Sections
Succession
Idea of community
stability usually includes two related concepts:
- Communities have some kind of
equilibrium state
- Communities return to a previous
state
- This chapter deals with the
regular return to a prior state
- Regular change in community
composition is called succession
- One set of species succeeds
a previous set
Clementsian
Succession
- Idea developed for terrestrial
plant communities
- Following a disturbance that
alters community composition but not the nature of the habitat, there will
be an orderly return to pre-disturbance community composition
- each of the stages of the
sequence are called a sere
- first sere called the pioneering
sere
- Community will return to a climax
community composition
- Climax communities are stable
and not prone to invasion by non-climax species
- Climax communities renew
themselves
- Climax community composition
is determined by the interaction between plant species and abiotic factors
(climate, soil moisture, etc.)
- Earlier communities alter habitat
so that plants in succeeding communities can invade
- Facilitation
is the process of paving the way for your successor
- as species are introduced,
they modify the soil, the light, and may alter the conditions that
allowed them to get a start. The new conditions, now not so favorable
to the pioneering species, are favorable for species that could not
have invaded at first.
- Climax communities have soil
requirements that are built by earlier communities (organic material in
soil must build up so that moisture is retained, pH must be correct, etc.)
- Facilitation is not the only
process underlying succession
- Studies have shown that replacement
of one species by another is sometimes due to:
- facilitation (as Clements
proposed)
- nodulating species can raise
the nitrogen content of the soil to a point that other species can invade
- on marine hard surfaces,
one set of organisms can facilitate the settlement of organisms that replace
the original holders of the space
- in this case, the first species makes
its successor more likely to settle on the surface
- if the situation is more emphatic
such that a species cannot invade unless another species has arrived
and flourished first, the the first species Enables the second
species
- competition -- some species simply
compete for a resource such as light, without either species facilitating
the invasion of the other
- Thus few present day ecologists
accept that Clementsian succession is the only means by which one community
can replace another
- Primary
succession - succession that begins on a surface with no preexisting
communities present
- new islands
- new volcanic rock
- new lakes formed by glacier retreat
- many microbial communities normally undergo
primary succession
- Secondary
succession
- succession that begins after a disturbance that has left some of the previous
community intact
- Allogenic
(literally, "other origin"} succession is caused by an abiotic
disturbance
- Fire and storms often lead to secondary
succession
- Human disturbance (agriculture,
forestry, development) can lead to secondary succession once the land
is abandoned by humans
- Autogenic
(literally, "self origin") succession is caused by a disturbance
due to organisms within the system
- locust plague can cause a disturbance
large enough to initiate succession once the grasshopper populations
have returned to low levels
- outbreak herbivores (gypsy moth
in oak forests, spruce budworm in spruce forests, army worm, etc.)
can lead to widespread loss of the climax tree species and initiate
succession
- Succession has been expanded
to other situations beyond primary succession and "old field" succession
- Decomposition of leaf litter
or of accumulated materials on lake and ocean bottoms
- Leaves in streams
- Leaf litter in forest
- Carcass decomposition
- Colonization of marine hard
substrates (rocky areas)
- Regrowth of forests as glaciers
retreat
- Regrowth after devastating
natural disasters (volcanic explosions)
Problems
with Clementsian succession
- Facilitation
does not always occur
- Climax
communities vary and particular conditions may prevent
the predicted climax community from developing
- Soils may
never be correct because the parent material will not
support the climax community normally found in the area
(example of Lake Michigan sand dunes in book)
- Recurring fire or other disturbance
may prevent formation of climax
- in this case, the outcome
may be that which community occurs is not predictable, in which case the
community is called Non-equilibrium
Alternatives
to Clementsian succession
Assembly
Rules
- If most of the species in a
community are found because they are either enabled by
another species or are missing because they have been
competitively excluded by another species, than it may be
possible to predict the composition of a community
through the use of Assembly Rules
- Assembly rules
- a set of rules used to choose a subset of a
larger set of species that can coexist
- Assembly rules are
useful if they work:
- indicator species can
be used to predict community composition
- However, assembly rules are
subject to criticism
- The rules are usually
made from collection data by observing which
species never or always occur together
- Because of
this, we do not know why they do or do
not occur together
- Hard to tell if the
associations between species are significant or
just patterns that randomly arise if you have
enough data
- This problem
lead to a debate about which was the
appropriate null model to use for the
statistical tests
- If you
want to know if the pattern of
species co-occurrences is due to
competition, then the null model
must not contain any effects of
competition
- However,
both competition that is ongoing
and competition that occurred in
the past might both have
something to do with the pattern
observed.
