Recent evidence of change has led to a nonequilibrium model, which describes communities as constantly changing after being buffeted by disturbances.
A disturbance is an event that changes a community, removes organisms from it, and alters resource availability.
Fire is a significant large scale disturbance in most terrestrial ecosystems. It is often a necessity in some communities.
The intermediate disturbance hypothesis suggests that moderate levels of disturbance can foster greater diversity than either high or low levels of disturbance.
The large-scale fire in Yellowstone National Park in 1988 demonstrated that communities can often respond very rapidly to a massive disturbance.
(a) Soon after fire
(b) One year after fire
Ecological succession is the sequence of community and ecosystem changes after a disturbance, over time.
Primary succession occurs where no soil exists when succession begins. Pioneer organisms, such as lichen, are the foundation of the community and soil building.
Secondary succession begins in an area where soil remains after a disturbance / disaster such as fire or field abandonment.
Early-arriving species and later-arriving species may be linked in one of three processes:
Early arrivals may facilitate appearance of later species by making the environment favorable
They may inhibit establishment of later species
They may tolerate later species but have no impact on their establishment
Glacier retreating -- predictable pattern of ecologial succession …
Pioneer stage = soil builders / fireweed dominant
Dryas stage grasses and shrubs
Alder stage: trees and shrub
Spruce stage = Climax Community STABLE
Succession is the result of changes induced by the vegetation itself.
On the glacial moraines, vegetation lowers the soil pH and increases soil nitrogen content.