1958: Bryce Canyon National Park

1970

1991

Vegetation dynamics

• Also known as plant succession– Sequence of compositional and structural

vegetation changes through time• Why study succession?

Succession

• Pragmatic reasons: forest management and restoration

Succession

• Study of succession raises other questions: – How is nature organized?– Are communities highly

integrated or are they more individualistic?

Succession

• Two types of succession

– Primary

– Secondary

Recovery following eruption on Mount St. Helens---primary or secondary? Or is this an overly simplistic dualism?

Early views on ecological change

• Pervasive order, nature as clockwork of God• Nature has telos, a guiding hand to

development• Change is to admit imperfection• Succession (and evolution) runs counter to

religious doctrine• Nature as a cathedral, holy and timeless,

without change, a static view of nature

Early views on ecological change

• Darwin (1859, On the Origin of Species)– Originally held religious, static views of nature– Observation of competition in nature

contradicted the perfection of holy design– Competition, natural selection drives change– Change and flux in nature accepted– No telos: there is no external, god-like entity

orchestrating natural selection towards an endpoint in a rigid, predetermined way.

History of plant succession

• Holistic, equilibrial, organismal views (Clements)• Individualistic views (Gleason, Whittaker, Watt,

Egler)• Ecosystem ecology and resurgence of holistic,

organismal views (Odum)• Disturbance and non-equilibrium ecology• Synthesis views that combine holistic and

individualistic perspectives

Frederic Clements(1874-1945)

Key terms associated with hisfacilitation model of succession:

immutabledeterministicequilibrialorganismalholistic superorganismalorderlyintegrated

Contributions of Clements

• Defined primary and secondary succession• Popularized a misleading concept often abused: nature

will always grow back to its climax state• Immutable pioneer-to-climax sequence brought out

critics who saw natural disturbance as overlooked phenomena

• Introduced idea that evolution works at higher levels than the individual

• Superorganismal concept has been discredited, but he still receives undue criticism---he had a much more nuanced conception of a climax

Henry Allen Gleason(1882-1975)

Key terms associated with hiscontinuum concept:

individualisticreductionistrandomcontingentnon-equilibrialdisorganized

Contributions of Gleason

• Major works published in mid 1920’s, but not acknowledged for 30 years because of the shadow of Clements

• Contributed to development of non-equilibrium ecology• His work allowed for a much richer possibility of new and

novel plant communities • Idea of loosely organized plant communities has been

abused: if nature is unorganized, then why worry about human impacts, right?

The decline of Clementsian dominance and the turn toward Gleasonian

individualism

• Dust Bowl (1930’s)

• Chestnut blight (1950’s)

• Scholars– R.H. Whittaker – Frank Egler– A.S. Watt

American chestnut (Castanea dentata)

http://www.patacf.org/history.htm

R.H. Whittaker (1920–1980)

IndividualisticGleasonianMathematicalGradient analysisSmoky Mountains

• Individual species, not entire populations, replace each other during succession (time) and across space (ecotone).

• In absence of disturbance or environmental discontinuities (a sudden change in soil type or topography) boundaries between plant communities are not sharp.

A.S. Watt (1947)

Gap phase dynamicsEndogenous disturbanceStability through constant changeDynamic equilibriumSpace time substitution

Gap phase dynamics

Space-time substitution in a chronosequence

F. Egler’s (1954) Initial relay floristics model of succession

IndividualisticContingentNoveltyLife history traits

Life history traits

Initial relay floristics (Egler)

• Grasses and forbs (A)

– r-selected, efficient long-distance dispersal, fast-growing, shade intolerant

• Pines (B)

– Shade intolerant, intermediate growth rate• Oaks ,hickories (C)

– C: k-selected, more local dispersal, slow growing, shade tolerant

The rise of ecosystem approaches to the study of succession: revising Clementsian

ideas of development and equilibrium

Eugene Odum(1913-2002)

Systems ecologyEcosystemsHolisticCommunity controlledEquilibriumBiomassMutualismsZero growth economyHomeostasisScale

Odum: the strategy of ecosystem development (1969)

• Succession is orderly, directional, and predictable• Succession is community-controlled, though physical

environment often sets limits • Culminates in a stabilized ecosystem in which a

maximum in biomass and mutualisms are maintained for the available energy flow.

• Strategy of succession is increased control of, or homeostasis with, the physical environment to achieve maximum protection from its perturbations.

Rejection of equilibrium views and questions about how to incorporate

exogenous disturbance

Inhibition model of succession

Connell and Slatyer (1977)

Did the Gleasonian individualistic paradigm go to far?

Tom Vale (Plants and People,1982)