Essentials of Ecology

SUPPLEMENT 2 S7

2. 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

1. 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

1. 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?


2. 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)