IIIC. 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 successively 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
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
A simple definition (Grime 1979) "...mechanisms which limit the plant biomass by
causing its partial or total destruction."
1. Endogenous disturbances originate within the plant community itself (e.g. tree death
due to old age and the creation of a gap).
2. Exogenous disturbances originate outside the plant community (e.g. volcanic ash
deposition).
Can a disturbance event always be classified as being exclusively endogenous or
exogenous?
C. Terms describing succession
1. Primary succession occurs on sites that did not previously support a plant cover.
2. Secondary succession occurs on sites that previously supported a plant cover, and
some remains and “legacies” of that plant cover persist at the site.
3. Autogenic mechanisms of succession are changes in the environment caused by the
plants themselves which drive or trigger successional change.
4. Allogenic mechanisms of succession are changes in the environment caused by
physical processes which are relatively independent of the vegetation itself.
5. Biogenic mechanisms of succession refer to a sudden change in the biota which has
a major influence on succession (e.g., introduction of a plant disease which removes a major
plant species, introduction of an herbivore which significantly affects plant populations).
6. A sere is a sequence of plant communities that develop over the course of
succession; seral stages are sometimes recognized. Sere is the “product” of succession
whereas the term “succession” is usually used to refer to the process of the change.
Is it appropriate to describe a sere as being “autogenic succession,” “allogenic
succession,” or “biogenic succession?”
Classical Theories of Succession
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.
Clementsian 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. succession is 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."
Tansley’s 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 substitute 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).
Main advantages of using steady-state terminology (e.g. compositional equilibrium) include:
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.