What structures communities? Biotic & Abiotic factors

24 Pages·2009·0.62 MB·English

by  Joshua Whorley

Community structure
What structures communities?
Biotic & Abiotic factors
Are communities predictable
or stochastic (random)?
• Are communities highly organized or just
loose assemblages?
– Do communities have a predictable number
of trophic levels?
– If a disturbance erases a community, will
identical species recolonize? Will they have
the same interactions with each other?
Competing Hypotheses
• H : Communities are stable & integrated
1
with predictable compositions (Clements)
– Communities pass through predictable stages
of development & species composition
– Those stages end in a Climax Community
– Determined by climate& species interactions
– If disturbed, the same species will arrive &
reconstitute the community
Climax
Competing Hypotheses
• H : Communities are loose, unpredictable,
2
chance assemblages of species
– The presence of each species is due to chance
– History& abioticfactors determine which
species are present
– If disturbed, the chance of identical
reconstitution is small
Climax 1
Climax 2
Climax 3
Climax 4
Climax 5
Climax 6
Species interactions
• Bob Paine (UW)
• Intertidal ecology
• How do communities
respond to altered
species interactions?
– Predatory interaction
between Pisaster&
Mytilus
– What effect does
predator removal have
on Mytiluspopulations
Biotic interactions do
structure communities
• Removal of predator
leads to:
– increased prey densities
– decreased species
diversity
• algae & invertebrates
disappear
• Mytiluspopulations
explode
• Keystone species
– Has greater impact on
surrounding species than
its abundance suggests
Historical data
• Pollen record suggests that species
composition at a given site changes
through time (NA).
– Some tree species appear and disappear
together.
– Others appear to come and go independently
of each other.
– Is community composition static or dynamic?
– Is it predictable or dependent on history &
chance?
More experimental data
• Pond communities!
• Examined richness &
diversity of
planktonic species
• 12 identicalponds
– Same location, filled
& sterilized @ same
time
• Prediction if H is
1
correct?
• Prediction if H is
2
correct?
Results
• 60 possible species
• Ponds contain ~ 34
species
• Most species occur in
most or all of the
ponds, but NO
assemblage is identical
• Why?
– Dispersal ability?
– Occupied niches?