Community%20Ecology

1
Community Ecology

• Chapter 9

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Succession

• Temporal patterns in communities
• Replacement of species by others within
  particular habitat (colonization and extinction)
• Non-seasonal, continuous, directional

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Degradative succession

• Decomposers breaking down organic matter
• Leads to disappearance of everything, species
  included

4
Autotropic succession

• Does not lead to degradation
• Habitat continually occupied by living organisms

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Two types of autotropic succession

• Allogenic succession
• Autogenic succession

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Allogenic succession

• Serial replacement of species driven by changing
  external geophysical processes
• Examples
• 1) silt deposition changing aquatic habitat to
  terrestrial habitat
• 2) increasing salinity of Great Salt Lake

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Autogenic succession

• Change of species driven by biological processes
  changing conditions and/or resources
• Example organisms living, then dying, on bare
  rock

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Autogenic succession can occur under 2 different
conditions

• In an area that previously supported a community,
  but now does not
• Secondary succession
• Example terrestrial habitat where vegetation was
  destroyed, but soil remained

• In an area that previously did not support any
  community
• Primary succession
• Example terrestrial habitat devoid of soil

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Primary succession

• Volcanic eruptions
• Glaciers

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Secondarysuccession

• Floods
• Fires

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Rate of succession

• Primary - slow - may take 1000s of years
• Secondary - faster - fraction of the time to
  reach same stage

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Autogenic succession begins

• First community comprised of r-selected
  species - pioneer species

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r-selected species

• Good colonizers
• Tolerant of harsh conditions
• Reproduce quickly in unpredictable environs
• Example lichens

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Pioneer species

• Carry out life processes and begin to modify
  habitat
• Extract resources from bare rock
• Break up/fragment rock with roots
• Collect wind-blown dust, particles
• Waste products accumulate
• Die and decompose
• Soil development begins

15
Continuing change

• Colonizers joined by other species suited for
  modified habitat
• Eventually replace colonizers
• Better competitors in modified habitat
• Less r-selected, more K-selected

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More change

• Communities gradually become dominated by
  K-selected species
• Good competitors, able to coexist with others for
  long periods of time

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Stability

• Communities may become stabilized
• Reach equilibrium
• Little or no change in species composition,
  abundance over long periods of time
• Climax community
• End stage of succession

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Will climax stage be reached?

• Rarely is climax stage reached quickly
• Slow succession most common, climax stage almost
  never achieved
• Community usually affected by some major
  disturbance (e.g., fire) before climax stage is
  reached
• Resets succession, forces it to start again from
  some earlier stage

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Terrestrial succession
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Predictability of Succession
Deterministic- process with a fixed outcome
Community restoration via succession?
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Relay Floristics
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Relay Floristics
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Lake or pond succession
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Disturbances - A Force Structuring Communities

• Non-biotic disturbances can structure a
  community
• Kill off individuals and open up gaps the same
  way a predator might
• But gaps may result from removal of many
  species/individuals, not few as in most predators

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Disturbances

• Relatively discreet event in time that causes
  abrupt change in ecosystem, community, or
  population structure
• Changes resource availability, substrate
  availability, or the physical environment

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Disturbances

• Intensity, size, frequency
• Small disturbances of low intensity are much more
  frequent than large disturbances of high intensity

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Disturbances

• Fire
• Wind
• Water
• Animals
• Earthquakes, volcanoes
• Disease
• Humans

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Gap Colonizers

• Gaps opened by disturbances are colonized by
  nearby, remaining organisms
• Size of gap determines how it is filled

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Gap Colonizers

• Small gaps may be taken over by single
  individual, or leaning of neighbor
• Conditions may be suitable for many species, but
  first-come, first-served (lottery)

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Gap Colonizers

• Conditions present may only favor one species
• Gap remains until colonized by that species

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Gap Colonizers

• Large gaps may be colonized by many
  individuals/species, and proceed through a
  successional process
• Secondary?

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Succession in a Gap
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Succession in a Gap

• Fairly predictable change in species
• Additional disturbance knocks community back to
  an earlier stage in succession

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Multiple gaps

• If single disturbance produces multiple gaps,
  these proceed through succession in phase with
  one another
• May impact structure of entire community

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Multiple gaps

• If multiple gaps develop over time from different
  disturbances, succession occurs out of phase
• Little impact on overall community structure

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Frequency of Disturbance

• Intermediate disturbance hypothesis - species
  diversity should be highest at intermediate
  levels of disturbance

Species richness
Disturbance frequency
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Frequency of Disturbance - example

• Intertidal algae communities on boulders (climax
  in 2-3 years)
• Small boulders - 42 chance of monthly
  disturbance
• Medium boulders - 9 chance of monthly
  disturbance
• Large boulders - 0.1 chance of monthly
  disturbance
• Medium boulders had highest species richness
  (lower on small and large boulders)

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Disturbance vs. Diversity?

• No relationship between disturbance frequency
  and diversity is the most common observation
• Only 16 of studies show expected pattern