
graph is straightforward. Simply run a line per-
pendicular to the bar you are reading from the
top of that bar (or the right or left end, if it lies
horizontally) to the variable value axis. In Figure
9, you can see that the NPP for continental shelf,
for example, is close to 1,600 kcal/m^2 /yr.
Questions
- About how many times greater is the NPP
in a tropical rain forest than the NPP in a
savannah?
to the independent and dependent variables is
to show them on two separate curves. This is
useful when an experiment takes place over a
long period of time, as did the Hubbard Brook
experiment. In Figure 8, the years in which this
experiment was conducted appear on the x-axis.
The range of values for presence of a soil nutri-
ent called nitrate appears on the y-axis. And two
curves were plotted: one showing the values of
the dependent variable in the uncut forest (the
control site), and the other showing the values
of the dependent variable in the clear-cut forest
(the experimental site).
Questions
- Approximately what was the maximum
amount of nitrate lost from the undisturbed
(control) forest? Approximately what was
the maximum amount of nitrate lost from
the clear-cut (experimental) forest? At the
point of maximum nitrate loss from the ex-
perimental forest, about how many times
more nitrate was lost there than in the con-
trol forest? - In what year do you think the experimental
forest was cut? How long did it take, once
nutrient loss started there, for the losses to
reach their maximum? How long did it take
for the forest to regain its pre-experimental
level of nutrients?
Bar Graphs
The bar graph is used to compare measurements
for one or more variables across categories. Un-
like the line graph, a bar graph typically does
not involve a sequence of measurements over
time or distance. The measurements compared
on a bar graph usually represent data collected
at some point in time or during a well-defi ned
period. For instance, we can compare the
20
40
60
Year
Nitrate (NO
- 3
) concentration
(milligrams per liter)
1964
Disturbed
(experimental)
watershed
Undisturbed
(control)
watershed
1963 19651966196719681969197019711972
Figure 8 Loss of nitrate ions from a
deforested watershed (upper curve),
mostly due to precipitation wash-
ing away the nutrients, compared
with loss of nitrate ions from an
undisturbed forest (lower curve).
(Data from F. H. Bormann and
Gene Likens)
Swamps and marshes
Tropical rain forest
Temperate forest
Northern coniferous forest (taiga)
Savanna
Agricultural land
Woodland and shrubland
Temperate grassland
Tundra (arctic and alpine)
Desert scrub
Extreme desert
Aquatic Ecosystems
Estuaries
Lakes and streams
Continental shelf
Open ocean
Terrestrial Ecosystems
800 1,600 2,400 3,200 4,000 4,800 5,600 6,400 7,200 8,000 8,800 9,600
Average net primary productivity (kcal/m^2 /yr)
Figure 9
Estimated annual
average net pri-
mary pro ductivity
(NPP) in major
life zones and
ecosystems,
expressed as
kilocalories of
energy produced
per square meter
per year (kcal/m^2 /
yr). (Data from
R. H. Whittaker,
Communities and
Ecosystems, 2nd
ed., New York:
Macmillan, 1975)