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