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

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

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

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

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

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

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.

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

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

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

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

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

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)

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.

Succession Related to Biodiversity

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

The disturbance

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

 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

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