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 communityClimax

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 3Climax 4

Climax 5Climax 6

Species interactions• Bob Paine (UW)• Intertidal ecology• How do communities

respond to alteredspecies interactions?– Predatory interaction

between Pisaster & Mytilus

– What effect does predator removal have on Mytilus populations

Biotic interactions dostructure 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 frequentand 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 succession1. 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 successionalspecies traits:– Small– Good

dispersers/colonizers– Short-lived– Adapted to withstand

harsh abioticconditions

– High r

• Late successionalspecies traits:– Large– Poor

dispersers/colonizers– Long-lived– Good long-term

competitors for resources (adapted for success in bioticencounters)

– 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.