The nature of the plant community: a
reductionist view
JB Wilson
Chapter 4
Mechanisms of coexistence and stability
Liz Matthews and Matt Simon
Introduction
• How is coexistence between plant species
achieved? What are the mechanisms?
• Bastow’s fundamental statement: Plant
communities comprise persistent, coexisting
populations of several species
• Fundamental problem = how is this
achieved?
• Resource (light, nutrients, etc) and
competition (interference) limited
• ‘Paradox of the plankton’ and
monospecific stands of vegetation
(Hutchinson 1941;1961)
• HIGH diversity examples, coexisting
seems to be working pretty well here…
– Or are the niches just more
abundant, ie resources?
•“…we are concerned with mechanisms that allow species
to coexist locally, i.e. mechanisms that are not due to
imposed habitat heterogeneity within the area considered.”
-JB
JB Bastow’s 12 mechanisms of coexistence
Stabilizing mechanisms
Niche-differentiation
1. Alpha-niche Differentiation (type of resource and time of availability)
2. Environmental Fluctuation – season, decadal and gradual change
Balances
3. Heterotroph challenges: Pest Pressure
4. Circular competitive networks
Escape through movement
5. Allogenic Disturbance – disrupting growth mainly mechanically
6. Competition/dispersal Tradeoff
7. Initial Patch Composition
8. Cyclic Succession: movement of community phases:
Equalizing mechanisms
9. Equal Chance (neutrality)
10. Inertia - Temporal and Spatial Inertia (aggregation)
11. Coevolution of Similar Interference Ability
12. Spatial Mass Effect (vicinism)
What is a stabilizing coexistence
mechanism vs. an Equalizing one?
• Stabilizing mech. = increase-when-rare
– Include negative abundance/density-dependence to
counter species fluctuation
– Alpha-niche differentiation, environmental
fluctuation, balances, or escape through movement
- these mechansims lead to true coexistence
– If plant biomass is low then in order for it stay in
the mix, it must increase its relative growth rate
(fitness) – (Chesson in press) , this JB states is the
“one necessary and sufficient phenomenon for
maintaining a species in a mixture”
Equalizing Mechanism
• Mechanisms are: Equal Chance (neutrality), Inertia
(temporal and spatial), Coevolution of Similar
Interference Ability, Spatial Mass Effect (vicinism)
• Ways in which species may persist for a time in
unstable local coexistence, slowing exclusion by
interference
• A temporary competitive solution
Alpha-niche Differentiation
Mechanism No. 1 “impossible to disprove”
• Increase-when rare occurs if a species is rare, its
required resources will be more abundant = niche is
not full, there’s room to grow
– The potential to increase is there, but will this rare-species
necessarily increase?
• In contrast, populations are limited by full niche
occupancy, coexistence occurs when resource
requirements differ (niche seperation).
• Stronger the niche differentiation, greater chance of
coexistience between species….
Resource Gradients & Niche Differentiation
Types and Temporal
• Tilman (1976) “number of species able to coexist
is equal to the number of resources, and then only
if each species is limited by a different resource”
is there no contradiction?
• Vance (1984) coexistence on one resource occurs
only “if each species interferes less with resource
acquisition by the other than with resource
acquisition by itself”
– two species can coexist on one resource if: intraspecific
competition for the resource > interspecific “ “
Resource Gradient cont.
•
How can a resource gradient be used as a
niche differentiation mechanism?
1.
2.
3.
Soil resources at different depths
Seasonally pollinators
Light-capture, canopy shapes
(“subtle and devious ways...[of]…resource differentiation” JB)
4.
•
Temporal Resource gradient, two
examples
1.
2.
•
Rooting depth
Speed of reaction to resource availability,
opportunistic species establish quickly when
resources become available
Seasonal: species’ growth patterns can
separated by their response to length of day
and temperature
How much separation is needed?
Heterotroph-imposed niches
• Pollinators typically highly
specialized – robbers, mimics,
rewards, guides and warnings
• Allele effect can alter the pollination
niche and neutralize the increasewhen-rare mechanism
• Mycorrhizal fungi assocations, niche
differentiation is unlikely says JB.
