Download Distributions of Communities in time

Distributions of
Communities in time:
Disturbance and
Ecological
Succession
Outline
1. Definitions
2. Mechanisms of succession
3. Successional theory
Disturbance – any relatively discrete event that disrupts the
stand structure or changes resource availability (Pickett and
White 1985)
Endogenous – originate within the community (e.g. tree death
due to old age, fungal disease in soil, grazing)
Exogenous – originate outside the community (e.g. volcanic ash
deposition, floods)
Is wildfire endogenous or exogenous?
Denver Post
Succession – process by which a series of different plant
communities and associated animals and microbes successively
occupy and replace each other over time in a particular
ecosystem or landscape location following a disturbance
(Kimmins 2004)
Primary succession – ecological succession in an area that lacks
organic matter and has not been altered by living organisms
– Examples: establishment after a glacial retreat or volcanic activity
Secondary succession – ecological succession in an environment
already modified by biota
– Example: post-fire establishment, establishment after clearcut
Sere – sequence of communities that develop over the course of
succession
Seral stage – stages in the development of a sere
Climax – (controversial idea) community that has reached a
steady state or equilibrium. Species present are those best
adapted to the climate.
But is succession directional?
Mechanism of succession – process that contributes to
successional change; “proximate cause”
Autogenic succession - changes in the environment caused by the
species themselves which drive successional change
Allogenic succession – changes in the environment caused by
physical processes which are relatively independent of the species
Mechanism of succession
Three-pathway model of succession (Connell and Slatyer 1977)
1. Facilitation - a limited group of “early successional species”
modify the environment so that it is less suitable for new
recruitment of “early successional species” and more suitable
for “late successional species”
Possible modes of facilitation:
Amelioration of environmental stress (e.g. nurse shrub)
Increase in resource availability (nitrogen-fixers)
Nitrogen fixation: atmospheric nitrogen (N2) is converted to ammonia (NH3)
which is available to plants
Importance : Fixed nitrogen is required to create nucleotides (DNA, RNA)
and amino acids (proteins)
Plants with the ability to fix nitrogen can often survive in nutrient poor soil
and thus are often early successional species and are thought to facilitate
establishment of later successional species
Three-pathway model of succession (Connell and Slatyer 1977)
2. Inhibition - early occupants modify the site and inhibit both
early and late successional species.
Common modes of inhibition:
Deep litter
Reduction in light availability
Reduction in soil nutrients and moisture
Changes in soil pH
Allelopathy
Black walnut
Three-pathway model of succession (Connell and Slatyer 1977)
3. Tolerance - plants modify environment so that it becomes less
suitable for early successional species but has little or no effect
on subsequent recruitment of late successional species
Possible modes of tolerance:
Passive tolerance – succession occurs because of differences in the species
traits such as growth rate and longevity
Active tolerance – ability to endure low resource levels (shade-tolerance)
Generalizations about Connell and Slatyer’s 3-mechanisms of
succession model
1. Most successional sequences involve a mixture of
mechanisms (facilitation at early stages can be replaced by
inhibition at later stages)
2. More than one mechanism may operate simultaneously in
same community
3. The same species may be involved in more than one
mechanism at different life stages
Classical Successional Theories
Frederic Clements 1916 - Monoclimax theory
Arthur Tansley – Polyclimax theory
Henry Gleason – Individual concept of the plant association
Handout on classical successional theories
Frederic Clements
1874-1945
Clements, 1916 – Monoclimax Theory
“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.”
Clements, 1916 – Monoclimax Theory
“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.”
Bottom line - plant community is an organism (Organismal View)
Clements, 1916 – Monoclimax Theory
Reaction – modification of site by plants that guides or drives
succession
What mechanism of succession does this sound like?
“Reaction continues the development of the organism”“reaction is final and critical” "succession is due to biotic
reactions only, and is always progressive . . . succession being
developmental in nature, the process must and can be
progressive only”
Clements, 1916 – Monoclimax Theory
Reaction – modification of site by plants that guides or drives
succession
What mechanism of succession does this sound like?
“Reaction continues the development of the organism”“reaction is final and critical” "succession is due to biotic
reactions only, and is always progressive . . . succession being
developmental in nature, the process must and can be
progressive only”
Bottom line - Pioneer species colonize site and modify it to make
it better for later successional species and worse for itself.
Reaction (facilitation) drives succession forward
Clements, 1916 – Monoclimax Theory
“A sere is a unit of succession. It comprises the development of a
formation from the appearance of the first pioneers through the
final or climax stage. Its normal course is from nudation to
stabilization.”
Clements, 1916 – Monoclimax Theory
“A sere is a unit of succession. It comprises the development of a
formation from the appearance of the first pioneers through the
final or climax stage. Its normal course is from nudation to
stabilization.”
