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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
• H1: Communities are stable & integrated
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
• H2: Communities are loose, unpredictable,
chance assemblages of species
– The presence of each species is due to chance
– History & abiotic factors 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 Mytilus populations
Biotic interactions do
structure communities
• Removal of predator
leads to:
– increased prey densities
– decreased species
diversity
• algae & invertebrates
disappear
• Mytilus populations
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 identical ponds
– Same location, filled
& sterilized @ same
time
• Prediction if H1 is
correct?
• Prediction if H2 is
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?
Disturbance structures
communities
• An event that removes biomass from a
community
– Forest fires, disease epidemics, etc.
– Produce “vacancies” (unfilled niches)
• Components of disturbance (disturbance
regime)
–
–
–
–
Timing since last disturbance
Frequency of disturbance
Severity
Predictability
Some disturbances “make”
the community
• Tree rings mark time
• Scars mark fires
– In Sierra Nevada mtns,
fires are both frequent
and predictable
– Fire suppression reduced
species diversity,
understory, soil health &
primary productivity
– Suppression has given
way to tolerance &
promotion of small,
controlled burns
Succession structures
communities
• Recovery that occurs after disturbance
• Primary succession
– Occurs after severe disturbance (all organisms & soil
removed
– Volcanic eruptions, glaciations, floods
• Secondary succession
– Occurs after mild disturbance (some organisms
removed)
– Fire, logging
• Recovery often follows a predictable sequence,
but not always
Factors affecting succession
1. Species Traits
–
–
Critical @ early stages of succession
Abiotic conditions have strongest influence
2. Types of species interactions
–
–
More important as time-since-disturbance
increases
Biotic conditions have strongest influence
3. Historical & environmental context
–
Affects pattern & rate of succession
Secondary Succession Sequence
Species traits
• Early successional
species traits:
– Small
– Good
dispersers/colonizers
– Short-lived
– Adapted to withstand
harsh abiotic
conditions
– High r
• Late successional
species traits:
– Large
– Poor
dispersers/colonizers
– Long-lived
– Good long-term
competitors for
resources (adapted for
success in biotic
encounters)
– Low r
Biotic interactions
• Existing species influence future
colonizers in 3 ways
• Facilitation: encourages future colonizers
• Tolerance: no effect on future colonizers
• Inhibition: Inhibits future colonizers
Succession in Glacier Bay
• As glaciers retreated, distinct regions of Bay
become exposed for recolonization
How many successional pathways?
• Just 1: Willows, dryas -> alder, cottonwood ->
spruce, alder -> Spruce, hemlock
Or many?
All factors contribute
• Species traits
– Hemlock requires thick, organically rich soil & shade
for seed germination & growth
– Perhaps only the oldest sites have those conditions
• Species interactions
– Alder facilitates spruce, due to N2 fixing bacteria
living in alder roots
– Alder competes with spruce for light: Spruce are
inhibited at first, then overtop & out-compete alder
• History & environment
– Lower Bay has closer (geographically) sources of late
successional species
Patterns of Species Diversity
• Diversity decreases as distance from Equator
increases
• Explanation? What determines diversity?
– An abiotic factor that varies predictably with latitude?
– An abiotic factor that alters speciation, extinction,
immigration or emigration?
Time since Disturbance
• Observation: Tropical regions ice free
but higher latitudes were glaciated
– H3: Higher latitudes are less species
diverse because they’re in an earlier stage
of succession
– Less time for species interaction & niche
partitioning
Intermediate Disturbance
• Observation: Mid-successional
communities have higher species diversity
• H4: Regions with moderate disturbances
(frequency, severity) contain more species
because they will always have a mix of
pioneer & late-successional species.