Disturbance and succession
Effects of disturbances
Disturbances can result in the long –term maintenance of species diversity
Intermediate disturbance hypothesis
Disturbance in communities is the norm and not the exception
Disturbances
In the short term, disturbances can cause local extinctions of previously dominant species
What happens following disturbance?
Ecological succession
Change in the composition of species over time
Primary Succession
Ecological succession
Primary succession - A community begins to develop on a site previously unoccupied by living organisms
Pioneer Species
Species that colonize barren habitats
Lichens, small plants with brief life cycles
Improve conditions for other species who then replace them
Process of environmental modification (facilitation) by organisms is called ecological development
Primary succession
Ecological succession
Secondary Succession - An existing community is disrupted and a new one subsequently develops at the site
Old field succession
Succession
Clements: continuous, directional change in the species composition of a community … leading to a single ultimate community.
Current: not continuous, directional, nor is there a single endpoint
Sequential change in the relative abundances of species following disturbance
Climax community
Stable array of species that persists relatively unchanged over time
Succession does not always move predictably toward a specific climax community; other stable communities may persist
Cyclic changes
Cyclic, nondirectional changes also shape community structure
Equilibrium or Disclimax Communities - Never reach stable climax because they are adapted to periodic disruption
Tree falls cause local patchiness in tropical forests
Fires periodically destroy underbrush in sequoia forests
Mechanisms of succession
Tolerance
r-selected species are tolerant of harsh conditions
Facilitation
One species modified habitat in a way that facilitates invasion by another
Inhibition
One species modifies habitat in a way that inhibits another
Primary succession – Glacier Bay
1794, Capt. Vancouver said covered by mountains of ice
1879, Muir said was passable, noticed fossil trees
1916, Cooper started study of succession
Species change
Pioneer community
Alnus + Dryas (N fixing)
Populus and Picea (partial canopy)
Tsuga (hemlock forest = climax)
Glacier Bay Diversity