Competition and Mutualism
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Reading:
1. Textbook Chapters 6 and 7
2. Tilman, D. 1997. “Mechanisms of Plant
Competition”. pp239-261 in: M. J. Crawley (ed)
Plant Ecology, Second Edition. Blackwell
Publishing, Oxford. (on reserve in library).
3. Stachowicz, J.J. 2001. Mutualism, facilitation,
and the structure of ecological communities.
Bioscience, 51:235-246.
Lab Feb 24
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Papers for discussion:
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Bais HP, Vepachedu R, Gilroy S, Callaway RM,
Vivanco JM . 2003. Allelopathy and exotic
plant invasion: From molecules and genes to
species interactions. Science 301 (5638):
1377-1380
TBA (Herbivory? Or choose your favorite
interaction)
Seminars
• EECB Colloquium: Thurs Feb 19. John
Maron, University of Montana. “Rapid
evolution in invading plant populations.
Evidence from Hypericum perforatum”
Outline
1.
2.
3.
4.
5.
6.
7.
Types of species interactions
Competition
Mechanisms of competition
Coexistence
Mutualism
Types of relationships
Example: mycorrhizae
Interactions organized in following way:
+
-
0
+
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0
Mutualism
Predation
Parasitism
herbivory
Competition
Commensalism
Predation
Parasitism
herbivory
Commensalism Amensalism
Amensalism
Neutralism
Competition
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Ability of one species to inhibit the survival
and/or growth of individuals of another
species = interspecific competition.
Ability of individuals to suppress growth or
survival of conspecifics = intraspecific
competition (density dependence, e.g.)
Effects can be substantial: e.g. 16-fold
decrease in growth of little bluestem in
presence of neighbors (Wilson and Tilman
1991).
Competition Models
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Consider joint carrying capacity of N1 + N2
Species 2 has an effect α on species 1.
Species 1 has an effect β on species 2.
Alter logistic growth equations as follows:
Species 1: dN1/dt=r1N1((K1-N1-αN2)/K1)
Species 2: dN2/dt=r2N2((K2-N2- βN1)/K2)
Effect and Response
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Competitive effect: ability of plant to
suppress other species.
Competitive response: ability of plant to
persist in presence of competitors (i.e.
resist suppression).
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How do they differ? How would they
affect alpha and beta?
Handout: tree tolerances. Does this describe
competitive effect or competitive response, or
both?
Competition Mechanisms
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Species vary in the nutrient concentrations
they require for positive net growth
Resources are limited
Changes in plant density/biomass alter
resource availability (rate of consumption)
This influences and survival and growth
Equilibrium concentration R*
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R* is resource concentration at which a
species “breaks even”. Differs among
species.
R* depends on species growth rate and
change in growth rate with change in
resources.
All things equal, a species will reduce a
resource to its R*.
Equilibrium concentration R*
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With one limiting resource, species with
lowest R* will “win”.
For soil resources (e.g. N): negative
correlation between R* and root mass (e.g.
tallgrass prairie, Tilman and Wedin 1991)
ZNGIs
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Zero net growth isoclines: graphical
representation of resource use and
competition
Show that at equilibrium coexistence is
possible if each species limited by a
different resource
Coexistence
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Why are communities diverse?
Coexistence
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If tilmans models are right, only as many
species as there are different limiting
resources should coexist. Why are
communities diverse?
Coexistence
1.
Neighborhood effect: deplete resources in
spatially discreet area; low R* in plant’s
immediate vicinity.
2. Spatial heterogeneity: resource supply
variable at very small scale.
3. Resource fluctuations in time: “storage
effect”
4. Non-equilibrium conditions: disturbance,
herbivory, etc.
Coexistence
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If tilmans models are right, only as many
species as there are different limiting
resources should coexist. Why are
communities diverse?
Apparent competition
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More complex 3rd order interactions
create appearance of competitive effects:
Example: Species 1 is primary food source
for herbivore; Species 2 is secondary.
Presence of Species 1 can suppress species
2 via shared herbivore.
Competition in range and
forest ecosystems
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What is the importance of competition?
Mutualism
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Mutually beneficial interactions.
More widespread and important than often
given credit for; much less studied than
competition.
Very important in range and forest
ecosystems. Examples?
Mutualism
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Four main types:
1. Nutritional: breakdown and supply of nutrients
(e.g. mycorrhizae to plant)
2. Protection: from extreme site conditions or
predators/pathogens (e.g. fungus to alga, nurse
plants)
3. Transport: dispersal of seed, pollen, etc. (e.g.
fruit bats to banana trees, bees to orchids)
4. Energy: supply of photosynthetic energy by
plants to associates (e.g. plant to mycorrhizae;
alga to fungus)
Mutualism models
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Mathematical models not as well developed
as for competition or predation but…
Can convert “Lotka-Volterra” competition
models into mutualism models (+ effect).
Species 1: dN1/dt=r1N1((K1-N1+αN2)/K1)
Species 2: dN2/dt=r2N2((K2-N2+βN1)/K2)
Mutualism models
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But: this causes positive feedback and rapid
exponential growth. “Orgy of mutual benefaction”
(May 1973)
Better models: economic models and game theory.
Cooperation is mutually beneficial.
But: cheaters may prosper. Must be mechanisms
for “policing” : e.g. Yucca moths. Cheaters oviposit
but don’t pollinate. Solution? Yucca selectively
aborts flowers with many moth larvae (Pellmyr and
Huth 1994. Nature 372:257-260)
Example: mycorrhizal associations
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Ectomycorrhizae – form extensive hyphal
mats that grow between but not within
cells. Usually Basidiomycotes and
Ascomycotes. Common on woody forest
species (e.g. pines, fir, hemlock). Modify
external appearance of root.
Example: mycorrhizal associations
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Ectomycorrhizae – form extensive hyphal
mats that grow between but not within
cells. Usually Basidiomycotes and
Ascomycotes. Common on woody forest
species (e.g. pines, fir, hemlock). Modify
external appearance of root.
Endomycorrhizae – hyphae penetrate root
cells. Visible only by microscope. VAM=
vesicular arbuscular mycorrhizae (named
for structures in root cells).
Benefits and extent
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Fungus receives carbohydrate from plant
Plant receives increased “root area” with
substantial gains in water and nutrient gathering
capacity (P, N).
Most plants form some sort of mycorrhizal
association.
Trees have ecto, sometimes VAM or both. Most
range plants have VAM.
Most common and important in low-resource areas.
Colonization may be decreased in high nutrient or
disturbed areas. Can be affected by herbivory.
Importance
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May affect competitive interactions and
invasion success.
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Maintenance of dense grassland
Some invasions might fail because of lack of
symbionts
Importance
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May affect competitive interactions and
invasion success.
Often necessary for tree growth: example –
failed afforestation attempts in areas of
grassland
Importance: example
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White Fir in Klamath mountains…
– Clearcut caused shift in soil microbial community
(fungal to bacterial)
– Some clearcuts cannot be re-vegetated despite
numerous attempts since 1960
– Planting seedlings along with soil from healthy
forest increased survival 50% in first year.
– After 3 years, 100% mortality without inoculum.
– In some cases, tree growth stopped at edge of
inoculated hole.