Visualizing Environmental Science

374 CHAPTER 15 Biodiversity and Conservation

Species Richness and Biological Diversity

LEARNING OBJECTIVES

species are not readily replaced in isolated environments

such as islands or mountaintops. Isolated areas are often

small and possess fewer potential ecological niches.

Generally, species richness is also inversely related to

the environmental stress of a habitat. Only species capable

of tolerating extreme environmental conditions can live

in an environmentally stressed community, such as a pol-

luted stream or a polar region exposed to a harsh climate.

Species richness is also reduced when one species occu-

pies a decided position of dominance within a community

because that species may appropriate a disproportionate

share of resources, thus crowding out other species.

Species richness is usually greater at the edges of

adjacent communities than in their centers. This is

because an ecotone—a transitional zone where communi-

ties meet—contains all or most of the ecological niches of

1. Describe factors associated with species richness.


2. Define biological diversity and distinguish
   among species richness, genetic diversity, and
   ecosystem diversity.


3. Relate several important ecosystem services
   provided by biological diversity.

A

species is a group of more or less distinct
organisms that are capable of interbreeding
with one another in the wild but that do not
interbreed with organisms outside their
group. We do not know exactly how many species exist.
In fact, biologists now realize how little we know about
Earth’s diverse organisms.

How Many Species Are There?
Scientists estimate that there may be as few as 5 million
or as many as 100 million different species inhabiting
Earth. According to the International Union for Conser-
vation of Nature (IUCN), about 1.7 million species have
been scientifically named and described to date, includ-
ing nearly 310,000 plant species, more than 64,000 verte-
brate animal species, and 1 million insect species. About
10,000 new species are identified each year.
Species richness varies greatly from one community
to another. It is determined by
several factors: the abundance of
potential ecological niches, geo-
graphic isolation, habitat stress,
dominance of one species over
others, closeness to the margins of adjacent communi-
ties, and geologic history.
A complex community, such as a tropical rain forest or
a coral reef, offers a greater variety of potential ecological
niches than does a simple community such as a mountain
chaparral. A similar effect is seen in comparisons between
more structurally complex and less structurally complex
areas within the same type of habitat (Figure 15.1).
Species richness is inversely related to the geo-
graphic isolation of a community. Isolated island
communities are much less diverse than communities in
similar environments found on continents. This is partly
the result of the difficulty many species have in reaching
and successfully colonizing an island, and locally extinct

species richness

The number of

different species in a

community.

Structural complexity of vegetation

0 0.4 0.8 1.2 1.6 2.0

Number of bird species 10

20

30

40

After M.L. Cody and J.M. Diamond, eds.,

Ecology and Evolution

of Communities

. Harvard University, Cambridge (1975).

Effect of community complexity on species

ÀˆV…˜iÃÃÊUʈ}ÕÀiÊ£x°£Ê

The structural complexity of chaparral vegetation in California

(x-axis) is based on vegetation height and density, from low

complexity (very dry scrub) to high complexity (woodland). Note

that species richness in birds increases as vegetation becomes

more structurally complex.

Interpreting Data

What is the percentage difference between the number

of bird species supported by the least structurally

complex areas and the number supported by the most

structurally complex areas?