3.3 Succession: How Ecosystems Change over Time

Unit A

Science 7

Key Terms

Pioneer species

Succession

Primary succession

Climax community

Secondary succession

Pioneer Species

• Pioneer Species are those organisms, usually plants, that colonize barren rock or lava during primary succession.

• Pioneer species physically break up the rocks, extract minerals, and provide organic matter that will decompose and become the soil for later successional species.

Examples o

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Lichens, mosses, ferns, and weeds are often pioneer species. Lichens can secrete acids that help breakdown rock and provide opportunities for other types of plants to move in. Growing seedlings can help split boulders weighing more than your family car! Without pioneer species paving the way, ecosystems of a climax community would never develop.

Moss

Give It A Try –p. 62

• Look at Figures 3.8 a) – c). Try to decide which one(s) are pioneer species. Explain.

Succession

• Succession is the process of community change and development that happens most obviously when a natural community is disturbed or when new land become available to life. But biological communities are always changing. Many lakes and wetlands gradually become filled in and over thousands of years become dry land.

• Slowly, slowly, generations of sedge-mats die and sink to the bottom of the lake and decompose. After centuries of plant matter building up on the lake bottom, the lake becomes shallower. Given enough time, the lake becomes a cattail marsh or sphagnum peat bog.

• Eventually there is no open water left. The lake has transformed from an aquatic community to a wetland community and will eventually become a wet meadow or, further north, a black spruce forest growing on peat.

                                                                          

             

This muskeg or peat bog was once a lake

•The lake at first (after glaciers formed it) is surrounded by essentially scoured bare land. After a time, pioneer plants establish themselves there. After more time, a forest grows there. Near the shores of the lake grow reeds and cattails and floating mats of sedges, which are cousin to grasses.

Primary Succession

• Some succession (primary succession), on the other hand, is the process of life colonizing dead or sterile areas such as volcanic lava flows and new sand dunes, or rock left behind by retreating glaciers, and transforms them into living communities.

Secondary Succession• Some succession

(secondary succession) is an ecosystem's response to an injury, the way we heal a cut. – Such succession

transforms a disturbed or damaged part of a community. Example: a tree falls and creates an opening in the canopy of leaves. Succession fills in the opening.

Climax Community

• A climax community is a stable group of plants and animals that is the end result of the successional process.

Redwood Forest

The redwood forest of California is a climax community. Those trees are huge and can out-compete any invading tree species. The redwoods also provide a stable habitat for other organisms that live in the forest. The redwoods serve as the foundation that supports the rest of the ecosystem. Huge trees are the anchor for literally thousands of other species of organisms.

C and R - p. 64

• Do all questions.

C and R Answers –p.641. Students should recognize that plants (or plantlike living things such

as lichens) are always the pioneer species. Plants, as producers, must colonize an area before animals can become established because animals, being consumers, need producers to feed on.

2. Answers will vary, but students may identify stages like those below (for primary succession). Encourage students to provide as much detail as possible for each stage. Look in particular for evidence of their understanding that pioneer species must be producers, and are usually small and low-growing.

a) bare rockb) rock covered with lichensc) mosses and other small plants growing in the developing soild) grasses and small shrubs/bushes growing in more developed soile) trees growing in fully developed soil.Note: for grasslands, the climax community will be d.

C and R Answers –p.64

3. In primary succession, a new community develops in an area where no community existed before. In secondary succession, a new community develops in an area where a previously existing community has been disturbed or destroyed by natural or human activity.

4. Students may suggest lichens, ferns, or mosses for pioneer species. For climax species, they may suggest any large trees such as pine, spruce, or poplar. However, if they are describing grasslands, they should suggest grasses or shrubs, not large trees.

C and R Answers –p.64

5. The most logical sequence is fireweed, grass, mouse, birch tree, bear. Ask students to identify this sequence as an example of primary or secondary succession, with reasons.

6. These photos would be an example of secondary succession. Because the land is no longer being used for farming, native plants, shrubs, and trees have moved back to re-establish themselves. Students should be able to find many examples in their communities. They just need to look at the edge of the schoolyard or open lot to see examples of plants that are re-establishing themselves.

AYL – p. 65

• Do All Questions

AYL Answers –p.65

1. a) Students may suggest floods, drought, tornadoes, hurricanes, or other violent weather-related events.

b) The details students provide will vary, but most answers will likely focus on loss (or reduced numbers) of producer and/or consumer populations, and the effects of those losses or reductions (e.g., an increase in other populations, which might eventually lead to further reductions if food becomes scarce). Students should provide sufficient detail to explain the causes and effects they identify.

AYL Answers –p.65

2. Answers will vary but illustrations or written work should be accurate and clearly show how primary succession takes place. Primary succession begins with the arrival of a plant to an area that has not had any life before. Over time, this pioneer plant helps to break down rocks into soil. As the soil becomes more fertile, other plants move in.

AYL Answers –p.65

3. Population fluctuations can cause many changes in an ecosystem. For example, with the lynx/hare cycle, an increase in the food supply for the hares means that more hares survive. When more hares survive, more lynx can survive. However, when the hares decimate their food supply, their numbers will decrease and then so will the lynx numbers. The populations of organisms are closely connected through the food chains.

4. Some of the following interactions can cause changes in an ecosystem: predation, human impact, bioinvasion, and resource competition.

AYL Answers –p.65

5. Secondary succession can occur in any ecosystem where a community has been destroyed by natural occurrences or human activities.

6. The removal of the bears would cause significant changes in the ecosystem, because they are an important part of the food chain. Students’ plans could list measuring the biotic factors before the bears were removed and then after the bears were removed. Students’ plans may also provide a specific example of how the removal of the bears will affect all the organisms in the park.

Review Key Terms

Pioneer species

Succession

Primary succession

Climax community

Secondary succession