Do you agree?
Chiloglottis trapeziformis, mimicry
Niche extended by reaction
• Species react to their environment and construct their own
niche…
• Species density patterns reflected in habitat create resource
patchiness in the landscape, or is it the other way around?
– Pelletier et al. (1999) found beech litter reduced soil [Ca]
– Ehrenfeld et al. (2001) higher pH was found below two exotic species
than native vaccinium
– Other examples closer to home? Acidic coves…black walnut
• Root plasticity and nutrient uptake
• The tree canopy alters shade/temperature/humidity etc.
Environmental Fluctuation
Seasonal, annual and decadal change
• In order for Env. Fluctuation to cause coexistence
there has to be interaction between growth and
resource supply
• ‘Species can also separate along niche axes of
vegetation phenology’ examples?
• JB says “Separation in flowering times will reduce
competition for pollinators, giving coexistence based
on niche differentiation”
– Difference between this and heterotroph-imposed niches?
• ‘Flowering and fruiting niche gradient mediated by pollinators and
dispersers’
Relative non-linearity vs. sub-additivity: two temporal
variations leading to coexistence
•
Relative non-linearity: two species, two levels of
response to resources
– Fluctuating levels of a resource lead to increase-whenrare mechanism
•
Storage Effect - considerably stronger effect than
relative non-linearity
1.
2.
3.
4.
•
Species must be competing
affected by an environmental factor and respond different
covariance b/w env. factor and intensity of competition
Subadditivity, interaction b/w env. factor and effect of
competition
Timescale of resource depletion determines scale at which
env. fluctuation affect coexistence
Resource level [R]
Pest (pathogens and herbivores) Pressure
Heterotroph challenges
• This increase-when-rare mechanism can occur with
pests if three conditions are met
– Impact is significant
– Pest is specific to a particular species
– Abundance (density)-dependence,
pressure must be less on sparse
• Abundant species will be kept in check
When sparse the species will benefit
from lack of infestation & fitness will increase =
increase-when-rare effect
Pest Pressure, cont.
Abundance-dependance
•
JB cites the following examples of abundance-dependance
mechanisms (Boudreau and Mundt 1997):
1.
Decreased abundance of palatable/susceptible plants, which inhibits
the dispersal of herbivores, disease spores, or disease-vectors
2. Flypaper effect for disease spores and possibly for insect pests, virus
vectors and hence for the viruses they carry, in which the pest is
caught by a passive surface
3. Alteration in the air flow and microclimate ?
4. Chemicals from associated species that repel herbivorous insects
5. Promotion by an associated species of natural enemies of the
herbivorous insects, i.e their predators - used in gardenning for
multiple species, but what’s another example?
Jb then give various seedling mortality examples in which the ‘JanzenConnell’ hypothesis may explain
Pathogens
the considerable impact of fungal pathogens..
• No doubt that the impact can be significant.
• Can pathogens that act in the soil be species specific?
How about aerial transmission? JB balks…
How about sudden oak death,
chestnut blight, dogwood blight…
How are these considered
coexistence mechanisms?
Are they too catastrophic?
Coast live oak in Marin County, CA, May 2000
• These are all probably more density-dependent, but some do
seem to target a particular species.
Some empirical evidence and arguments for
host specificity
• C.E. Mitchell et al. (2002 and 2003)
– At the Tilman established Cedar Creek site, found that
infection dropped as species richness increased
• C.E. Mitchell and Power (2006) “the transmission
of specialist pathogens can be highly sensitive to
the identity of other host species in the
community”
• For below-ground pathogens this has not been
proven, yet…has it?
Herbivory, general
• Potential mechanism for
coexistence via herbivory
is similar to that for pathogens
• Grover (1994) used the
keystone concept, subordinate
species are able to survive
because of a controlling
herbivore munching down the
dominant
Yucca moth in Yucca flower, not herbivory…
Herbivory, of disseminules and seedlings
• Herbivores can impact adult population
abundance by limiting the seed input to the
seedbank
– But is there specificity?
