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Transcript
Disturbance – discrete punctuated killing, displacement, or damaging of one or more individuals (or colonies) that creates an opportunity for new individuals (or colonies) to become established. Agents of disturbance: – Physical – water motion (logs, tearing, roll boulders), desiccation, flood, freezing – Biological – predation, grazing, bulldozing Disturbance regime: • Areal extent • Frequency • Predictability • Turnover rate Repopulation/Colonization of Disturbed Patches • Source of colonists and distance of that source – Asexual (colonial) vs. sexual (individual) – Lecithotrophic larvae vs. planktotrophic larvae • Time of patch creation – Must be competent larvae available • Size and shape of disturbed patch – Influence of surrounding organisms • Nature of disturbance – Physical vs. biological (selective/nonselective mortality Intermediate Disturbance Hypothesis – Disturbance at intermediate intensity or frequency will allow high diversity by maintaining communities in a nonequilibrium state (Connell 1978) -- disturbance-mediated coexistence Time Evidence for Intermediate Disturbance Hypothesis from marine systems: • Age of disturbed patches – link to diversity • Boulders – intermediate-sized boulders are most diverse Disturbance (opens space, releases resources) 1 Of those that arrive, only “early successional” species survive 2, 3 Of those that arrive, any that establish themselves can survive 2 Growth of early species modifies the environment to allow later species 1 = Facilitation 2 = Inhibition 3 = Tolerance 3 Environment modified so less suitable for any other colonist Environment modified so some early arrivers can no longer survive Original colonists persist until damaged or destroyed Less tolerant species die and/or are forced out by better competitors Patchiness on hard substrates: • Disturbance • Larval site selection • Fluctuations in availability of larvae • Heterogeneity in substrate • Territoriality Differences Subtidal vs. Intertidal • Subtidal always covered by water • No refuge from predation in subtidal • Space appears 100% occupied in subtidal • Dominant role of colonial species in subtidal John Sutherland • Subtidal hard substrate (pier pilings in North Carolina) • Followed what happened after a disturbance – wasn’t always the same • Why? Tunicates (Sea squirts) Styela Ascidia Colonial tunicate Encrusting bryzoan Aplidium (Sea Pork) Schizoporella Bryzoans (fleshy) Bugula Hydroids Tubularia Sutherland’s experiment • Followed colonization of plates he placed underwater at different times, or cleared at different time • Results: – Schizoporella dominated when fish predators were common (fish eat Styela recruits) – Styela dominated when few fish were present, Tubularia and Bugula were present (Styela recruits hide under these species Sutherland’s conclusions: • Community on any given patch depends on: – Larval recruitment – Fish predation intensity • Once established, the dominant maintains self/excludes others by feeding on larvae, overgrowth on margins • Overturn by: – Styela sloughing off – disturbance Multiple State Points/Alternate Stable States • Have a different community on the same physical site over long period of time • Each persists – remains stable – until the next major disturbance Is not one “mature” community, but several Inhibition model of succession Other examples: • Horizontal faces – with urchins, without urchins – coralline algae, or foliose algae • Tidepools – with Littorina or without Littorina