Environmental Engineering Reference
In-Depth Information
nonessential species. We need much more research
into this important area of ecology.
quality because trout need clean water with high lev-
els of dissolved oxygen.
Birds are excellent biological indicators because
they are found almost everywhere and are affected
quickly by environmental changes such as loss or frag-
mentation of their habitats and introduction of chemi-
cal pesticides. The population of many bird species is
declining.
Butterflies are good indicator species because their
association with various plant species makes them vul-
nerable to habitat loss and fragmentation. Some am-
phibians are also believed to be indicator species, as
discussed next.
6-2
TYPES OF SPECIES
Types of Species in Communities
Communities can contain native, nonnative, indicator,
keystone, and foundation species that play different
ecological roles.
Ecologists often use labels—such as
native, nonnative,
indicator, keystone,
or
foundation
—to describe the major
niches filled by various species in communities. Any
given species may play more than one of these five
ecological roles in a particular community.
Native species
are those species that normally live
and thrive in a particular community. Other species
that migrate into or are deliberately or accidentally in-
troduced into an community are called
nonnative
species, invasive species,
or
alien species.
Many people tend to think of nonnative species
as villains. In fact, most introduced and domesti-
cated species of crops and animals such as chickens,
cattle, and fish from around the world are beneficial
to us.
Sometimes, however, a nonnative species can
crowd out native species and cause unintended and
unexpected consequences. In 1957, for example, Brazil
imported wild African bees to help increase honey
production. Instead, the bees displaced domestic hon-
eybees and reduced the honey supply.
Since then, these nonnative bee species—popu-
larly known as “killer bees”—have moved northward
into Central America and parts of the southwestern
United States. They are still heading north but should
be stopped eventually by the harsh winters in the cen-
tral United States unless they can adapt genetically to
cold weather.
The wild African bees are not the fearsome killers
portrayed in some horror movies, but they are aggres-
sive and unpredictable. They have killed thousands of
domesticated animals and an estimated 1,000 people
in the western hemisphere. Most of their human vic-
tims died because they were allergic to bee stings or
because they fell down or became trapped and could
not flee.
Case Study: Why Are Amphibians
Vanishing?
The disappearance of many amphibian species may
indicate a decline in environmental quality in many
parts of the world.
Amphibians (frogs, toads, and salamanders) live part
of their lives in water and part on land, and some are
classified as indicator species. Frogs, for example, are
especially vulnerable to environmental disruption at
various points in their life cycle, shown in Figure 6-3
(p. 112). As tadpoles, they live in water and eat plants;
as adults, they live mostly on land and eat insects that
can expose them to pesticides. Frogs' eggs have no pro-
tective shells to block ultraviolet (UV) radiation or pol-
lution. As adults, they take in water and air through
their thin, permeable skins, which can readily absorb
pollutants from water, air, or soil.
Since 1980, populations of hundreds of the world's
estimated 5,280 amphibian species have been vanish-
ing or declining in almost every part of the world, even
in protected wildlife reserves and parks. According to
the World Conservation Union, one-fourth of all
known amphibian species are extinct, endangered, or
vulnerable to extinction.
No single cause has been identified to explain the
amphibian declines. However, scientists have identi-
fied a number of factors that can affect frogs and other
amphibians at various points in their life cycles:
■
Habitat loss and fragmentation
(especially from
draining and filling of inland wetlands, deforestation,
and development)
■
Prolonged drought
(which dries up breeding pools
so few tadpoles survive)
■
Pollution
(particularly from exposure to pesti-
cides, which can make frogs more vulnerable to
bacterial, viral, and fungal diseases and cause sexual
abnormalities)
■
Increases in ultraviolet radiation
caused by reduc-
tions in stratospheric ozone (which can harm young
embryos of amphibians in shallow ponds)
Indicator Species: Biological Smoke Alarms
Some species can alert us to harmful changes taking
place in biological communities.
Species that serve as early warnings of damage to a
community or an ecosystem are called
indicator spe-
cies.
For example, the presence or absence of trout
species in water at temperatures within their range of
tolerance (Figure 3-11, p. 43) is an indicator of water
