Environmental Engineering Reference
In-Depth Information
and tolerate a wide range of environmental conditions.
Flies, cockroaches (Science Spotlight, p. 69), mice, rats,
white-tailed deer, raccoons, coyotes, copperheads,
channel catfish, and humans are generalist species.
Specialist species
occupy narrow niches (Fig-
ure 4-4, left curve). They may be able to live in only
one type of habitat, use one or a few types of food, or
tolerate a narrow range of climatic and other environ-
mental conditions. This makes specialists more prone
to extinction when environmental conditions change.
For example,
tiger salamanders
breed only in fishless
ponds where their larvae will not be eaten. Threatened
red-cockaded woodpeckers
carve nest holes almost exclu-
sively in longleaf pines that are at least 75 years old.
China's highly endangered
giant pandas
feed almost ex-
clusively on various types of bamboo. Some shorebirds
also occupy specialized niches, feeding on crustaceans,
insects, and other organisms on sandy beaches and
their adjoining coastal wetlands (Figure 4-5).
Is it better to be a generalist or a specialist? It
depends. When environmental conditions are fairly
constant, as in a tropical rain forest, specialists have an
advantage because they have fewer competitors. Un-
der rapidly changing environmental conditions, the
generalist usually is better off than the specialist.
Natural selection can lead to an increase in spe-
cialized species when several species must compete in-
tensely for scarce resources. Over time one species
may evolve into a variety of species with different
adaptations that reduce competition and allow them
to share limited resources.
This
evolutionary divergence
of a single species into a
variety of similar species with specialized niches can be
illustrated by considering the honeycreepers that live
on the island of Hawaii. Starting from a single ancestor
species, numerous honeycreeper species evolved with
Specialist species
with a narrow niche
Niche
separation
Generalist species
with a broad niche
Niche
breadth
Region of
niche overlap
Resource use
Figure 4-4
Overlap of the niches of two different species: a
specialist and a generalist. In the overlap area, the two species
compete for one or more of the same resources. As a result,
each species can occupy only a part of its fundamental niche;
the part it occupies is its realized niche. Generalist species
have a broad niche (right), and specialist species have a nar-
row niche (left).
Generalist and Specialist Species
Some species have broad ecological roles; others have
narrower or more specialized roles.
Scientists use the niches of species to broadly classify
them as
generalists
or
specialists.
Generalist species
have broad niches (Figure 4-4, right curve). They can
live in many different places, eat a variety of foods,
Brown pelican dives for fish,
which it locates from the air
Black skimmer
seizes small fish
at water surface
Avocet sweeps bill through
mud and surface water in
search of small crustaceans,
insects, and seeds
Scaup and other
diving ducks feed on
mollusks, crustaceans,
and aquatic vegetation
Flamingo
feeds on
minute
organisms
in mud
Oystercatcher feeds on
clams, mussels, and
other shellfish into which
it pries its narrow beak
Louisiana heron wades into
water to seize small fish
