Species Richness and Biological Diversity 375John Prior Images/Alamy© blickwinkel/AlamyAnnie Griffiths Belt/NG Image Collectiona. The variation in color pattems is evidence of the genetic
diversity in the land snail species Cepaea nemoralis, common
in western and central Europe.c. A scenic view in Switzerland reflects ecosystem diversity
across central Europe.b. Species richness in a European grassland or woodland is
represented by the land snail species, the grasshopper, and
the grass.the adjacent communities as well as some niches unique to
the ecotone. The change in species composition produced
at ecotones is known as the edge effect.
Geologic history greatly affects species richness. Trop-
ical rain forests are probably old, stable communities that
have undergone few climate changes in Earth’s entire
history. During this time, myriad species evolved in tropi-
cal rain forests, having experienced few or no abrupt cli-
mate changes that might have led to their extinction. In
contrast, glaciers have repeatedly altered temperate and
arctic regions during Earth’s history, as the climate alter-
nately cooled and warmed. An area recently vacated by
glaciers will have a low species richness because few spe-
cies will as yet have had a chance to enter it and become
established. Scientists expect that future climate change
will alter species richness, although it is difficult to pre-
dict the impact in any given area.
Why We Need Biodiversity
The variation among organisms is referred to as biologi-
cal diversity, or biodiversity. Biological diversity occurs at
all levels of biological organization,
from populations to ecosystems
(Chapter 5). It takes into account
three components: genetic diver-
sity, the genetic variety within all
populations of that species; species
richness; and ecosystem diversity,
the variety of ecosystems found on
Earth and the variety of interactions
among organisms in natural com-
munities (Figure 15.2). For example, a forest community—
with its trees, shrubs, vines, herbs, insects, worms, vertebrate
animals, fungi, bacteria, and other microorganisms—
has greater ecosystem diversity than a cornfield.
Humans depend on the contributions of thousands of
species for their survival. For example, insects are instru-
mental in several ecological and agricultural processes,
including pollination of crops, weed control, and insect
pest control. Bacteria and fungi provide us with foods, anti-
biotics and other medicines, and biological processes such
as nitrogen fixation (see Chapter 5). However, relatively few
species have been evaluated for their potential usefulness
to humans. At least 250,000 of the approximately 310,000
known plant species have yet to be assessed for industrial,
medicinal, or agricultural potential. The same is true for
most of the millions of microorganisms, fungi, and animals.
biological
diversity The
number and variety
of Earth’s organisms;
consists of three
components: genetic
diversity, species
richness, and
ecosystem diversity. iÛiÃÊvÊL`ÛiÀÃÌÞÊUÊ}ÕÀiÊ£x°ÓÊ