14.7.1 Community Disturbance: Succession

Question 1

Community Disturbance: Succession

Answer 1

• Examples of community level disturbances are glaciation, fire, flood, earthquake, and human impact.
• After the retreat of a glacier, stages of primary succession are:
· bare rock—pioneers such as bacteria, fungi, algae, and lichens
· short grasses—weeds and short shrubs
· short trees—willows and alders
• An example of facilitative replacement occurs when lichens build soil on bare rock, enabling the colonization of short grasses.

Question 2

ecological succession and facilitative replacement

Answer 2

• Question: Are communities capable of changing?
• Ecological succession is the change in community structure over time. Disturbances such as fire, flood, glaciation, and human activity are possible causes for ecological succession.
• In facilitative replacement, a species improves conditions of the environment for another species. For example, an organism that increases the acidity of soil improves conditions for conifers.

Question 3

primary succession

Answer 3

• begins in an area that completely lacks
  community structure. Primary succession can result from a catastrophic event like glaciation, earthquake, or forest fire.
• After a glacier retreats, the exposed rock is completely barren of life. Successional stages on the rock include
  pioneer organisms, short grasses, and short trees. pioneer organisms—bacteria, fungi, algae, and lichens—are the first to colonize the rock. The biological activity of the organisms, especially the lichen, builds soil on the rock. Hundreds of years later, enough soil is present to permit the growth of short grasses. The growth of short trees, such as willows and alders, represents the third stage of the ecological succession.

Question 4

note

Answer 4

• A lichen (not pictured) is a symbiotic association of millions of photosynthetic algae cells enmeshed in fungal hyphae. The fungi absorb water and minerals, and the algae fix carbon through photosynthesis. The lichen attaches itself to a rock via the fungal hyphae. When the lichen dries out, it releases acids that break down the rock to form soil.
• The picture on the far left shows the colonization of short grasses. Eventually short trees such as willows develop. The willows increase the nitrogen content of the soil, favoring the growth of alder trees. The graph on the immediate left shows a spike in the number of alder trees (shaded red) when soil conditions are favorable.

Question 5

True or false?

Succession involves changes in the identity of the species in a community but not their abundances.

Answer 5

• false

Question 6

In primary succession, why are the initial pioneers overtaken by other species?

Answer 6

• The pioneer community changes the environmental conditions, ultimately making it unfavorable for themselves.

Question 7

The following chart depicts various stages in primary succession on bare rock. Arrange the stages in chronological order and choose the correct order of answers.

Answer 7

• C, B, D, E, A

Question 8

A key factor in changes in species composition during succession is

Answer 8

• facilitative replacement

Question 9

Which of the following graphs correctly depicts the change in species number over the course of primary succession?

Answer 9

• graph D

Question 10

Which of the following is an example of a substrate that will not be subject to primary succession?

Answer 10

• soil inhabited by a community of microorganisms