AEC 360 Ecology

Succession and Disturbance

  1. The basics
  2. Community dynamics - stability and change
  3. How does succession operate?

 

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?

Disturbance:

“Resets” all or part of a system to an earlier or alternate state

Removes individuals

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

Climax communities

 vs.

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

Facilitation

Tolerance

Inhibition

1- Facilitation

•         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

 

2 – Tolerance

•         Null model

•         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.

 

3 – Inhibition

-          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