Communities in Transition

Ecological Succession – The replacement of one community by another until a climax community is reached

May take hundreds or thousands of years

Each new community makes it harder for the old communities to survive.

New environmental conditions allow one group of species in a community to replace other groups.

All communities change their structure and composition in response to changing environmental conditions.

During succession: Pioneer species arrive first.

(Examples only – Write what will help you) A forest hundreds of years old may have

been a shallow lake thousands of years ago.

A dead tree falls and allow sunlight to reach the forest floor. Sunlight allows seeds to germinate. Wildflowers and shrubs grow.

Moss, shrubs, and small trees cover the concrete of an old city building.

Pioneer Species – The first organisms to colonize any newly available area and begin the process of ecological succession. The pioneer organism will alter the

environment so that a bigger and better organism can move in.

Climax Community – A final and stable community.

A climax community can only be disrupted by a catastrophe.

After the catastrophe it will then rebuild itself back up again.

Primary Succession – Succession that occurs on a surface where an ecosystem did not previously exist

Secondary Succession – Succession that occurs on a surface where an ecosystem had previously existed.

Starting from Scratch

Fig. 7-11, p. 156

Exposed rocks

Lichens and mosses

Small herbs and shrubs

Heath matJack pine,

black spruce,

and aspen

Balsam fir,

paper birch,

and white spruce

forest

community

Time

DEFINITION: Succession that occurs on a surface where an ecosystem did not previously exist Examples: Bare rock, newly cooled lava,

abandoned highway or parking lot

Takes a long time because there is no fertile soil to provide the nutrients needed to establish a plant community.

Begins when pioneer species arrive and attach themselves to bare rock. Examples of pioneer species: Lichens and

mosses

Fig. 7-11, p. 156

Exposed rocks

Lichens and mosses

Small herbs and shrubs

Heath matJack pine,

black spruce,

and aspen

Balsam fir,

paper birch,

and white spruce

forest

community

Time

Starting Over With Some Help

Succession that occurs on a surface where an ecosystem had previously existed. Examples: Abandoned farmland, burned/cut

forests, heavily polluted streams, and land that has been flooded

New vegetation can usually germinate within a few weeks from seeds in the soil and those brought in from nearby plants by wind or by birds and other animals.

Secondary succession occurs in ecosystems that have been disturbed or disrupted by humans, animals, or natural processes.

Fig. 7-12, p. 157

Time

Annual

weeds

Perennial

weeds and

grasses

Shrubs

and pine

seedlings

Young pine forest

with developing

understory of oak

and hickory trees

Mature oak-hickory forest

Secondary Succession: Post-fire

Secondary Succession: Old Field

Secondary Succession: Post-Volcano