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Disturbance and Succession. Intermediate Disturbance Hypothesis.

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intermediate disturbance hypothesis
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
slide7

Species diversity on intertidal boulders with

different degrees of disturbance – from 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
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
  • 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
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.
facilitation succession
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
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
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.
slide39

Ulva – above and

Gigartina overgrowing Ulva – right

typical succession
Typical Succession
  • In most successional sequences, all three mechanisms operate at different times in the sequence.