Essentials of Ecology

CONCEPT 5-2 107

5-2 How Can Natural Selection Reduce Competition

between Species?

CONCEPT 5-2 Some species develop adaptations that allow them to reduce or

avoid competition with other species for resources.

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Some Species Evolve Ways

to Share Resources

Over a time scale long enough for natural selection to

occur, populations of some species competing for the

same resources develop adaptations through natural

selection that allow them to reduce or avoid such com-

petition (Concept 5-2). In other words, some species

evolve to reduce niche overlap. One way this happens

is through resource partitioning. It occurs when spe-

cies competing for similar scarce resources evolve spe-

cialized traits that allow them to use shared resources at

different times, in different ways, or in different places.

For example, through natural selection, the fairly broad

and overlapping niches of two competing species (Fig-

ure 5-7, top) can reduce their niche overlap by becom-

ing more specialized (Figure 5-7, bottom).

Figure 5-8 shows resource partitioning by some

insect-eating bird species. In this case, their adapta-

tions allow them to reduce competition by feeding

in different portions of the same tree species and by

feeding on different insect species. Figure 4-13 (p. 93)

shows how bird species in a coastal wetland have

Resource use

Number of individuals

Species 1

Region

of

niche overlap

Species 2

Resource use

Number of individuals

Species 1 Species 2

Figure 5-7 Competing species can evolve to reduce niche overlap.

The top diagram shows the overlapping niches of two competing

species. The bottom diagram shows that through natural selection,

the niches of the two species become separated and more special-

ized (narrower) as the species develop adaptations that allow them

to avoid or reduce competition for the same resources.

Blackburnian Warbler Black-throated Green Warbler Cape May Warbler Bay-breasted Warbler Yellow-rumped Warbler

Figure 5-8 Sharing the wealth: resource partitioning of five species of insect-eating warblers in the spruce forests

of the U.S. state of Maine. Each species minimizes competition for food with the others by spending at least half

its feeding time in a distinct portion (shaded areas) of the spruce trees, and by consuming different insect species.

(After R. H. MacArthur, “Population Ecology of Some Warblers in Northeastern Coniferous Forests,” Ecology 36

(1958): 533–536.)