Download Notes 30: Community and Ecosystem Ecology I

4/20/11
Review
Community and Ecosystem
Ecology
by Dr. W.
• A population is the set of all members of one
species in a defined area.
• A community is the set of all populations in a
defined area.
• An ecosystem includes a community, together with
the non-living factors that affect it.
– In a land ecosystem, these might include temperature,
rainfall, amount of sunlight, seasonal changes, etc. . .
We’ve already seen how populations in a community may
affect each other through predation. . . . . . and also through symbiosis, such as parasitism
and mutualism.
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Other forms of competition are more subtle. The creosote bush
(Larea tridentata), from the Mojave Desert of the western US, has
roots that are so efficient at getting water that they outcompete any
other plant. This is why this picture looks so evenly planted: each
creosote bush is surrounded by a “dead zone”, corresponding to the
expanse of its own roots. And a number of plants produce chemicals in their roots or leaves
that block the growth of other plants. For example, black walnut
trees (Juglans nigra) produce a chemical (called juglone) that
keeps most other plants from growing nearby. Life's a Niche. . .
Life's a Niche. . .
• Each species has a certain set of conditions under
which it will grow. These make up its niche.
• If two populations share the same niche, one
of two things may happen. . .
– Difference between fundamental niche (set of
conditions under which a species can grow) and
realized niche (conditions under which it actually does
grow, given the existence of competitors).
– Competition results between members of any two
populations whose niches overlap, because the two
populations are using the same resources.
– One population may drive the other to
extinction. This is competitive exclusion. – Another response to competition is resource
partitioning — organisms change over time in
such a way as to share a resource and minimize
niche overlap. 2
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Case Study: In 1931, G.F. Gause took populations of two
species of single-celled protist, Paramecium aurelia and
Paramecium caudatum. When grown separately, both
populations grew to carrying capacity. . .
Paramecium aurelia
When Gause grew both species in the same test tube,
however. . . one species always dominated, and the
other one went down to extinction.
Paramecium caudatum
In other cases, different species sharing a niche will partition
it into sub-niches. In Canadian conifer forests, these three bird
species all live in the same fir trees, but each uses different
space and feeds on different insects.
Case Study: Two species of barnacles (related to
shrimp and crabs, but live permanently attached to
rocks) live on Pacific rocky coasts: larger Balanus
and smaller Chthamalus. 3
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Where both of them live together, Balanus
outcompetes Chthamalus. However, Chthamalus can
survive for longer out of the water, and thus lives on
higher rocks, where Balanus would dry out. Ecological Succession
• Ecological succession is a series of changes over
time in the communities present in an ecosystem.
– Imagine a plowed field that's been abandoned. First
you get small weeds growing there. . .
– . . . and then tall grasses. . .
– . . . and then shrubs and small trees. . .
– . . . and then large trees. . .
– . . . and, eventually, dense forest with no underbrush.
This is an example of ecological succession.
Eventually—
possibly after centuries
—the ecosystem is
dominated by a climax
community, and
succession ceases. Oldgrowth forests, like the
redwood forest at Muir
Woods, California, are
one example of a climax
community.
It’s often useful to distinguish between primary succession (in a
location where there has never been life) and secondary succession.
An example of primary succession: Surtsey, a volcano that appeared
above the ocean surface off the coast of Iceland in 1963.
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Secondary succession, on the other hand, refers to ecological
succession taking place in a region that has been disturbed but has
never been void of life—like this abandoned farm field in Canada.
The lava and ash that make up Surtsey were originally completely
barren. Today, there are over 60 species of plants on Surtsey, 15
species of birds, plus spiders, insects, slugs, and seaweeds and
marine life on the rocky shores. Ponder how they got there. . .
In some ecosystems, repeated disturbances keep the
community from continuing ecological succession all the way
to the climax community. Prairie is a good example of this—
prairies are subject to fire.
Cherokee Prairie Natural Area, Franklin County, Arkansas
Prairie fires may be caused by lightning—but today, prairies
are often deliberately burned by managers, in order to keep
them from becoming forest.
Cherokee Prairie Natural Area, Franklin County, Arkansas
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r-selected species are often the first to appear in succession—
because they reproduce abundantly and spread rapidly. Later,
they'll be replaced by more K-selected species.
Another example of an ecosystem that never reaches the climax
community because of periodic fires: chaparral, a mixture of
grasses and scrubby oaks, pines, and other trees. This is typical of
southern and central California hills. Terre Noire Natural Area, Clark County, Arkansas
Chaparral species are adapted to survive wildfires and come back
quickly. When chaparral wildfires are suppressed, however, dead
vegetation builds up and eventually fuels huge fires—such as the
Oakland Firestorm of October 1991, which caused 25 deaths and
destroyed over 3000 homes. 6