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Community Ecology
Chapter 9
Succession
Temporal patterns in communities Replacement of species by others within
particular habitat (colonization and extinction)
Non-seasonal, continuous, directional
Degradative succession Decomposers breaking down organic
matter Leads to disappearance of everything,
species included
Autotropic succession Does not lead to degradation Habitat continually occupied by living
organisms
Two types of autotropic succession Allogenic succession
Autogenic succession
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
Autogenic succession Change of species driven by biological
processes changing conditions and/or resources
Example: organisms living, then dying, on bare rock
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
Primary succession Volcanic eruptions
Glaciers
Secondarysuccession Floods
Fires
Rate of succession Primary - slow - may take 1000s of years
Secondary - faster - fraction of the time to reach same stage
Autogenic succession begins… First community comprised of r-selected
species - pioneer species
r-selected species Good colonizers Tolerant of harsh conditions Reproduce quickly in unpredictable
environs
Example: lichens
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
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
More change Communities gradually become dominated
by K-selected species Good competitors, able to coexist with
others for long periods of time
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
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
Terrestrial succession
Lake or pond succession