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Community and Ecosystem Ecologyby Dr. W.

Review

•  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. . .

•  Each species has a certain set of conditions under which it will grow. These make up its niche.– 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.

Life's a Niche. . .

•  If two populations share the same niche, one of two things may happen. . .–  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.

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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 Paramecium caudatum

When Gause grew both species in the same test tube, however. . . one species always dominated, and the

other one went down to extinction.

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.

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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. Old-growth 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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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. . .

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.

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.

Terre Noire Natural Area, Clark County, Arkansas

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.

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.