Succession and Disturbance
Community assembly: the processes through which species form an interacting network within an ecosystem
I.e. the formation of biological communities
What is succession?
Succession: the change in species composition, structure, and function of a biotic community through time
Succession example: establishment of forests
Succession example: Filling of lakes
Succession example: Species turnover on carcasses and dead trees
Succession example: Fermentation and maturation of sour beer!
What is disturbance?
Disturbance: Any disruption of a community, or ecosystem that alters the availability of resources, the presence of substrate, or the presence of individuals.
Disturbances vary in intensity and scale
And can be natural or anthropogenic
How are succession and disturbance linked?
Resets all or part of a system to an earlier or alternate state
Frees resources (including space)
Reduces competitive exclusion
Two general kinds of succession
Primary succession: proceeds on a substrate that has not previously supported a biotic community
Secondary succession: proceeds on a substrate that has previously supported a biotic community
Primary succession: invertebrate community formation on new pier pilings
Primary succession: volcanic disruption (Krakatau, Mt. St. Helens)
Primary succession: lake formation in impact crater or abandon quarry
Secondary succession: Old-field succession
Secondary succession: post-burn regrowth
Glacial retreat: primary or secondary succession?
When glaciers that have covered underlying land in tens to hundreds of meters of ice for hundreds to thousands of years recede, they leave behind scoured, almost Martian landscapes. The exposed land has not seen life since shortly after it was covered in ice. However, much of the soil can be intact (if displaced), and, often, seeds that were buried for centuries are still viable and will germinate in the glacial till(!). Is this primary or secondary succession? It depends on your timeframe, which illustrates that the boundaries can be blurry here. Remember, these terms are descriptive tools, and no tool is perfect.
II. Community dynamics stability and change
Seral stages: periods in community assembly (succession) that are distinct from one another with respect to species dominance, community structure, and ecosystem function.
Some general trends
- Traits of early and late successional species
Early: good dispersers, fast growth and reproduction, short lived, tolerant of exposed conditions, less tolerant of shade
Late: slower to disperse, slow growing, long lived, less tolerant of exposed conditions, more tolerant of shade
- Abiotic conditions vs. competition
Early: During early stages of succession, abiotic conditions tend to be more limiting
Late: During later stages, the limiting role of competition increases
- Biomass and complexity
Early: Biomass and complexity are often lower
Mid: Biomass and complexity may peak
Late: Biomass and complexity may decrease and stabilize
Stable state: the point at which a community develops long-term resistance to change in structure, function, and species composition.
Example: Mature redwood forest
Alternative stable states
Climax community: The end point of a successional sequence ; a community that has reached a steady state under a particular set of environmental conditions (From Ricklefs)
Alternative stable states: Different stable community types that are possible for a given ecosystem
Alternative stable states:
Plant community with large herbivores absent v.s. large herbivores present
Healthy coral reef vs. bleached, algae dominated reef
III. How does succession operate?
Mechanisms of successional change:
Verbal (and mathematical) models
Early species modify the environment, make it more suitable for later species
Late species dependent on early species, may subsequently exclude earlier species once established
Facilitation example: Lupine on Mt. Saint Helens add organic nitrogen to substrate, facilitating other species
Early species have little or no effect on later species
Succession depends on the dispersal abilities and life history traits of individual species as well as environmental characteristics
Tolerance example: Grasses growing among lichen covered rocks, lichens and grasses do not compete with or facilitate one another.
- early species resist invasion of later species
- succession pattern dependent on who gets there first (initial composition)
- change limited to periods following removal of early colonizers by disturbance or senescence
Inhibition example: Sessile marine invertebrate communities
Take home messages:
- Succession refers to the process of community development and change
- Primary succession refers to community development on new substrates
- Secondary succession refers to community development where communities existed
- Disturbance is a natural and necessary part of all ecosystems
- Disturbance starts and often determines the outcome of succession
- Multiple (alternative) stable states exist for most ecosystems, and many factors determine which one is arrived at