Download Disturbance Stable States

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