Community Dynamics- Disturbance and Succession

Learning Goals:
* Understand the basic properties of disturbances (intensity, frequency), with special emphasis on fire, and how they determine the successional responses of communities
* Be able to describe the abiotic and biotic determinants of community response to disturbance and exposure of new substrate = primary and secondary succession

Composition of communities changes constantly, in response to changes in both abiotic (e.g. climate) and biotic (e.g. species interactions) factors
Pace of change varies considerably; human time perspective fools us into thinking stasis is the norm, i.e. things don’t change

Disturbances = abiotic factors that kill organisms, influence population sizes and community composition, and open up an opportunity for species to become established
 * Impact of the disturbance on a community is determined  by the type, intensity, and frequency
* Many disturbances occur with regular frequencies- e.g. floods, fire
 * There is usually a negative relationship between frequency and intensity (the magnitude of the impact) – Low intensity disturbances are more frequent than high intensity disturbances

Common disturbances include wind, floods, drought, human land use, air pollution, geomorphic factors, and fire- many have regular frequencies that vary in intensity
Fire provides a useful example for examining intensity and frequency; related to the requirements:
1) Fuel: plants, both live and dead; moisture content and chemical composition determine flammability
2) Ignition source
3) climatic conditions conducive to carrying a fire-warm, dry, windy conditions
Fire intensity categories:
1) surface fires- low intensity; burn understory and litter, leaving soil and overstory trees relatively unaffected
2) crown fire- more intense; burns understory as well as canopy trees
3) ground fires- limited to soils with very high organic matter content  e.g. Boreal Forests, Arctic, swamps; slow moving, but can be intense and last for long periods of time
Fire Frequency dependent on both the rate of fuel accumulation and appropriate climatic conditions

Historic Range of Variability
The variability in the environment is an important determinant of community development and ecosystem function
Applies to disturbance regimes, i.e. the spatial and temporal characteristics of a disturbance

Biological response to disturbances = Succession, the change in species composition in communities over time
Primary succession is community establishment and change on a site that was devoid of life
Secondary succession involves the reestablishment of communities in which some, but not all organisms have been destroyed by a disturbance

Studying succession may utilize “space for time” approach, examining different areas that have undergone succession for different amounts of time
Study of succession has at times been contentious
Some (e.g. Frederick Clements) believed succession occurred in a predictable, orderly fashion with community replacement driven by facilitative promotion from one to the next
Others (e.g. Henry Gleason) believed succession was driven by the responses of individual organisms and was subject to the environmental conditions at the time

Conceptual models of factors controlling succesion:
Facilitation, inhibition, tolerance, and chance
Early conceptual models of succession emphasized the role of facilitation as a control of succession; one sere prepared the way for the next (e.g. Clements)

Additional observations and experiments indicated additional biotic interactions and chance may play important roles in succession:
1) facilitation: increasing soil resource availability, lessening the climatic extremes
2) inhibition: competition, herbivory, and allelopathy
3) tolerance:
    a) ability to survive the disturbance e.g. Pines and fire
    b) climatic adaptations- ability to survive the post- disturbance environment
4) life-history characters- reproductive output, longevity, and reproductive maturity
5) chance events: e.g. in what season did the disturbance occur; what direction was the wind blowing? (which plant arrives first?)
led to development of  Tolerance-competition model (Connell and Slatyer)

Facilitation tends to be more important in the early stages of primary succession
The life history characters of the plants were important for  determining the "path" of succession- what species occurred in each successional community
Biotic interactions were important for determing the rate of succession- i.e. how fast community turnover occurred

Rates of predation/herbivory can affect the rate and outcome of succession, and can influence final stable community