Chapter 18: Interactions of Living Things

Figure 11 Population density
can be shown on a map. This map
uses different colors to show varying
densities of a population of
northern bobwhites, a type of bird.
Topic: Human Population
Visit green.msscience.com for Web
links to information about human
population and densities.
Activity Select at least three
different areas of the world and
prepare a bar graph to compare
population density of each area.
Compare the population density
of where you live to the three
areas of the world you select.
Figure 12 Some populations,
such as creosote bushes in the
desert, are evenly spaced throughout
an area.
Pacific
Ocean
540 CHAPTER 18 Interactions of Living Things
Dan Suzio/Photo Researchers
Average Count per km 2
< 1
1–3
4–10
11–30
31–100
> 100
C A N A D A
U N I T E D S T A T E S
Atlantic
Ocean
Population Density At the beginning of this chapter, when
you figured out how much space is available to each student in
your classroom, you were measuring another population characteristic.
The number of individuals in a population that
occupy a definite area is called population density. For example,
if 100 mice live in an area of one square kilometer, the population
density is 100 mice per square kilometer. When more individuals
live in a given amount of space, as seen in Figure 11, the
population is more dense.
Population Spacing Another characteristic of populations is
spacing, or how the organisms are arranged in a given area. They
can be evenly spaced, randomly spaced, or clumped together.
If organisms have a fairly consistent distance between them,
as shown in Figure 12,
they are evenly spaced.
In random spacing, each
organism’s location is independent
of the locations of
other organisms in the population.
Random spacing of
plants usually results when
wind or birds disperse seeds.
Clumped spacing occurs
when resources such as food
or living space are clumped.
Clumping results when animals
gather in groups or
plants grow near each other
in groups.