Community Ecology

1
Community Ecology

• Chapter 9

2
Succession

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

3
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

5
Two types of autotropic succession

• Allogenic succession
• Autogenic succession

6
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

7
Autogenic succession

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

8
Autogenic succession can occur under 2 different
conditions

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

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

9
Primary succession

• Volcanic eruptions
• Glaciers

10
Secondarysuccession

• Floods
• Fires

11
Rate of succession

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

12
Autogenic succession begins

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

13
r-selected species

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

14
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

16
More change

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

17
Stability

• Communities 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

19
Terrestrial succession
20
Lake or pond succession