Biol 121- 2010.01.15- Ecology- Community Ecology- Community Structure (Ch. 53).docx

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Biol 121 225
Freeman 1209-1217 (Ch. 53) Jan 15th, 10
Limits on research on species
-usually focuses on two at a time, but biological communities can contain
thousands of species
-to understand how communities work, biologists must broaden the scope of
research and explore how combinations of many species interact
-if community destroyed by disturbance and allowed to recover, diversity and
abundance of species should be identical when recovery complete
-but if communities can be made up of many diff combo of species, depending on
which arrive earlier/later, then community composition will be difficult to predict
Climax community (Frederick
-proposed by Frederick Clements
-Clements argued that communities develop by passing through a series of
predictable stages dictated by extensive interactions among species and that this
development culminates in a stable final stage known as a climax community
Henry Gleason
-contended that community in an area is neither stable nor predictable
-claimed that plant and animal communities just happen to share similar climatic
-largely a matter of chance whether similar community develops in the same area
after a disturbance occurs
Factors that determine community
-Gleason’s view is closer to being correct
-although both biotic interactions and climate are important in determining which
species exist at a certain site, chance and history also play a large role
Keystone species
-a species that has a much greater impact on the surrounding species than its
abundance and total biomass would suggest
-for example, introducing wolves can reduce elk pop due to predation, and the
food that elks eat (aspen, willow, cottonwood) increase, which increases number
of beavers that competed with elks for these resources
-also, wolves might not tolerate presence of coyotes, so decline of coyotes which
increases mouse pop which then increases hawk pop
(What do disturbances affect in
-any event that removes some individuals or biomass from a community (biomass
is the total mass of living organisms)
-e.g. fires, windstorms, floods, fall of large canopy tree, disease epidemics, short-
term explosions in herbivore numbers
-these events are important because they alter light levels, nutrients, unoccupied
space, etc.
What factors determine the impact
of disturbance? (3)
1. Type of disturbance
2. Frequency
3. Severity (e.g. speed/duration of flood, intensity of heat during fire, earthquake
-most communities experience a characteristic type of disturbance, and in most
cases, disturbances occur with a predictable frequency and severity
Disturbance regime
- a disturbance’s impact on a community (or the range of the impact)
-it can be predicted b/c disturbances usually occur with a predictable frequency
and severity
-the recovery that follows a disturbance
-name was inspired by observation that certain species succeed others over time
Primary succession
-occurs when a disturbance removes the soil and its organisms as well as the
organisms that live above the surface
-e.g. glaciers, floods, volcanic eruptions, landslides, earthquakes that open up a
canyon, glacier retreat
Secondary succession
-occurs when disturbance removes some or all of the organisms from an area but
leaves the soil intact
Biol 121 225
Freeman 1209-1217 (Ch. 53) Jan 15th, 10
-e.g. fire, logging, farming
Successional pathway
-specific sequence of species that appears over time
What are the 3 factors used to
predict succession outcome in a
1. Particular traits of the species involved (like dispersal capability, and ability to
withstand extreme dryness)
2. How species interact
3. Historical and environmental circumstances (such as size of area involved and
Stages of a successional
Characteristics of successional
-there are different stages during succession that follow a disturbance
-two types of successional communities: early and late successional community
-early successional community dominated by short lived small species that
disperse their seeds over long distance
-late dominated by spp that tend to be long lived, large, good competitors for
resources such as light and nutrients
Pioneering species
-species that arrive after a disturbance (first species to arrive in the area)
-they usually have “weedy” life histories (a weed is a plant adapted for growth in
disturbed soils)
-usually have good dispersal ability
-devote most energy to reproduction and little to competitive ability
-small seeds, rapid growth, short lifespan, reproduce at early arge
What affects successional pathway
after colonization is under way?
-course of succession starts to depend less on how species cope with aspects of
the abiotic environment
-starts to depend more on how species interact with other species
-this change occurs because plants that grow early in succession change abiotic
conditions in a way that makes the conditions less severe
-e.g. plants provide shade that reduces T and increases humidity
-their dead material also adds organic material and nutrients to soil
-as abiotic conditions improve, biotic interactions become more important
Existing species can have 3 effects
on subsequent species
1. Facilitation
-takes place when presence of early arriving species makes conditions more
favourable for arrival of certain later spp by providing shade/nutrients
2. Tolerance
-existing species do not affect probability that subsequent spp will be established
3. Inhibition
-presence of one spp inhibits establishment of another
-e.g. plant species that requires high light levels to germinate may be inhibited in
late succession by presence of large mature tree
Other effects on pattern and rate
of succession
-historical and environmental context
-in summary, species traits and species interactions tend to make succession
predictable, while history and chance events contribute a degree of
unpredictability to succession