Page 296 - Environment: The Science Behind the Stories

Biodiversity encompasses multiple levels offspring. Biologists use differing criteria to distinguish one
species from another. Some biologists emphasize characteris-
Biodiversity is a concept as multifaceted as life itself, and tics that species share because of common ancestry, whereas
biologists employ different working definitions according to others emphasize the ability to interbreed. The spirited debate
their own aims and philosophies. Yet scientists agree that the on this topic could easily fill a textbook in itself! In practice,
concept applies across the major levels in the organization of however, scientists generally agree on species identities.
life (Figure 11.2). The level that is easiest to visualize and most We can express species diversity in terms of the number
commonly used is species diversity. or variety of species in a particular region. One component
of species diversity is species richness, the number of spe-
Species diversity A species (p. 67) is a distinct type of cies. Another is evenness or relative abundance, the extent to
organism, a set of individuals that uniquely share certain char- which species differ in numbers of individuals (greater even-
acteristics and can breed with one another and produce fertile ness means they differ less).
Speciation (p. 71) generates new species, whereas extinc-
tion (pp. 76–77) diminishes species richness. Immigration,
emigration, and local extinction may change species richness
locally, but only speciation and extinction change it globally.
Biodiversity exists below the species level in the form
of subspecies, populations of a species that occur in different
geographic areas and differ from one another in some char-
acteristics. Subspecies arise by the same processes that drive
speciation (pp. 71–72) but result when divergence stops short
of forming separate species. As an example, the black rhino-
ceros has diversified into four subspecies, each of which live
in different parts of Africa. The eastern black rhino, which is
native to Kenya and Tanzania, has characteristics slightly dif-
ferent from the western black rhino, the southwestern black
Ecosystem diversity rhino, and the southern-central black rhino.

Genetic diversity Scientists designate subspecies when
they recognize substantial, genetically based, differences
among individuals from different populations of a species.
However, all species consist of individuals that vary geneti-
cally from one another to some degree, and this genetic
diversity is an important component of biodiversity. Genetic
diversity encompasses the differences in DNA composition
(p. 47) among individuals.
Genetic diversity provides the raw material for adapta-
tion to local conditions. In the long term, populations with
more genetic diversity may be more likely to persist, because
their variation better enables them to cope with environmental
change.
Species diversity Populations with little genetic diversity are vulnerable
to environmental change because they may happen to lack
genetic variants that would help them adapt to novel condi-
tions. Populations with low genetic diversity may also be CHAPTER 11 • Bi odiv ER si T y A nd Cons ER vAT i on Bi ology
more vulnerable to disease and may suffer inbreeding depres-
sion, which occurs when genetically similar parents mate and
produce weak or defective offspring. Scientists have sounded
warnings over low genetic diversity in species that have
dropped to low population sizes, including American bison,
elephant seals, and the cheetahs of the East African plains.
The full consequences of reduced diversity in these species
remain to be seen. Diminished genetic diversity in our crop
plants is a prime concern to humanity (pp. 270–271).

Ecosystem diversity Biodiversity encompasses levels
Genetic diversity above the species level, as well. Ecosystem diversity refers to
Figure 11.2 The concept of biodiversity encompasses the number and variety of ecosystems (pp. 78, 128), but biol-
multiple levels in the hierarchy of life. ogists may also refer to the diversity of biotic communities 295

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