Climax is in
equilibrium
with climate
Bottom line - Climax stage is deterministic (climate), predictable,
repeatable
Arthur Tansley
1871-1955
English botanist
Tansley, 1935 – Polyclimax Theory
“Mature well-integrated plant communities (which I identified with plant
associations) had enough of the characters of organisms to be considered as
quasi-organism, in the same way that human societies are habitually so
considered. The comparison with an organism is not merely a loose analogy
but is firmly based. For Clements the plant community is an organism, and he
who does not believe it departs from the true faith. There is no need to weary
the reader with a list of the points in which the biotic community does not
resemble the single animal or plant. They are so obvious and so numerous
that the dissent expressed and even the ridicule poured on the proposition
that vegetation is an organism are easily understood.”
Tansley, 1935 – Polyclimax Theory
“Mature well-integrated plant communities (which I identified with plant
associations) had enough of the characters of organisms to be considered as
quasi-organism, in the same way that human societies are habitually so
considered. The comparison with an organism is not merely a loose analogy
but is firmly based. For Clements the plant community is an organism, and he
who does not believe it departs from the true faith. There is no need to weary
the reader with a list of the points in which the biotic community does not
resemble the single animal or plant. They are so obvious and so numerous
that the dissent expressed and even the ridicule poured on the proposition
that vegetation is an organism are easily understood.”
Bottom line - plant communities have some similarities to organisms, so an
analogy may be appropriate (quasi-organism) – modification of Clements’
ideas
Tansley, 1935 – Polyclimax Theory
“It is the fact that autogenic and allogenic factors are present in all
successions”
Tansley, 1935 – Polyclimax Theory
“It is the fact that autogenic and allogenic factors are present in all
successions”
Bottom line - believed succession was not always driven by the plant
community (disagreed with Clements)
Tansley, 1935 – Polyclimax Theory
“The usual view is that under the " typical " climatic conditions of the region
and on the most favorable soils the climatic climax is reached by the
succession [Clement’s climax community]; but that on less favorable soils of
special character different kinds of stable vegetation are developed and
remain in possession of the ground, to all appearance as permanently as the
climatic climax. These are called edaphic climaxes, because the differentiating
factor is a special soil type. Similarly special local climates determined by
topography (i.e., land relief) determine physiographic climaxes. But we may
go farther than this and say that the incidence and maintenance of a decisive
" biotic factor " such as the continuous grazing of animals may determine a
biotic climax. And again we may speak of a fire climax when a region swept by
constantly recurrent fires shows a vegetation consisting only of species able
to survive under these trying conditions of life... In each case the vegetation
appears to be in equilibrium with all the effective factors present, including of
course the climatic factors, and the climax is named from the special factor
differentiating the vegetation from the climatic climax.”
Tansley, 1935 – Polyclimax Theory
“The usual view is that under the " typical " climatic conditions of the region
and on the most favorable soils the climatic climax is reached by the
succession [Clement’s climax community]; but that on less favorable soils of
special character different kinds of stable vegetation are developed and
remain in possession of the ground, to all appearance as permanently as the
climatic climax. These are called edaphic climaxes, because the differentiating
factor is a special soil type. Similarly special local climates determined by
topography (i.e., land relief) determine physiographic climaxes. But we may
go farther than this and say that the incidence and maintenance of a decisive
" biotic factor " such as the continuous grazing of animals may determine a
biotic climax. And again we may speak of a fire climax when a region swept by
constantly recurrent fires shows a vegetation consisting only of species able
to survive under these trying conditions of life... In each case the vegetation
appears to be in equilibrium with all the effective factors present, including of
course the climatic factors, and the climax is named from the special factor
differentiating the vegetation from the climatic climax.”
Bottom line - There are many possible climaxes under the same climate
conditions (e.g. also depends on soil, topography, animals, disturbances)
Henry Gleason
1882-1975
American ecologist
Gleason, 1939 – Individualistic Concept of the Plant Association
“The vegetation unit is a temporary and fluctuating phenomena,
dependent in its origin, its structure, and its disappearance on the
selective action of the environment and on the nature of the
surrounding vegetation. Under this view, the association has no
similarity to an organism and is scarcely comparable to a species.”
Gleason, 1939 – Individualistic Concept of the Plant Association
“The vegetation unit is a temporary and fluctuating phenomena,
dependent in its origin, its structure, and its disappearance on the
selective action of the environment and on the nature of the
surrounding vegetation. Under this view, the association has no
similarity to an organism and is scarcely comparable to a species.”
Bottom line - A plant community (association) is not even
comparable to an organism (disagrees with Clements and
Tansley).
Plant associations are temporary, fluctuating.
Stochastic processes important.
Gleason, 1939 – Individualistic Concept of the Plant Association
“Of the various species which reach one spot of ground, the local
environment determines which may live, depending on the
individual physiological demands of each species separately.”
“In summary it may be stated that environment varies constantly
in time and continuously in space; environment selects from all
available immigrants those species which constitute the present
vegetation, and as a result vegetation varies constantly in time
and continuously in space.”
Gleason, 1939 – Individualistic Concept of the Plant Association
“Of the various species which reach one spot of ground, the local
environment determines which may live, depending on the
individual physiological demands of each species separately.”
“In summary it may be stated that environment varies constantly
in time and continuously in space; environment selects from all
available immigrants those species which constitute the present
vegetation, and as a result vegetation varies constantly in time
and continuously in space.”
