2. Ecological Succession  Progressive change in species composition, ecosystem function and structure following a disturbance  Minor changes in structure and function accumulate over time  Ecological function of ecosystem changes  Initiated by a disturbance  Directional change in structure  Follows a predictable pattern

3. Successional Classifications (seres)  Primary succession: establishment of plant communities on newly formed habitats lacking plants  Lava flows, sand dunes, landslides, etc.  Secondary succession: return to vegetation following a disturbance  There is some blurring of the classifications

4. Some examples  Tornado levels a strip of forest  Trees are all gone, but seed bank is still there.  Secondary succession will follow  Severe fire burns through organic layer of soil and destroys the seed bank and nutrients  Primary succession would proceed even though plants were there before  Disturbances vary along with their impact

5. Succession as a Deterministic Process  Frederick Clements (1916)  Distinct steps…ends in a climax community

6. Clementsian Succession  Established species alter the environment  Allow new species to move in and establish  Directional change in composition – aka. Community structure – is maintained by the continuous alteration of the environment  Within the community (specified):  Succession always follow the same pattern as it develops to climax community  If interrupted, it will follow the path again.  This is why considered deterministic

7. Stability and Climax Community  In Clement’s view,  The concept of climax community assumes:  The species colonizing and establishing themselves in a given region can achieve stable equilibrium  Stable equilibrium – forces to change system = forces to keep system the same…no change results  Therefore, the climate community = stable equilibrium

8. Clementsian Example  Hardwood trees, a climax community, has a disturbance in the form of deforestation.  The community will always proceed this way  The transition will occur in predictable manner.  Barring any further disturbances, the hardwood community will be reached and will be stable.

9. Is deterministic pathway the only way?  Idea of stable community fell out of favor  Individualistic Perspective (Gleason, 1926)  The relationship between coexisting species (communities) as the result of similarities in their requirements and tolerance to the environment.  Partly result of chance  Succession is not deterministic

10. The challenge  Gleason argued that Clements explanation of succession could not explain retrogressive successions  Plant community simplifies and loses biomass over time

11. Clements vs. Gleason Clements  Assume long term stability  Deterministic  Interdependence among species Gleason  Assume environment can deteriorate over time  Random  No relationship between species – together because of similar env. requirements

12. Stability Refuted  Succession viewed as a phenomenon that rarely attains equilibrium  Equilibrium related to nature of disturbance  Disturbance acts at variety of scales  Magnitude of disturbance varies  Many disturbances remove only part of the previous plant community

13. Disturbance  Variations in the definition (general agreement):  Any relative discrete event in space and time that disrupts an ecosystem, community, or population structure and changes resources, substrate, or the physical environment – Pickett and White, 1985  Discrete in time (as opposed to chronic stress or background environmental variability)  Cause a notable change (perturbation) in the state of the system

14. Consequences of Disturbance  Total habitat destruction  Creation of new habitat (transformation)  Fragmentation (loss of certain habitat, isolation of habitats)  Increase patch number, isolation, edge  Decrease patch size, connectivity, interior  Alter local climate/microclimate, hydrology, biota diversity, behavior, health, persistence)

15. The good in disturbances  Dependent on temporal scale of analysis  Example:  Forest fire  Short term = disturbance  Long term = required to maintain seed bank and regeneration of intermediate tree growth (pines) Intermediate disturbance hypothesis Highest diversity when disturbances occur at certain time intervals.

16. Succession Related to Biodiversity

17. An example  Mt. St. Helens eruption – May 1980  Good place to see the different aspects of succession and disturbance at work.

18. The disturbance

19. Eruption Survival  Survival of organisms was strongly influenced by characteristics of disturbance processes, local site conditions, and biological factors  Pyroclastic flow and avalanche debris:  Almost no organisms survived the blast  Those that survived:  Plants with underground buds, burrowing animals, and organisms protected by snow, topography, or other features  Having diverse refuges facilitated survival of some organisms

21.  Life history attributes:  Many organisms not present yet  Anadromous fish – still at sea  Migratory birds  Larger, local populations suffered higher mortality than smaller species or migratory species  Surviving groups included all of the primary trophic levels – herbivores, predators, scavengers and decomposers  Complex food webs quickly developed in the emerging ecosystems  Surviving species established new interactions and began to process the dead organisms from the pre-eruption system. Eruption Survival

22. Timing  Eruption in early morning  Allowed nocturnal animals to be protected in subterranean burrows  Eruption in early spring  Snow and ice created refuges and many plants had not broken out of winter dormancy at higher elevations  Early successional stage of many recently harvested forest sites  Profusion of wind dispersed seeds of pioneer plant species