Models of succession

Succession: (a working definition) an orderly process of development involving changes in species composition and community processes over time

General terms used for describing successional concepts:

Sere: the whole series (or gradient) of communities present on a site over time

Seral stage: a specific community occurring on a site at some point in time

Pioneer stage: initial seral stage

Climax stage: theoretical endpoint of successional; final seral stage that is (1) self-perpetuating, and (2) in equilibrium w/ its biotic and abiotic environment

Autogenic succession (syn. biotic succession):

organisms are driving forces in succession; habitat is altered by organisms to favor other organisms, which then replace previous occupants (e.g., temp., light, nutrients, soil physical properties)

The pattern of succession formalized by Clements was described by naturalists from the 1600s through the 1800s

Early scientific work: Cowles (1899) sand dunes; Clements (1901-1920s)

Clements (1916 book) described succession as a sequence of identifiable processes:

nudation--initiates succession

ecesis (successful establishment of plants, coming either from propagules--seeds, root fragments, or whole plants--remaining in the soil or migrating from somewhere else)--controlled by environmental conditions and the characteristics of plant species available at the site

competition --> elimination of some species

reaction (the change in environment as a result of plants growing and dying)

stabilization develops as long-lived species dominate a site

The traditional view that developed as a result of Clements' work is that of 'relay floristics':

Species prepare area to make it more suitable for other species

If, as Clements indicated, plant communities are superorganisms, they are capable of having a "strategy"

In 1971 Odum (Fundamentals of Ecology; also 1983 Basic Ecology) discussed succession under the heading "The Strategy of Ecosystem Development":

[S]uccession is an orderly process of community development; it is reasonably directional and, therefore, predictable ... it results from modification of the physical environment by the community; that is, succession is community- controlled.... Species replacement in the sere occurs because populations tend to modify the physical environment, making conditions favorable for other populations until an equilibrium between biotic and abiotic is achieved.

Thus, the prevailing paradigm in vegetation development from the early 1900s until relatively recently was that communities facilitate the development of other communities by altering site conditions:

This implies that late-successional species could not occupy the site w/o earlier-successional species occupying the site first

Frank Egler (1954) studied secondary succession in old fields

He concluded that, when disturbed, soil retains large and diverse propagule pool, representing various successional stages

i.e., all spp. are present from the beginning, and different spp. assume dominance over time

Egler called this 'initial floristics'--most species are present initially (as seedlings or seeds), and succession merely represents changes in dominance over time

absence of a species from the propagule pool --> this sp. will not be part of succession or will only become a part very slowly

Drury and Nisbet (1973) reviewed field evidence for relay floristics and initial floristics models of succession, and concluded:

1. many species which characterize later successional stages are present but inconspicuous at earlier stages (support for initial floristics model)

2. removal of annuals in first few years often enhances performance of perennials

3. similarly for forest successions, removal of early- successional pines speeds up dominance by late- successional hardwoods

4. inhibition against self is inconsistent w/ respect to natural selection--a sp. would not be expected to create a situation in which it can not survive and other spp. can

5. "most studies suggest early stages can be explained in terms of" differential growth in response to changing resource availabilities associated w/ succession; early plants often delay succession; later plants are present throughout sere

In general, Drury and Nisbet viewed succession as a process in which plant spp. are sorted along a gradient of resources

since each indiv. sp. has a unique optimum (of resources) for growth or reproduction, and

because resource availabilities change through time (e.g., light decreases, most nutrients change),

species replacement occurs

Drury and Nisbet's model represented a fundamental shift from succession as a vegetation-controlled phenomenon to a process based on properties of individual species

Pickett (1976) expanded Drury and Nisbet's resource gradient concept to include competition; thus, species replacements occur during succession as a result of changes in competitive "winners" in a changing environment

Connell and Slatyer (1977)

Developed 3 models for succession, then reviewed the literature to determine am't of evidence to support each

Facilitation ('Model 1')

colonists prepare environment for later successional spp. (no implication of strategy vis-a-vis Odum)

appears to be associated w/ primary succession

C&S attribute this model to relay floristics

Tolerance ('Model 2')

modifications that early-successional spp. impose on environment neither increase nor reduce rates of recruitment and growth of later-successional spp.

species sequence is solely a function of life history

late-successional spp. either arrive early or late, then grow slowly

late-successional spp. are able to grow and reproduce despite the presence of early- successional spp.

C&S attribute this model (and their 'Model 3') to initial floristics

concluded that few situations in the literature fit this model

Inhibition ('Model 3')

once early colonists secure a place and/or resources, they inhibit subsequent invasion by other spp., or suppress the growth of spp. invading at the same time

when an early colonist dies, space and/or resources are release for another individual (could be the same or different spp. --> succession)

driven by negative interactions

Nobel and Slatyer (1980)

attempted to define 'vital attributes' of spp. that would predict their performance during succession

based on:

method of arrival or persistence after disturbance

ability to enter an existing community and then grow to maturity

time required to reach critical stages in the species' life cycle

Pickett et al. (1987) expanded on the concept of vital attributes and developed a hierarchy of succession including causes of succession, contributing processes, and defining factors

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."