Plant Ecology - Chapter 12

Disturbance & Succession

Succession

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

Degradative succession

Decomposers breaking down organic matterLeads to disappearance of everything, species included

Autotropic succession

Does not lead to degradationHabitat 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 habitat2) increasing salinity of Great Salt Lake

Autogenic succession

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

Autogenic succession can occur under 2 different conditions

In an area that previously did not support any communityPrimary successionExample: terrestrial habitat devoid of soil

In an area that previously supported a community, but now does notSecondary successionExample: terrestrial habitat where vegetation was destroyed, but soil remained

Disturbances

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

Disturbances

Intensity, size, frequencySmall disturbances of low intensity are much more frequent than large disturbances of high intensity

Disturbances

GapsFireWindWaterAnimalsEarthquakes, volcanoesDiseaseHumans

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 colonizersTolerant of harsh conditionsReproduce quickly in unpredictable environs

Example: lichens

r-selected species

Primary - colonized by seeds, spores, via wind, waterSecondary - wind-dispersed seeds, seed banks

Pioneer species

Carry out life processes and begin to modify habitat

Extract resources from bare rockBreak up/fragment rock with rootsCollect wind-blown dust, particlesWaste products accumulateDie and decomposeSoil development begins

Continuing change

Colonizers joined by other species suited for modified habitatEventually replace colonizersBetter competitors in modified habitatLess r-selected, more K-selected

More change

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

Stability

Communities may become stabilized on some scaleReach equilibrium (dynamic)Little or no change in species composition, abundance over long periods of timeClimax communityEnd stage of succession

Will climax stage be reached?

Rarely is climax stage reached quicklySlow succession most common, climax stage almost never achievedCommunity usually affected by some major disturbance (e.g., fire) before climax stage is reachedResets succession, forces it to start again from some earlier stage

Terrestrial succession

Relay Floristics

Relay Floristics

Predictability of SuccessionDeterministic- process with a fixed outcome

Community restorationvia succession?