Environmental Engineering Reference
In-Depth Information
Animal and vegetable life is too complicated a problem for hu-
man intelligence to solve, and we can never know how wide a
circle of disturbance we produce in the harmonies of nature
when we throw the smallest pebble into the ocean of organic
life.
G
EORGE
P
ERKINS
M
ARSH
6-1 COMMUNITY STRUCTURE
AND SPECIES DIVERSITY
Community Structure: Appearance Matters
Biological communities differ in their structure and
physical appearance.
One way that ecologists distinguish between biological
communities is by describing their overall
physical ap-
pearance:
the relative sizes, stratification, and distribu-
tion of the populations and species in each community,
as shown in Figure 6-2 (p. 110) for various terrestrial
communities. There are also differences in the physical
structures and zones of communities in aquatic life
zones such as oceans, rocky shores and sandy beaches,
lakes, river systems, and inland wetlands.
The physical structure within a particular type of
community or ecosystem can also vary. Most large ter-
restrial communities and ecosystems consist of a mo-
saic of different-sized vegetation patches. Life is patchy.
Likewise, community structure varies around its
edges
where one type of community makes a transition
to a different type of community. For example, the edge
area between a forest and an open field may be sunnier,
warmer, and drier than the forest interior and support
different species than the forest and field interiors.
Increasing the edge area through habitat fragmen-
tation makes many species more vulnerable to stresses
such as predators and fire. It also creates barriers that
can prevent some species from colonizing new areas
and finding food and mates.
This chapter looks at the roles and interactions of
species in a community, the ways in which communi-
ties and populations respond to changes in environ-
mental conditions, and actions we can take to help
sustain populations, communities, and ecosystems.
What determines the number of species in a
community?
■
How can we classify species according to their
roles in a community?
■
How do species interact with one another?
■
How do communities respond to changes in envi-
ronmental conditions?
■
How do populations respond to changes in envi-
ronmental conditions?
■
How do species differ in their reproductive
patterns?
■
What are the major impacts of human activities on
populations, communities, and ecosystems?
■
What lessons can we learn from ecology about liv-
ing more sustainably?
■
KEY IDEAS
■
Biological communities differ in their physical struc-
ture, species diversity, and the ecological roles their
species play.
Species Diversity and Niche Structure
in Communities
Biological communities differ in the types and
numbers of species they contain and the ecological
roles those species play.
A second characteristic of a community is its
species
diversity:
the number of different species it contains
(
species richness
) combined with the abundance of indi-
viduals within each of those species (
species evenness
).
For example, two communities, each with a total of 20
different species and 200 individuals, have the same
species diversity.
But these communities could differ in their species
richness and species evenness. For example, commu-
nity A might have 10 individuals in each of its 20
species. Community B might have 10 species, each
with 2 individuals, and 10 other species, each with 18
individuals. Which community has the highest species
evenness?
Another characteristic is a community's
niche struc-
ture:
how many ecological niches occur, how they re-
semble or differ from one another, and how the various
■
Species interactions have profound effects on com-
munities and population size.
■
Communities change their species composition
and structure in response to changing environmental
conditions.
■
No population can continue grow indefinitely
because of limitations on resources and competition
between species for those resources.
■
Nature has survived for billions of years by relying
on solar energy, recycling nutrients, using biodiversity
to sustain itself and adapt to new environmental con-
ditions, and controlling population growth.
■
We can live more sustainably by mimicking the
four major ways nature has used to adapt and sustain
itself.
