General Ecology: Lecture 12
General Ecology: Lecture 19
November 18, 2005
I. Community dynamics
A. Succession: Temporal variation in community structure
1. Definition: Temporal change in community structure through time
2. Primary succession
a. Primary succession defined: the sequence of events that occurs when a completely bare area without remnants of an older ecosystem is colonized
• Where would primary succession occur?
b. Example of primary succession: Algal succession in the rocky intertidal (Sousa): Placed bare settlement plates in the intertidal [Fig. 21.4]
• Pioneer (early successional) species
← Usually r-selected: have qualities that allow them to quickly and effectively exploit newly available habitat [See. Lecture 11/Chapter 13 for r vs. K selected species]
← Example from Sousa study: Ulva
⇨ Initially dominates, but then disappears over time
• Late successional species (once called “climax community”)
← K-selected: have qualities that allow them to outcompete others (especially the r-selected species) and firmly establish themselves over time, until a disturbance occurs.
← Examples from Sousa study: Most all of the other species are K-selected, although the Gigartina canaliculata was the most successful
⇨ Note inverse correlation between the two Gigartina species. Possible explanation?
3. Secondary succession
a. Secondary succession: A disturbance occurs which wipes out the late-successional species, leading to a sequence of events similar to primary succession
b. Example: Secondary succession in a seagrass community [Fig. 21.13]
• Disturbance is caused by wave action or grazing by sea turtles and urchins
• Note that r-selected species initially colonize, whereas later successional stages increasingly favor K-selected species.
← Fast-growing macroalgae in first, followed by smaller grasses and finally the turtle grass
4. Cyclic replacement [Fig. 21.14]
a. Old field, Michigan: moss ( Canada bluegrass( dock ( lichen; Lichen killed by rain, frost, wind… back to moss.
5. Succession and shifting mosaics [Fig. 21.15]
a. Example: mid-intertidal of the Oregon coast [Fig. 6.30 and 6.26 in Nybakken, “Marine Biology”]
6. Autosuccession
a. Example: Chaparral communities of California
• After fire, mature shrubs “lost” to the fire are replaced by seedlings of the same species.
Study questions
1. What is succession? What is the difference between primary and secondary succession? Provide an example of each type of succession.
2. Describe succession in terms of r-selected vs. K-selected organisms and describe characteristics of each that account for their different timing/roles in succession.
3. Once the late-successional community is reached, does it stay that way forever? If not, why not?
4. What is meant by cyclic replacement? Provide an example.
5. What is a shifting mosaic? Describe in general terms and also provide an example.
6. What is autosuccession and how does this differ from both primary and secondary succession?
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