• Herbivory has been shown to be densitydependent, but its effects on density are not
always consistent
• Overall a weak case for for 2 out 3 of pest
pressure coexistence mechanisms
– Can any of his arguments be strengthened?
Vegetative Herbivory
•
•
Impact can be considerable
Selectivity varies even though
preferences do exist
–
–
•
Butterflies are often specialists
Cattle are selective,
but still generalists
Some clear examples:
1.
2.
–
Beetle plagues keeping
goldenrod pops. in check
Vertebrate grazing increasing
species diversity
BUT, says JB, “when the sward is higher, light
competition is more important…this has nothing to do
with any mechanism of coexistence between plant
species”
Pest Pressure Conclusions
• JB concludes that this mechanism most likely acts via
diseases
• Also concludes with a pretty good example of
coexistence and cyclic succession
– a pioneer grass is impacted by nematodes early in the
season (& succession)
– the pathogens and nematodes, with a synergistic effect,
reduce the growth of the next dominant grass
– The result is a patchy landscape of both species, with
different phases of vigor associated with pest phases
(4) Circular Interference Networks
species A
species C
species B
Interference relations between a set of species is intransitive, i.e. it cannot
be arranged in a pecking order such that a species higher in the order
always competitively excludes a species lower in the order
Precautions in the interpretation of
Circular Interference Network
Mechanism:
• Competitive abilities change per
environment
• RGR of species may change as community
composition changes
• Past studies exploring competitive abilities
have been flawed…
– Comparisons of performances in mixtures with
those in monoculture
Precautions in interpretation of
Circular Interference Network
Mechanism:
Table 4.2. Which species has the higher interference
ability? The starting biomass for both species was
1.00
Species
Biomass in
monoculture
Biomass
in mixture
A
3.00
Decreases to… 2.77 Winner in mixture
B
2.64
Increases to…
2.71
Connolly 1997
Various experiment do not find
intransitivity in plant communities:
•
•
•
•
Mouquet et al (2004)
Roxburgh and Wilson (2000)
Keddy et al (1998)
Silvertown et al (1992) and (1994)
So, do circular interference
networks exist???
Theoretical processes that could lead
to intransitivity:
(1)
(2)
A
A is taller
than B and
shades B out
C produces an
allelopathic
chemical,
toxic to A
C
(3)
A
o
Tree A is taller
than shrub B
and shades B
out
grass C lowers the
temperature, and
suppresses
seedlings of A
B
B is taller
than C
A
B
C
shrub B shades out grass
C, and is not affected by
lower temperature
(4)
A
C is shadetolerant, and
scavenges
nitrogen
A is taller
than B
and
shades B
out
C with A is taller
than it, and
shades it out
C
B
C
B is taller than C,
and fixes N
A is taller
than B
and
shades B
out
B
B with C is taller than it,
and shades C out
Any real world examples of this mechanism?
(5) Allogenic Disturbance
(disrupting growth, mechanically)
• A between-patch mechanism
• Newly disturbed patches and recovered
patches must exist…disturbance must be
frequent enough to produce patches at
various stages of recovery
…should this be considered coexistence?
If we accept “between-patch”
patterns as coexistence…
• Then allogenic disturbance mechanism assumes
distinct pioneer and climax species
– May not always be true: Peterson and Pickett (1995)
– Autosuccession
• According to W&A’s discussion, pioneer vs.
climax species traits appear to be most evident in
temperate forests
– Most tropical species are intermediate in respect to
these traits
– Poulson and Pratt (1996) found size of gaps in
Michigan did impact species composition of
regeneration
(6) Interference vs. Dispersal
Tradeoff
• competitive abilities vs. dispersal abilities
• Increase-when-rare:
– If our competitor is sparse, there are empty sites
for it to occupy and outcompete (due to
increased growth rate) our disperser
– If our disperser is sparse, there are many sites
left over by the dispersal-limited competitor for
it to occupy
• The assumption inherent in this trade-off
appears to be common
(7) Initial Patch Composition
• By chance, small transient patches are
occupied by species in different proportions
• Species in the majority will suppress
species in minority if inter-specific
interference > intra-specific interference
• Does this constitute a stabilizing coexistence mechanism? Is there an “increasewhen-rare” component?
