TRADITIONAL SUCCESSION AND CLIMAX CONCEPTS

PRELIMINARY DEFINITIONS:

A. Succession

Kimmins: “Ecological succession is the process by which a series of different plant communities and associated animals and microbes succesively occupy and replace each other over time in a particular ecosystem or landscape location following a disturbance to that ecosystem.”

Spurr and Barnes: "Succession is the replacement of the biota of an area by one of a different nature"

Importance of temporal scale: ecological succession at time scales of a few years to to 100s of years (occasionally over a 1000 years) versus biota replacements at geological time scales of > 1000s of years (evolution, long-distance migration of biota, major change in climate).

Succession requires directional change in floristic composition.

B. Disturbance

Grime 1979: "...mechanisms which limit the plant biomass by causing its partial or total destruction."

1. Monoclimax theory (Clements)

Late-19th century historical context--shift from "static" and "creationist" views of nature to "dynamic" and "evolutionary" views.

Clements emphasized:

a. convergence of floristic composition;

b. stability of the endpoint of succession (i.e. permanence of climax);

c. strong role of climatic control of the climax;

d. "life history of the climax formation" as the theoretical explanation of succession; and

f. "reaction" (community control, dominance of autogenic change) as the mechanism (proximate cause) of change.

Clementsion succession theory is:

1. an equilibrium or developmental model (i.e. assumes long-term site stability and progressive development towards a stable endpoint);

2. a deterministic model (highly predictable pattern of orderly change due to "driving forces" that leave little room for the influences of chance).

Henry Gleason's 1926 critique of monoclimax theory:

a. not as predictable as Clements claimed;

b. successional pathways are variable and not orderly.

2.Polyclimax Theory (Tansley)

a. During succession the floristic convergence is only partial.

b. There may be more than one type of stable endpoint.

c. These alternative stable endpoints are controlled by local factors such as topographic position or soil type (consequently, called topographic climax, edaphic climax, etc. as opposed to "climatic climax").

In contrast, Clements regarded these as "subclimaxes."

Polyclimax theory is:

a. an equilibrium model;

b. less deterministic than monoclimax theory.

3. Pattern climax theory (Whittaker 1953)

Premises:

a. landscape consists of environmental gradients;

b. tendency for a stable type of vegetation to develop will be different at each point along these gradients.

Therefore,

the climax vegetation will be a spatial pattern of vegetation which reflects the spatial variation in the underlying physical environment.

This is simply a combination of the continuum/gradient concept and the individualistic concept of the plant association. In areas of relatively homogeneous regional climate Whittaker recognized the most common climax type as the "prevailing climax."

STEADY-STATE CONCEPTS

Steady-state was proposed by Whittaker as a subsitute for climax terminology because the later carries numerous theoretical implications which lead to confusion.

TYPES OF STEADY-STATES

1. Steady-state ecosystem-- Defined by constant biomass and nutrient content.

2. Steady-state populations-- Defined by constant population sizes.

3. Steady-state community (or compositional equilibrium) -- Defined by species persistence.

Allows for fluctuations (i.e. a dynamic equilibrium) but requires "a relatively stable species composition."

Identified on the basis of regeneration status (persistent versus non-persistent species populations).

ADVANTAGES OF STEADY-STATE TERMINOLOGY (COMPOSITIONAL EQUILIBRIUM):

1. No theoretical or causal implication.

2. No implication of convergence towards a single climax type.

3. Can be applied to overstory separately from understory.

4. Can be applied at specified spatial and temporal scales.

5. Can incorporate periodic, repeated disturbance.