recap

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56.4 recap

Communities constantly undergo change in species composition over time as a result of abiotic and biotic factors, a process known as succession. Abiotic-induced changes to communities come in two forms: disturbance and stress. There are two types of succession: primary succession is the recovery of a community after a catastrophic disturbance kills all the organisms in it, and secondary succession is the reestablishment of a community when most, but not all, organisms have been destroyed. Both facilitation and inhibition influence succession. Communities under similar environmental conditions are sometimes subject to alternative successional pathways, leading to variations in community composition (alternative states).

learning outcomes

You should be able to:

  • Distinguish between disturbance and stress, and give examples of each.

  • Given specific examples, characterize biotic factors that might influence community structure, and describe their possible effects.

  • Describe, with examples, the conditions required for primary succession to occur, including the time intervals involved.

  • Describe, with examples, the concept of alternative states, or regime shifts, in a community, including how such states might arise.

Question 1

What is the difference between disturbance and stress?

Disturbance is an abiotic event that may physically or chemically injure or even kill some individuals, creating opportunities for other individuals to grow and/or reproduce. For example, hurricanes, forest fires, and volcanic eruptions are disturbances. Stress occurs when some abiotic factor reduces the growth, reproduction, and ultimately survival of some individuals. For example, drought or extreme cold can cause stress in organisms.

Question 2

What are some biotic factors that induce change in communities?

Some biotic factors that induce change in communities include species interactions, such as predation, competition, or disease. Others include physical damage by organisms, such as trampling or digging.

Question 3

True or false? “Primary succession is controlled only by facilitation.” Explain your answer.

False. Primary succession is controlled by facilitative and inhibitory processes acting together over time. The progress of succession depends on the early colonists, each of which can facilitate and inhibit other colonizing species. For example, in Glacier Bay Dryas and alders allowed spruce trees to become established, but later in succession, competition from spruce trees led to the decline of early successional species.

Question 4

How does an alternative state, or regime shift, occur? Give an example from the chapter of such a shift.

An alternative state, or regime shift, occurs when a disturbance or stress causes a community to follow a different successional trajectory, leading to an alternative community state. Examples include the extirpation and then reintroduction of wolves to aspen forests of Yellowstone National Park, the presence or absence of sea otters in kelp forests along the west coast of North America, and the effects of beavers on wetlands in Minnesota.

We have focused thus far on the dynamic nature of community structure and on the factors on which community membership depends. In the next section we will consider whether the successful functioning of communities depends on the number of species they contain. In other words, how does species richness affect community productivity and stability?