- Removing
the effect of present competition
from a model does not make it
null with respect to all
competition
Inhibition
Model
- Inhitition
is the competitive exclusion (see below, as
it is interference competition) of a species from a community
by the activity of another species
- can occur through the
production of a secondary chemical
- some plants
secrete allelochemicals that prevent
other plants from germinating or prevent
root growth
- some yeast,
hyphal fungi, and bacteria secrete
allelochemicals to kill other yeast,
hyphal fungi, and bacteria
- can occur through the
unequal effects of pathogens on two species, as
one species may be excluded by a disease
tolerated by another (so that the winning species
acts as a reservior for the disease)
- inhibition is the opposite of facilitation -
it explains the loss of species as the community changes
- Facilitation is not
the only process that is needed for succession
- many early
succession plants are prevented from
invading later successional stages by
competition (inhibition), so even if
facilitation is important for the forward
progress of seres, the next sere must
outcompete the previous sere for space in
the community
- Inhibition can lead to
communities that become resistant to invasion
- Expected climax
community never reached because some
intermediate community is resistant to
invasion by the climax species
- the climax
community may depend on who gets there
first and subsequently stops others from
invading, even though the environment is
otherwise suitable for non-climax species
Tolerance
Model
- A compromise
model in between facilitation and inhibition
- describes
a succession sequence that does not require
facilitation, although facilitation may occur
- a species would
invade eventually, even if no other
species were there to facilitate its
invasion
- Competitive
exclusion can affect the succession sequence but
no allelochemicals are needed
- perhaps scramble
competition is the mechanism, not
interference competition
Markov
Models of Succession
- A Markov approach to community
structure requires that one know the probability that one "state"
will change to another "state" in a given time period (similar to the assembly rules approach)
- States for communities are
all of the possible community compositions
- heart of the modeling process
is construction of a transition matrix
that has the probability of a state changing into any other state at a
given time
- transition matrix is a square table
with all of the species as row and column labels
- entries are the transition probabilities
If these transitions are such
that any state can be reached from any other (directly or along a path
that goes through other states) then the process is a regular Markovian
process
- Regular
Markovian processes have the advantage that they will reach a
"steady state" as time goes on, no matter which state (community
composition) is the beginning state
- Useful because one can see succession
as a Markovian process
- The steady state is the climax
community
- Approach does not require that
one know anything about facilitation or other biotic interactions
Horn's
Replacement model
- First applied
to canopy trees
- gathered
data on the trees under the canopy trees in the
forests around Princeton
- assumed that one of
the understory trees would replace the
canopy tree
- also assumed that
the choice would be random among the
understory trees, so that the species
with the most trees under a particular
canopy tree would be the most likely
species to replace the canopy tree
- Horn ran his
model until a steady state was reached and then compared
the steady state prediction from the model to the actual
composition of the forests around Princeton
- The
model predicted the known succession of tree
species in the local forests
- The
forest species composition data agreed with the
model's prediction for the climax community
Restoration
Ecology
- The science of managing lands
or bodies of water so that they return to their previous community composition
after a disturbance
- disturbances of interest
are usually those caused by human activity
- industrial activities such as mining
- restoring natural water regime to
areas that have been drained or where river channels have been altered
- restoring land after it has been
used by farmers or ranchers
- sometimes natural disturbances
are the cause (fire, storm, etc.)
- sometimes simply waiting for
natural processes will often restore a community or species
- recovery from a bottleneck
- reinvasion through migration
- sometimes this means active intervention
- releasing members of a species
from another population in a similar area
- preventing some species from
overgrowing the missing species of interest
- important to know what normal
succession sequence will be so that one can take appropriate action (and
not waste effort)
- Text lists some concerns
for restoring terrestrial communities, take a look at them.
- some efforts at clean-up after
a disturbance have slowed, instead of sped-up, the rate of succession
- washing oil off of Alaskan
shores covered by oil spills has slowed recovery
- removal of topsoil contaminated
with a chemical slowes recovery unless new topsoil is brought in
Generalities
about succession from Walker and Chapman (1987)
- Seed dispersal only important
where there is no seed bank in the soil (or where there is no root stock present)
- Stochastic (chance) differences
in community composition, mutualisms (lichens, plant-mycorrhizae) and facilitation
more frequent in communities under greater abiotic stress or where resources
are low
- Competition more important than
facilitation in most communities
Literature Cited
Walker, L. R. and F. S. Chapman.
1987. Interactions among processes controlling successional change. Oikos
50:131-135.
Terms
succession, Clementsian Succession, climax community, Facilitation, Enablement, Assembly Rules, Primary succession, Secondary
succession, allogenic, autogenic, sere, pioneering sere,
disturbance, Inhibition Model, Tolerance Model, Restoration
Ecology, Markov Models, transition matrix, Regular Markovian
processes, Horn's Replacement model
Last updated October 24, 2006