Download Examples of ecological succession so far concern how communities

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Examples of ecological succession so far concern how communities develop following
disturbance of a pre-existing community. Ecologists often distinguish between
(re)development of communities following disturbance and development of ecological
communities on new substrates where conditions are not shaped by a pre-existing
community. The former process is called secondary succession (e.g., dynamics
following a forest fire or blowdown); the latter is primary succession (the initial
development of ecological systems where none existed previously). Unstabilized
dunes of pure sand can be a substrate for primary succession.
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Historical interlude: Primary succession on dunes was first studied by Henry C.
Cowles (middle, upper photo) at the University Chicago in the years around 1900.
Many photos of his botany classes are archived at the Library of Congress ‘American
Memory’ website. They’re interesting for the field costumes. Also note the high
proportion of women in the botany classes; at this time, botany was considered an
appropriate field of study for women; conversely, it was sometimes regarded as
inappropriate for men…
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In large dune fields along the south and west shores of L. Michigan, Cowles
recognized that dunes closest to the lake were youngest and dunes progressively
further from the lake were older. He reasoned that the vegetation along such a series
of dunes might represent different stages along a sequence of vegetation development
(assuming that the different dunes have developed under relatively similar
circumstances). This approach to studying long-term dynamics – comparing current
samples from different places that are presumed to be at different stages of a similar
process – is a common approach now, and is referred to as a space-for-time
substitution (or, sometimes, a chronosequence study).
The youngest dunes are a stressful place for plant growth. They’re not very
fertile, but, more importantly, the substrate is extremely unstable; the sand is
continuously shifting in the wind, and roots of plants might be exposed, or the plants
smothered. Most types of plants can’t survive this. Some have special adaptations
that allow them to cope. Ammophila breviligulata or beach grass (found also along the
east coast), for example, is able to send out new roots along the stem as it gets buried,
and to spread by sending out horizontal underground stems (or rhizomes). As beach
grass spreads across the dune, its roots tend to bind the loose sand and gradually
stabilize it.
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Other plants can do similar things; wild strawberry (top) can also spread with
aboveground stolons. As the sand is stabilized, woody plants can become established.
Willows are one of the earliest types of woody plants that can colonize the dunes; they,
too, are able to root along buried stems and survive some burying. Note that
accumulation of a mound of wind-blown sand around the willow shrub at lower right.
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This process is not irreversible; severe storms (or foot or vehicular traffic) can disrupt
the root systems and cause ‘blow-outs’ where the sand becomes loose and mobile
again.
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But, generally, older, stabilized dunes are colonized by additional species of woody
plants, including trees; as these grow and spread, they shade out the early successional
grasses and willows.
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The forest in the background occupies one of the older dunes, and is dominated by
beech and maple – shade-tolerant species. Cowles was one of the first people to
recognize the potential for plants in early succession to alter habitat conditions in
such a way as to make the establishment of later-successional species possible when
those species might have been entirely unable to survive on the site initially. This is
very different from most secondary successional dynamics; there is no reason why
spruce and fir can’t grow on a newly burned site, for example; they just, typically,
don’t get there and grow as quickly as jack pine and aspen. But beech and maple trees
would be entirely unable to survive on a new sand dune. The process of habitat
alteration that permits (and is essential for) establishment of later-successional species
(superior competitors) is called facilitation. It can be important in primary
successions.
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" The developmental study of
vegetation necessarily rests upon
the assumption that the unit or
climax formation is an organic
entity. As an organism the
formation arises, grows, matures,
and dies... Furthermore, each
climax formation is able to
reproduce itself, repeating with
essential fidelity its development."
