Disturbance & Succession

Primary succession is a process that generally involves:

1) the accumulation of organic matter- wind blown debris, insects migrating in, etc.

2) Small plants (long-distance dispersal, small seeds)

3) Accumulation of more material, deeper rooted species- more habitats for wildlife

4) Continued increase in organic matter, the formation of rudimentary soil, eventually “real” soil, vegetation and the accompanying animal species.

12.16 Lava field on Hawaii, showing a young tree fern that has colonized the bare ground

12.8 The 1980 eruption of Mount St. Helens lead to primary succession

Henry Chandler Cowles- Lake Michigan Dunes, 1899

Glacier Bay

Secondary Succession

12.5 A large Dipteryx panamensis has created a gap

Gap formation & colonization are important for maintaining biodiversity.

-Forest floor heterogeneity

-Increased light at forest floor

-Colonization space for shade intolerant species

12.6 Locations of all tornadoes recorded the United States from 1981 to 1990

http://www.youtube.com/watch?v=rFeufWFPVm4

Tornado damage “Oak Openings”

12.9 Disturbance and early succession in a forest in northern lower Michigan (Part 1)

12.9 Disturbance and early succession in a forest in northern lower Michigan (Part 3)

12.11 Annual growth rings show clearly in this cross section of the trunk of a white pine

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Quercus macrocarpa

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Quercus macrocarpa

Some theories of succession… equilibrium forces, facilitation, inhibition and tolerance…

F. E. Clements

Put forth the idea that ecosystems arrive, ultimately, at a stable composition/structure, that is dictated by climate. The “Climatic Climax” community. When to exhaustive lengths describing the orderly phases ecosystems move through prior to arrival at the climax.

Was influenced by vegetation zonation, where if you at a certain elevation and aspect you see the same vegetation type over, and over, again across the landscape.

This idea is a predecessor to the idea of “Equilibrium Communities.” A more nuanced idea, to be sure, because it implies “balance” but, still emphasizes a deterministic process leading to a stability

http://people.wku.edu/charles.smith/chronob/CLEM1874.htm

1 & 2 are “equilibrium concepts.”

In number one the systems is passing through an orderly series of stages arriving at a common endpoint (box D) after that point the system does not change.

In two, the system is a bit less orderly, but still arrives at a stable endpoint.

Another way to think about equilibrium is a ball and cup model.

Manipulating ecosystem dynamics is an essential part of ecological restoration.

In many cases, you start from a system that is complete disarray- equilibrium forces are what you need to manage to drive the system toward the desired state.

But, what if the system has multiple-equilbria as demonstrated in 4 (or in a ball and cup diagram)?

Or, what if the system had no equilibrium! This is demonstrated in 3, (or via a ball and cup diagram)

In fact, the idea of a climax community and equilibrium conditions has been all but annihilated in the ecological literature. Clements has been over-simplified and is now something of a joke. As an ecologist it is a bit embarrassing to be caught in something that smacks of being Clementsian!

Many have put for the idea that ALL ecosystems are disequilibrium communities. There are no stable enpoints, and things are constantly changing.

This is particularly prevalent among paleoecologists who point out that ecosystem drivers are in constant flux. Climate and anthropogenic processes have been dynamic for 15,000 years in eastern North America and ecosystems are constantly reacting to them.

Nevertheless, there are some real distinct patterns- (Ecosystems near Dayton pretty much want to turn into hardwood forest… for instance.)

Equilibrium forces (succession)

Non-equilibrium forces (disturbance)

12.14 Predictable and unpredictable successional trajectories

Convergence, Divergence and Stability…

Is succession deterministic or stochastic?

12.13 F. E. Egler’s theories of succession diagrammed for a hypothetical abandoned field in NC (1)

12.13 F. E. Egler’s theories of succession diagrammed for a hypothetical abandoned field in NC (2)

Some theories of succession-

1) Facilitation- Each species creates opportunities for the next species. The system cycles through species until a species no longer provides this opportunity and become dominant.

2) Inhibition- Species hold onto the site via competition (and other negative interactions) and only give way to other species due to disturbance or death.

3) Tolerance- Species that make up the final composition of the community are those best able to tolerate environmental conditions. They are neither hindered (inhibition) or helped (facilitation) by other species

12.12 A saguaro cactus grows in the shade of its nurse plant, a palo verde tree

Watts Model of Vegetation Dynamics

Implications of succession…

Implications of succession- ecosystem processes…..

Disturbance & Succession