Models of succession (continued)

Lecture graphics

Westoby et al. (1989) proposed a state-and-transition model of succession

This model acknowledges that successional pathways can be very complex ... e.g., within a single soil type in a local area:

Succession may lead to convergence at a single community

Alternatively, succession may produce divergence

Multiple climaxes; no climax (cyclic succession) are examples of non-equilibrium communities

Tilman's (1990) "trade-offs" approach grew out of his resource- ratio hypothesis of competition

Tilman recognized 4 constraints to plant establishment and growth:

colonization (incl. many of Pickett et al.'s constraints)

availability of limiting soil resources

availability of light

sources of death (e.g., herbivores, pathogens)

Using his data from Cedar Creek Natural History Area (Minnesota), Tilman concluded that a 3-way tradeoff between colonization, nutrient competition, and light competition "drives" old-field succession there

Tilman systematically eliminated other alternative hypotheses via experimentation

colonization and competitive "strength" for N determine successional pathway for grasses

transition from grassland to oak woodland "seems" best explained by the nutrient:light ratio hypothesis

Tilman's model reflects modern consensus that succession:

is tightly linked w/ interactions between plants

is complex, and therefore cannot be described w/ one model for all situations and locations:

"Other plant communities will have other constraints, and other successions will be explained by other processes."

Modern definition: succession involves recruitment of a suite of species which is different from (or at a different rate than) mortality of a different set of species--death of some spp., w/ replacement into the community by other spp. --> changes in spp. composition which we call succession

Factors affecting rate and direction of succession

1. Type of disturbance

2. Intensity of disturbance

3. Frequency of disturbance

4. Scale of disturbance

5. Community structure at time of disturbance (affects soil development, seed bank, vegetative regeneration)

McIntosh's (1980) article on the history of succession research contains several insightful statements about succession and science:

"The search for clarity if not unity in succession has daunted ecologists from the beginning."

He cites Frank Egler as saying "ecology may not only be more complicated that we think, it may be more complicated than we can think."

He cites Frank Golley (who was editing a volume trying to provide an overview of succession) with: "A simple mechanistic explanation of succession is not possible."

Nonetheless, McIntosh encourages what he calls "the search for satisfying regularity and simplicity ... traditional in science, and [indicates] there is no reason to forgo that search."