Page 96 - Environment: The Science Behind the Stories

Figure 4.2 Competition may
force an organism to play a
more limited ecological role
or use fewer resources than
it would in the absence of its
Species 1 competitor. With no com-
Species 1 Species 2 petitors (a), an organism can
exploit its full fundamental niche.
When competitors restrict what
Resource 2 Resource 2 Species 3 an organism can do or what
resources it can use (b), the

Resource 1 Resource 1 organism is limited to a realized
niche, which covers only part of
(a) Fundamental niche (b) Realized niche its fundamental niche.

exclude another species from resource use entirely. This out- partitioning because the species partition, or divide, the
come, called competitive exclusion, occurred in parts of the resources they use in common by specializing in different
Great Lakes as zebra mussels displaced native mussels—and ways (Figure 4.3).
it is happening now as quagga mussels displace zebra mussels. Resource partitioning can lead to character displacement,
Alternatively, if neither competitor fully excludes the in which competing species come to diverge in their physical
other, the species may continue to live side by side. This characteristics because of the evolution of traits best suited to
result, called species coexistence, may produce a stable point the range of resources they use. For birds that specialize on
of equilibrium, at which the relative population sizes of each eating larger seeds, natural selection may favor the evolution of
remain fairly constant through time. larger bills that enable them to make best use of this resource,
Coexisting species that use the same resources tend to whereas for birds specializing on smaller seeds, smaller
adjust to their competitors to minimize competition with bills may be favored. This is precisely what extensive recent
them. Individuals may do this by changing their behavior so research has revealed about the finches from the Galápagos
as to use only a portion of the total array of resources they are Islands that were first described by Charles Darwin (p. 68).
capable of using. In such cases, individuals do not fulfill their
entire niche. A species’ niche reflects its use of resources and
its functional role in a community, including its habitat use,
food consumption, and other attributes (p. 79)—it is a kind of Yellow-bellied
multidimensional summary of everything an organism does. sapsucker drills
rows of holes
The full niche of a species is called its fundamental niche and consumes
(Figure 4.2a). An individual that plays only part of its role or White-breasted sap and insects
nuthatch climbs
uses only some of its resources because of competition or down trunk stuck in sap
other types of species interactions is said to display a realized looking for insects
niche (Figure 4.2b), the portion of its fundamental niche that
is actually “realized,” or fulfilled. The quagga mussel can
occupy a wider range of water conditions and substrates than
the zebra mussel, so it is thought to have a larger fundamen-
tal niche. The quagga mussel also appears to be reducing the
zebra mussel’s realized niche, as it displaces the zebra mussel
in many areas. CHAPTER 4 • S PEC i ES i n TERA CT i on S A nd Co mmuni T y E C ology
Species experience similar adjustments over evolu- Pileated Brown creeper
woodpecker
tionary time. Over many generations, the process of natu- digs deeply climbs up
ral selection (pp. 68–71) may respond to competition by into wood to trunk looking
favoring individuals that use slightly different resources find large insects for tiny
insects
or that use shared resources in different ways. If two bird
species eat the same type of seeds, individuals that prefer
eating larger or smaller seeds might minimize competition
and thereby survive and reproduce more effectively. If the
seed-eating tendencies are inherited, then these preferences
may be passed on to offspring, and over time natural selec-
tion may drive one species to specialize on larger seeds and Figure 4.3 When species compete, they may partition
the other to specialize on smaller seeds. Natural selection resources, each specializing on a slightly different resource
might also favor one bird species becoming more active in or way of attaining a shared resource. Many types of birds for-
the morning and the other more active in the evening, thus age for insects on tree trunks, but they use different portions of the
minimizing interference. This process is called resource trunk and seek different foods in different ways. 95

M04_WITH7428_05_SE_C04.indd 95 12/12/14 2:55 PM