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FIGURE 8.1 Levels of biodiversity. Biodiversity exists at
four scales. (a) Genetic diversity is the variety of genes among
individuals of a species. (b) Species diversity is the variety of
species within an ecosystem. (c) Habitat diversity is the variety of
habitats present in an ecosystem. (d) Ecosystem diversity is the
variety of ecosystems within a region.

viable and fertile offspring. This last requirement is impor-
tant because sometimes individuals from different species
can mate, but they typically produce offspring that do not
survive or cannot breed. Based on this definition, species
diversity indicates the number of species in a region or in
(a) Ecosystem diversity a particular ecosystem. For example, in our discussion of
terrestrial biomes in Module 2 we learned that the tundra
,
and taiga biomes have a low species diversity of plants and
animals while tropical rainforests have a very high species
diversity. Similarly, a temperate deciduous forest typically has
dozens of tree and shrub species, but a commercial timber
plantation typically has just one or a few species of trees that

are grown for their high-value lumber ( FIGURE 8.2 ). In “Do
the Math: Converting hectares and acres” you can see how
scientists examine species diversity in terms of land area.

Habitat Diversity
Habitat diversity is the variety of habitats that exist in a

given ecosystem. Some species are specialists which means
,
(b) Habitat diversity
that they only live under a narrow range of biotic or abiotic
conditions. Examples include herbivores that only consume
a single plant species, such as the koala ( Phascolarctos cinereus )
that live among eucalyptus trees and feed only on eucalyp-
,
tus leaves. In contrast, other species are generalists which
means they can live under a wide range of biotic or abiotic
conditions. White-tailed deer are an example of a generalist
because they can live in a wide range of climates through-
out North and South America and feed on a large number
of different plants. For example, consider a region that has
a mixture of farms growing crops, farms with pastures for
cattle, abandoned farms with small trees and shrubs, and for-
ests with large trees. Each of these habitats support different
suites of specialist and generalist plant and animal species.
(c) Species diversity
The combination of the four habitats supports many more
species. As a result, a greater diversity of habitats supports a
greater diversity of species.

®
AP Exam Tip
®
When mentioning a “habitat” on the AP Environmental
Science Exam, you should always specify which type
of habitat and name a particular organism or group of
organisms that live there. Forests provide habitat (this
won’t earn credit). Forests provide nesting habitats for
some species of birds (this will earn credit).
(d) Genetic diversity

FIRST PASS
fried_es4e_08_01_0801 - January 27, 2022
98 UNIT 2 ■ The Living World: Biodiversity
Uncorrected proofs have been used in this sample.
Copyright © Bedford, Freeman & Worth Publishers.
Distributed by Bedford, Freeman & Worth Publishers. Not for redistribution.

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