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Frederic (no ‘k’; the photo caption is wrong) Clements was particularly
impressed by what seemed to be the orderliness and predictability of
successional sequences like those described by Cowles. The notion of
facilitation (he called it ‘reaction of organisms on the environment)
struck him as particularly important. It made the process of succession
look like a programmed developmental sequence, like that of a
developing organism. Each stage ‘prepared the way’ for the next, and
was necessary before the next could develop. Succession would
continue to a ‘mature’ stage, which Clements called the climax
community; the climax community should be stable. Ultimately,
Clements believed successional dynamics would alter site conditions
so that all communities in a given region would succeed towards a
single climax type, determined only by regional climate (the ‘climatic
climax’). However, many trajectories were possible, depending on
starting point and other things; he gave all of these names… He
extended the analogy of communities (= formations) with organisms by
suggesting that communities came in discreet and deterministic ‘types’
which could be named and organized in a taxonomic structure, just as
individual organisms could be grouped like with like into species. For
this reason, Clements’ model is often referred to as the
‘pseudoorganism’ model of communities. This also implies that there
should be distinct boundaries between communities; he called them
‘ecotones’.
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Thus, under the same climate, successional processes on a lava flow or in a pond
should eventually modify their respective habitats (by building soils on weathering
rock in one case, by filling in basins and developing new soils in sediments/peats in
the other) until communities converged on the same climax. Clements, of course,
gave these successional sequences names – ‘xerarch’ for dry starting points, ‘hydrarch’
for wet. Each sequence of successional change was called a sere, and successional
communities along the way might be referred to as ‘seral’.
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Clements’ models prevailed for decades, from about 1900 until the 1960s, perhaps
because they were practically and esthetically appealing, perhaps because he had a
dominating personality, and trained dozens of students who were influential in various
agencies and organizations. The map, here, is from a book by E. Lucy Braun on the
deciduous forests of eastern North America; the mapping units are ‘climatic climax’
formations in the Clementsian sense. Schematically (lower left), Clementsian
communities should have relatively sharp boundaries/transitions (ecotones) along an
environmental gradient (note that horizonal axis is NOT geographical, but represents
continuous change in some environmental factor). The curves represent abundances of
different species, and three distinct associations or communities occur along this
section of gradient.
“An association is not an organism,
scarcely even a vegetational unit, but
merely a coincidence.”
The Individualistic Concept of the
Plant Association
Henry Allan) Gleason
(1882-1975)
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Henry Allan Gleason challenged Clements’ model in his 1926 paper,
“The individualistic concept of the plant association”, in which he
suggested that ecological and evolutionary understanding indicate that
species should be distributed according to their individual tolerances
and adaptations, and that there is no reason that their distributions
along a gradient should be coherent with one another – that there’s no
mechanism that would tend to make communities act as integrated
wholes (he was also one of the first to recognize that long-term climate
change and range shifts would make the Clements model more
unlikely). Species’ distributions might be limited, he thought, by
competition with ‘neighboring’ species of similar life-style (this
foreshadows the notion of ecological niche and guild), but this would
simply have individual species replacing one another – not whole
communities. Gleason also believed that ‘chance’ factors (like seed
dispersal and weather) might have large influence on successional
sequences, so that they would be less predictable than Clements
thought. (It’s not clear whether he thought there would ultimately be a
deterministic ‘climax’). The graph here is similar to the one on previous
page, but reflecting the ‘species individualistic’ hypothesis.
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Contrasting predictions of species distribution patterns across environmental gradients
provide a means of testing between Gleasonian and Clementsian models – but it was a
long time before anybody gave serious effort to implementing such tests; the
prevailing Clementsian model was simply taken as dogma…
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But in the late 1950s, Robert Whittaker, as a graduate student at the University of
Illinois, conducted a massive study of plant communities in the Great Smoky
Mountains in Tennessee. He looked at distribution of woody species with elevation
(vertical axis in these graphs) and across what he called a ‘topographic moisture
gradient’ – from moist protected ‘coves’, to progressively more exposed slopes, to dry
ridgetops (left to right on the horizontal axis). The dashed lines indicate general
habitat categories; the solid lines are ‘contour maps’ of species abundance (stem
density per sample plot). Acer rubrum, for example (bottom left), is most abundant
(the highest ‘topographic’ contour) at lower elevations in ‘mid-moisture’ conditions.
