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Disturbance and Succession. Disturbance. Disturbance - any agent which causes complete or partial destruction of the community resulting in the creation of bare space
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Disturbance • Disturbance - any agent which causes complete or partial destruction of the community resulting in the creation of bare space • Disturbance agents: both physical and biological processes may cause disturbances, though we usually focus on physical processes - • Physical - fires, ice storms, floods, drought, high winds, landslides, large waves • Biological - severe grazing, predation, disease, things that inadvertently kill organisms - digging and burrowing
Wildfire – Southern California October 22, 2007
Intermediate Disturbance Hypothesis • Disturbance maintains communities in a "non-equilibrium state" (never reach equilibrium) and by renewing colonizable space, disturbance allows the persistence of species that might otherwise go extinct due to competitive exclusion. – from Joe Connell
Waves, boulders and disturbance Wayne Sousa
Species diversity on intertidal boulders with different degrees of disturbance – from Sousa
Species diversity on intertidal boulders with different degrees of disturbance – from Sousa
In an ecosystem, disturbance 1) clears space and interrupts competitive dominance 2) changes relative abundance of species 3) is a source of spatial and temporal variability 4) is an agent of natural selection in terms of life history characteristics
Succession • Succession is the non-seasonal, directional and continuous pattern of colonization and extinction on a site by populations of species - this definition incorporates a range of successional sequences that occur over widely different time scales and have very different mechanisms.
Types of Succssion • Primary - succession on a site that has not experienced life before - extremely severe disturbance may have killed all life so no seeds or roots or individuals survive - lava flow, volcanic explosion, glacial retreat, landslides, weathering of bare rock • Secondary - succession on a site that may have remnants of previous life on it - some survivors of the disturbance - fire, floods, windstorms, wave battering, severe grazing • Degradative - succession in which the substrate is decaying and being exploited by various organisms - succession of decomposers on carcass, rotting log, etc.
Body Farm – University of Tennessee FBI Forensics Class
Facilitation Succession • Early species change community or ecosystem in a way that allows later species to move in and changes the system so that the early species can no longer survive there.
Tolerance Succession • All species arrive at start of succession, but longer lived individuals eventually outlive short lived species and grow to dominate in the succession - long lived species can tolerate shade and competition early in life.
Inhibition Succession • First species to arrive occupies space and prevents the settlement of later arriving species - the first species are replaced only after they die.
Ulva – above and Gigartina overgrowing Ulva – right
Typical Succession • In most successional sequences, all three mechanisms operate at different times in the sequence.
