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Ecological succession:
lecture topics
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Plant (and animal) communities develop in often
predictable ways following disturbances, a
process known as succession
Several kinds of species interactions alter the
process of succession
 Facilitation
 Inhibition
 Tolerance
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Species richness changes predictably with
successional changes
Disturbance magnitude
Kinds & qualitative characteristics of
disturbances that impact communities
Asteroid impact
volcanism
Glaciation
Hurricane
Landslide
Animal carcass
ice storm fire
treefall
Disturbance frequency
Glaciers are a kind of disturbance that influences
the development of biological communities (Grinnell
Glacier, Glacier N.P.--photo by T. W. Sherry)
Thin soils on glacial till indicate short
time since release from glaciation (photo
by T.W. Sherry)
Aftermath of extensive fire in Yellowstone N.P.
(photo T. W. Sherry)
Fir waves in alpine zone are selfperpetuating dead zones (disturbances)
that create cyclic successional
sequences (from Ricklefs 2001)
Community succession
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The flipside of disturbance is community development
Define succession = community development in
response to disturbance
 Primary
succession involves colonization of new sites,
devoid of plants (e.g., bare soil from glacial recession;
bare rocks)
 Secondary succession involves the re-colonization,
regrowth, and/or germination of recently cleared ground
 Sere or seral stage is a “unit” or subset of succession
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Succession is omnipresent set of complex processes
 Organism
adaptations to disturbed environments attests
to importance, persistence of succession
 Succession may end in climax community
Kinds & examples of succession
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Most ecological communities are probably always in
some phase of recovery from disturbance, i.e.,
undergoing systematic change = a kind of
resilience
Terrestrial succession best studied, but aquatic, &
marine succession certainly important (e.g., animal
fauna changes in deep sea whale carcass;
progression of invertebrate species on rocks)
Most studies deal with plant succession, but animal
succession is also well known (as in above marine
examples; also succession of detritivores in rotting
tree trunks)
Examples of succession in next few slides
Lichens on
rocks,
initiating
primary
succession;
Glacier
National Park,
Montana (photo
by T. W. Sherry)
Succession examples: 1º succession,
Indiana sand dunes
Marram grass settlement
Accumulation organic nutrients
Establishment shrubs
Shrubs replaced by trees
Examples of succession: old-field (2º)
succession, oak-hornbeam, Poland
After forest cleared
After 7-years
After 15 years
After 30 years
After 95 years
After 150 years
2º Succession examples: bog
succession, Ontario, Canada
Beaver dam forms pond
Bog fills in from edges
Quaking bog covers almost entire wetland
Several models of succession...
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Facilitation = a process in which one or more
species alter environment, making it more suitable
for one or more other species to invade
 Earliest
ecologists to study succession thought this
process was predominant; this was only original
model
 Facilitation thought to lead progressively and
inexorably to climax community (stable end point,
comprised of K-selected species that replace
themselves, maintaining identical community)
 Climax then determined by local climate, soil
characteristics
Example: Alder
trees (shrubs)
facilitate
succession by
fixing nitrogen
in soils,
making them
more suitable
for invasion by
birch, aspen
spruce trees
Coralline
algae facilitate
seedling
establishment
of surfgrass
(Phylospadix)
Climax
community is
rarely any
deterministic,
fixed endpoint
of succession,
but rather a
continuum of
endpoints,
depending on
soil conditions,
among others
(from Ricklefs 2001)
Ecological models of succession,
continued
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Second of these models is inhibition
Defined as the negative effect of one species on
another, preventing it from establishing as quickly
(or at all) during succession
Basically involves interspecific competition as
mechanism
 Often
involves preemption of space
 Alternatively, may involve allelochemical interactions
 Third mechanism involves soil-borne pathogens
associated with particular plant that helps outcompete plant earlier in succession
Seral position of plants is related
to life-history adaptations: In this
2º succession of N. Carolina old
fields, Crabgrass (summer) and
Horseweed (winter) disperse well
(r-selected), and tolerate sunny,
bare soil conditions, but are
easily outshaded by perennials
such as broomsedge (example of
inhibition by broomsedge)
Broomsedge inhibits establishment of
aster by scavenging water, nutrients
from soil near parent stems
Ecological models of succession,
continued
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Third such model = tolerance
 Defined
by a lack of an effect on other species
 According to this model of succession, sequence can
start out with any of several disturbance-tolerant
species, and it eventually ends up with a climax species
that can persist indefinitely
 Example? Floral succession (2º), in which all species
present as seed bank in soil
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Fourth such model = random colonization
 Accordingly,
any species can colonize, and any species
can be endpoint--it is determined stochastically (i.e.,
colonization lottery)
 Example--colonization of corals by herbivorous fish?
Any concensus today about models
of succession?
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Succession is complex, and no one process is preeminent
Multiple processes involved in many successions,
and these processes manifest themselves at
different successional (seral) stages
E.g., 2º succession of abandoned tobacco fields in
North Carolina Piedmont region
 Inhibition
of white aster, ragweed by crabgrass
 Facilitation of broomsedge by white aster, ragweed
 Random colonization by loblolly pine, hardwoods,
white oak, dogwood, hickory in later seral stages
Early successional species
tend to be r-selected
Note tradeoff: shade tolerance of late-successional plants at
expense of slow growth rate, few (large) seeds
Survival of seedlings in shade is
directly related to seed weight--later
seral stages tend to have larger seeds
Some “climax” communities are
maintained by extreme conditions,
such as frequent fires -> “fire climax”
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Longleaf pine savannas of southeast are maintained
indefinitely as the endpoint of succession, as long as
fires are allowed to burn
 Depend on summer fires to burn off (kill) fireintolerant hardwood tree species
 Adapted exquisitely to tolerate (even promote?) fire
 In absence of fire, hardwoods replace pine savanna
 E.g., river bottoms such as Camp Independence
along Tangipahoa River
 Covington & Mandeville, where fires are
suppressed (see slides)
Fire used to control shrubbery, restore
pine savanna in Kisatchie Nat’l Forest,
LA (photo T. W. Sherry)
Longleaf
pine
savanna,
showing
aftermath
of ground
fire; &
seedlings
that
survived
light
burning
(from Ricklefs
2001)
Hardwoods replacing longleaf pine,
Kisatchie Nat’l Forest, LA (photo T. W.
Sherry)
Patterns of species richness
with succession?
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Species richness often increases with seral stages
 This
results from the increased time available for
different species to colonize sequence
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Species richness often reaches asymptote at later
successional stages, and can even decrease
dramatically (to one or a few species that are
competitive dominants, depending on level of
disturbance)
Dominance of fire-disturbed longleaf pine savanna
is example of how diversity decreases
 However,
shrubs and grasses in understory remain
very diverse--due to disturbance preventing any
particular species from dominating completely
Conclusions:
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Disturbance and community succession are flip sides
of same coin
Succession involves complex combination of multiple
processes: facilitation, inhibition, tolerance, random
colonization
Succession can be primary changes associated with
newly created environment, or much more frequent
secondary changes associated with disturbances to
well developed community
Climax communities can result from facilitation in late
seral stages, or from frequent, severe disturbances
such as fire; however few communities have any one
type of climax
Life history traits closely associated with seral stages
Acknowledgements:
Some illustrations for this lecture from
R.E. Ricklefs. 2001. The Economy of
Nature, 5th Edition. W.H. Freeman and
Company, New York.