Succession. Main types of changes (see Morin 13-1) in community/ecosystem properties that you should know:

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1 Bio 147/247 Community Ecology Succession 1. General patterns 2. Mechanisms that may explain patterns. a) How can we test/identify the different mechanisms? b) For any given system: which mechanisms operate?...and c) under what conditions? Types of succession: Allogenic: Autogenic: 1 o 2 o Main types of changes (see Morin 13-1) in community/ecosystem properties that you should know: Variable Early Succession Late Succession Biomass Low High Food chains Linear (simple) Web-like Species richness (S) Low High Species Low High evenness/equitability (J) Niche specialization Broad Narrow Size of plants Small Large Life cycles Short, simple Long, complex Growth form Weeds/fugitive/colonists Long-lived Nutrient conservation Poor (leaky) Good Stability Low High 1

2 A Brief History of Succession 1. ORDERLY SEQUENCE ESS to Climax Community Climax community is a stable end point Mechanism: habitat modification 2. NOT AS ORDERLY Individualistic: communities are not tightly organized, but composed of individualistic spp. Initial Composition explains some of the variation in patterns a. Many spp are present before disturbance these determine sequence (Storage effect) b. Any species can recruit c. Life histories of spp are important stronger competitors win d. Growth of some spp may prohibit others from colonizing successfully 3. EVEN LESS ORDERLY: DIFFERENT MECHANISMS at DIFFERENT TIMES Interactions between the same species may differ at different life stages Any species may inhibit later stages (e.g., annuals prevent perennial forbs from growing) Any successional sequence may be driven by different mechanisms What ARE the mechanisms of succession? Connell & Slatyer 1978: Formalized/Summarized Mechanisms Methodology to test/distinguish among the mechanisms 2

3 Mechanisms of Succession (after Connell & Slatyer) Succession Disturbance 3

4 Number of Gigartina canaliculata Experiment to test for the mechanism of succession: Do: Facilitation Tolerance Inhibition Prediction if: Interpretation: Two examples: experimental studies of succession. 1. Intertidal algae (Wayne Sousa) ESS species: green algae (Ulva) MSS species: several red algae (including Gigartina leptorhyncos) LSS species: one red alga (esp. Gigartina canaliculata) a) # of LSS/MSS algae remove ESS b) # of LSS algae remove MSS - ESS - MSS Control Control Time Mechanism of Succession: Mechanism of Succession: 4

5 c) Why is the LSS stage eventually reached? How is Inhibition broken? Habitat Conditions Notes Solid rock a. Later stages: better survival in harsh physical conditions (see graph below). b. Herbivores prefer ESS. c. MSS can be overgrown by other plants (epiphytes) on their surface. Cobbles (moving rocks) a. Scrape the surface and remove algae as they are moved around by waves b. Smaller cobbles move more than larger c. Larger cobbles may move only in larger disturbances Harsh physical conditions: LSS MSS ESS 5

6 Example 2: Terrestrial succession in Glacier Bay (FS Chapin see reading list). The main points of this study: a. Life histories are critical: Ex: some species out-live others (tolerance model) b. MSS and LSS seeds can be found (although numbers are small) even in early stages of succession. See Fig 7 in Chapin pt 1. c. The same species may have different interactions at different life history stages: Ex: young Alder tolerates Dryas, but Alder eventually out-competes Dryas (via litter and shade). d. All 3 mechanisms play roles in the succession of this community: effects on spruce seedlings. (See Chapin readings, part 2) 6

7 Summary (from Chapin et al 1994): General Summary of Succession: At Glacier Bay, the community would reach the same endpoint (e.g., spruce) because of life history traits (tolerance) rather than the other mechanisms, but facilitation and inhibition affect the rate of succession. The interactions of mechanisms that we learned from the Glacier Bay study may be very general and apply to many successional sequences in any habitat. But, for some successional sequences, one mechanism may predominate (e.g., intertidal algae). 7