Bottom line –Site acts as a filter to determine which species that
have dispersed to the site can survive (element of chance – which
species were able to disperse?).
Communities change in space and time.
Gleason, 1939 – Individualistic Concept of the Plant Association
“A piece of vegetation which maintains a reasonable degree of homogeneity
over an appreciable area and a reasonable permanence over a considerable
time may be designated as a unit community. Within such an area and during
such a period similarity in environmental selection tends toward similarity in
vegetation.”
“Since every community varies in structure, and since no two communities are
precisely alike, or have genetic or dynamic connection, a precisely logical
classification of communities is not possible.”
Gleason, 1939 – Individualistic Concept of the Plant Association
“A piece of vegetation which maintains a reasonable degree of homogeneity
over an appreciable area and a reasonable permanence over a considerable
time may be designated as a unit community. Within such an area and during
such a period similarity in environmental selection tends toward similarity in
vegetation.”
“Since every community varies in structure, and since no two communities are
precisely alike, or have genetic or dynamic connection, a precisely logical
classification of communities is not possible.”
Bottom line – It is possible for communities to persist over a long time period
due to stable environmental conditions (no climax/equilibrium language!).
Since communities vary in time and space and cannot be classified.
Pollen records show communities change over time, species
migrate independently of each other, and they form new and
different associations.
Who’s theory does this support?
Although climate is important in determining the types of vegetation,
disturbances can also determine community distribution
Example: Bond et al. (2004) used models to estimate ecological change
in the absence of fires.
Results - Without fire, savannas in Africa will eventually develop into
forests (savannas are fire-mediated ecosystems)
Who’s theory does this disprove?
Successional views - late 1970s – mid-1980s
• Clements’ ideas persisted through the 1970s
– Climax/equilibrium model - ignores importance of disturbances
– Deterministic model - left little room for influences of chance (e.g.
dispersal)
• “Recent” recognition of role of disturbances
– Gleason supported non-equilibrium viewpoint
• Shift from climax/equilibrium to non-equilibrium ideas
National Geographic
Pumice Plain
Pumice Plain
Pumice Plain
Spirit Lake
Spirit Lake
Toutle River
Toutle River
Zhou Quan
Pyroclastic Flow
Pyroclastic Flow
USGS
Blowdown
USGS
Blowdown
Fawn Lake
Blowdown
Will primary or secondary succession occur?
What biota could survive the pycroclastic flow?
Aug 29, 1979
NASA
Sept 24, 1980
NASA
Aug 23, 1981
NASA
Aug 18, 1982
NASA
May 22, 1983
NASA
June 17, 1984
NASA
June 22, 1985
NASA
Sept 11, 1986
NASA
Aug 29, 1987
NASA
Aug 31, 1988
NASA
Sept 3, 1989
NASA
Sept 22, 1990
NASA
Sept 9, 1991
NASA
Aug 26, 1992
NASA
Aug 29, 1993
NASA
Set 1, 1994
NASA
Aug 19, 1995
NASA
Aug 21, 1996
NASA
July 23, 1997
NASA
Aug 27, 1998
NASA
July 29, 1999
NASA
Aug 16, 2000
NASA
Aug 19, 2001
NASA
July 21, 2002
NASA
Aug 25, 2003
NASA
Aug 11, 2004
NASA
Aug 14, 2005
NASA
Aug 14, 2005
NASA
Sept 2, 2006
NASA
Sept 21, 2007
NASA
Sept 7, 2008
NASA
Sept 10, 2009
Aug 12, 2010
NASA
Succession usually studied using a chronosequence
(several sites with similar characteristics except time
since disturbance – different sites represent different
successional stages)
Space for time substitution (data can be collected
during a short time period instead of having to study
one plot over many hundreds of years)
Wood and del Morals (1987)
• Unique opportunity to study primary succession in real time
• Observation – limited invasion of plants onto barren surfaces
– Limited by lack of seed rain?
– Limited by lack of tolerance of physical conditions?
– Is facilitation occurring?
Who argued facilitation is
the most important
mechanism that drives
succession?
Wood and del Morals (1987)
Data collection:
Seed Rain (seed traps)
Determined location of seedlings
– Measured survival over 2 years
– Distance to adults species (seed source)
– Existing plant cover (facilitation?)
Experiment: assess species tolerances to conditions
– Planted seed from 22 subalpine plant species in 264 plots
on volcano
– Monitored survival over 2 years
Estimated seed-dispersal
distance for each species
with distance of seedlings
to nearest adults
How far can most species
disperse?
Pappus
Species have different dispersal capabilities and thus may
colonize site at different rates
Some species such as Hieracium gracile and Antennaria were
usually present in abundance in the seed traps, but not
established in plots. Why not?
Which successional theory do these results support?
Distribution of plots into
percent adult cover
classes (open bars) and
percent 1st year seedlings
in each adult cover class
(black bars; 3 year
means).
0 = no veg cover
7 = high veg cover
Why are there more seedlings in areas with
intermediate adult cover and few in open areas?
Don’t know if tolerance or inhibition were occurring for certain species
since they were not tested.