Whittaker plotted abundances for over 100 species, and found that they were
individualistically distributed; there were no coherent groupings with multiple species
showing congruent ‘topographies’ of abundance. He concluded that the Gleasonian
model was more accurate, at least for the spatial patterns he observed.
At the same time, John T. Curtis, at the University of Wisconsin, was
examining patterns of plant communities across the state with respect to moisture and
climatic gradients. He ultimately concluded the same thing; plant species were
individualistically distributed across moisture gradients.
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Whittaker continued to analyze distributions of tree species in other regions of the
country and found similar results repeatedly
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And one more similar study.
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Margaret Davis’s work (see paleoecology notes), showing that tree species that
currently co-occur broadly had strongly differing distributions during the glacial
maximum >14,000 ybp, first published in the mid-70s, was considered by many to be
the ‘final straw’ in rejecting Clementsian models of ‘coherent’ community-types –
configurations of species are reorganized when climate changes, and no communitytypes remain consistent over time.
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The predictions of Clements’ model for successional patterns would look something
like the upper graph; those of Gleason might look something more like the lower
(species arrive when they happen to get there, and they increase and decrease in
abundance or dominance depending on their individual competitive abilities relative to
other species. In the upper model, facilitation is necessary: later ‘groups’ (or ‘seral
communities’) can’t establish until earlier communities have altered environment to
permit it. In the lower model, facilitation might be involved, but it’s not inherently
required. Dispersal and chance can plan a large role as well. Frank Egler used a
different terminology: he called the upper (Clementsian) model ‘relay floristics’,
implying groups of plants ‘handing over dominance’ from one community to the next,
and he called the lower an ‘initial floristics’ model, implying that most of the species
involved in succession were there from the beginning – they jus
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Empirical studies seem to be more consistent with an ‘individualistic’
model of community change during succession as well. This is a study
of bird populations during old-field succession in Georgia; species do
not enter and leave the community in concert, and their abundance
peaks at different times in succession. The cluster of sharp ups and
downs at about 15 years corresponds to when a more or less
continuous canopy of trees was established – in other words, from a
bird’s perspective, an abrupt change in environment (‘individualistic’
models say community composition should change gradually with
environment; if environment changes abruptly, so will community
composition)
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In some instances successional trajectories are not what’s expected. For example, there
are situations where forest regeneration has ‘failed’ following major disturbances – here
logging followed by ‘slash-fire’ about 100 years ago in northern Michigan (the stumps are
remains of large white pine; large hardwood stumps have completely rotted away).
Hypotheses proposed for absence of ‘normal’ successional dynamics include: earlyestablished species prevent tree seedling survival by toxic (allelochemical) or mechanical
(barriers to rooting) effects; changes to soil properties (loss of organic matter and
resulting loss of fertility) due largely to intense post-logging fires; loss of water-holding
capacity for same reasons so that soil is too dry for seedling establishment. The first
mechanism would be an example of inhibition – slowing of successional dynamics by
early colonists; the latter two would suggest that re-establishment of forests will require
facilitation (rebuilding of soil properties by earlier successional species).
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In the northeast, some old fields show similarly slow establishment of tree canopies; here,
shrubby species of dogwood and viburnums (Cornus, Viburnum) may either establish
very dense shade or perhaps exert allechemical effects, again inhibiting succession.
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Noting phenomena like all of those described above, Connell and Slatyer suggested
THREE categories of successional processes – facilitation (as suggested in Clements’
original model) is alteration of habitat by stress-tolerant early successional species
allowing establishment of other species; tolerance is more related to Egler’s notion of
‘initial floristics’ – species establish when they can get there, and those more tolerant of
competition simply out-persist the others; and inhibition where the effects of some earlyarriving species actually modify environment so as to inhibit establishment of species that
would otherwise become established and dominant. These increasingly complex
conceptual models reflect recognition that successional dynamics are driven by diverse
interactions among species of diverse life-histories and changing site environments.
Other models (including the SORTIE simulation, for example) are much more complex.