• Why include it in the list?
(8) Cyclic Succession: movement of
community phases
• Similar to mechanism 4 (circular
interference networks), but involves the
whole community
• Again, a between-patch scale is necessary
(9) Equal chance: neutrality
“…any one of a number of species is equally
likely to occupy and pre-empt by reaction a
particular microsite.”
• Dispersal, for example, is so unpredictable
in practice that we can invoke chance
– Unpredictable climate
– Unpredictable disturbance events
W&A’s discussion of equal chance:
• “One cause would be that the probability of a
disseminule reaching a site is proportional to
its abundance.”
– Interference ability are equal, “equivalence of
competitors”
So this mechanism implies there are no trade-offs?
The more abundant species has a “better” (non-equal)
chance of establishing…is this equal chance? What
brought about abundance in the first
place…competitive abilities, dispersal abilities,
etc?
W&A’s discussion of equal chance:
• Equal chance sometimes used when species
composition of a community does not
appear to be correlated with an
environmental factor, past or present
– Perhaps explanatory variables were not
measured
– “…used as an excuse for failing to find
vegetation/environment correlations is the last
resort of the scoundrel.”
• Does equal chance have a place in
describing coexistence mechanisms?
Chance and IBG:
• MacArthur and Wilson model (1963) based
on probabilistic immigration and extinction
• W&A claim this hasn’t been proved often in
field experiments
(10) Inertia
• Equalizing mechanism
• Exclusion by interference of a superior
competitor is slow and may allow
stabilizing mechanisms to occur
(10) Inertia: temporal
• Small-scale dispersal switch due to niche
construction
• May be a result of dispersal limitation
– Dalling et al 1998 found that seedlings on BCI were
denser beneath conspecific adults, but this correlation
was weaker for small-seeded spp.
• May be a result of a seed bank and the storage
effect
• May be due to a switch
(10) Inertia: temporal
• May not apply equally to all species in a
community
– Short lived vs. long lived species
• Inertia as a mechanism does not explain
original coexistence: “if there is no
coexistence, inertia cannot prolong it.”
(10) Inertia: spatial
• Spatial aggregation may delay exclusion,
since exclusion occurs only at patch
boundaries
(11) Coevolution of Similar
Interference Abilities
• Aarssen (1983): selection pressure on the
less-competitive species would cause it to
become the better competitor
• “unbelievable because it involves continual
increases in interference ability…”
(12) Spatial Mass Effect (vicinism)
• A population may be maintained by
constant immigration into a patch where the
species cannot otherwise maintain itself
• Rarely quantified
• Metapopulation dynamics
Conclusions
• Stabilizing vs. equalizing mechanisms
– Short-term vs. long term coexistence
• What is the relative importance of each
mechanism?
Conclusions
“…if the difference between the
interference abilities of species is large,
even the presence of a stabilizing
mechanism may not prevent exclusion by
interference and in this situation an
equalizing mechanism might reduce the
difference in interference ability between
two species so that the stabilizing
mechanism is able to cause coexistence.”
(Chesson 2000).
Bastow and Agnew’s order of
importance:
•
•
•
•
•
•
•
•
•
•
•
•
Alpha-niche differentiation
Environmental fluctuation
Pest pressure
Spatial mass effect
Allogenic disturbance
Interference/dispersal tradeoff
Temporal and spatial inertia
Cyclic succession
Circular interference networks
Equal chance
Co-evolution of similar interference ability
Intitial patch composition
Do we agree with this order???
Any other comments or
reactions to the chapter?
Ideas for next week
• Expand upon metapopulation ideas
• Expand upon equal chance/neutral theory:
– Hubbell, S.P. (2005) Neutral theory in community ecology and the
hypothesis of functional equivalence. Functional Ecology, 19, 166172