Models of succession

Lecture graphics

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