61BL3313 Population and Community Ecology

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61BL3313Population and Community Ecology

• Lecture 08 Commensalism and mutualism
• Spring 2013
• Dr Ed Harris

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Announcements - Tutorial today - comments /
questions?
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This time Commensals and mutualists -Commensali
sm -Mutualism -Mutualism types -Some
examples
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This time Commensals and mutualists
Species A Species B
Competition - -
Predation -
Herbivory -
Parasitism -
Disease -
Mutualism
Commensalism neutral
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Commensalism

• Defined as an ecological relationship in which
  one species benefits from other species, which is
  itself not affected one way or the other by the
  relationship
• This is thus a , 0 relationship
• Examples
• -spanish moss (epiphyte) on trees in Louisiana,
  cattle egrets, and cactus wren nesting in ant
  acacia trees

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Spanish moss Tillandsia usneoides
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Commensalism between cattle (as food beaters) and
cattle egrets (3 white birds, 2 sitting on cows)
in Jamaica
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Commensalism between cattle (as food beaters)and
cattle egrets in Jamaica
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Cactus wren
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-phoretic commensals "hitching a ride"
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Mutualism is an interaction between two species
in which both participants benefit Mutualism
thus has a , interaction Contrast with
competition (-,-), predation, parasitism (both
,-)
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Mutualism is one kind of symbiosis Symbiosis
close (ecologically interdependent) relationship
of two or more species Other kinds symbiosis
involve parasites, predators Distinguish
obligate from facultative mutualism...
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Trophic mutualism formed by coral reef symbionts
Coelenterates zoozanthellae
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Trophic mutualism comprised of Rhizobium
(bacteria are red, false-color image in right
figure) in soybean root nodules (left figure)
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Defensve mutualism between cleaner organism in
this case a prawn (Lysmata amboiensis, a shrimp
relative) and moray eel prawn gets food, eel
gets parasites removed
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Defensive mutualism ants and acaciasbulls horn
acacia (Acacia trees Pseudomyrmex)
Newly developing bulls horns (evolutionarily
enlarged thorns) Filled with a pith that ants
easily remove, creating hollow interiors Ants
chew small hole into each thorn for use as
home Plants also provide ants with extra-floral
nectar, secreted from glands at base of leaves
(arrows)
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Older, hollowed-out bulls horns of Acacia
cornigera, next to main trunk
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Plants also supply ants with protein and fat-rich
food in the form of Beltian bodies, shown here
being harvested by ants (arrows) from the tips of
newly expanding leaflets of Acacia cornigera
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Small grove of Acacia cornigera trees in Costa
Rica, showing ground cleared around base of trees
by a single colony of Pseudomyrmex ants
Pseudomyrmex ants provide two services to Acacia
trees -24-hour patrolling of leaves for
protection against herbivorous animals (insects
and mammals) by stinging biting -Clearing of
plants from ground and from Acacia trees
themselves as protection from competitors (for
water, nutrients)
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Ant-acacia system, Costa Rica
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Ground cleared by ants around Acacia tree in
Costa Rica
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Dan Janzen (1966) tested ecological impact of
ants on plantsCo-evolution of mutualism
between ants and acacias in Central America.
Evolution 20 249-275.

• -Fumigated randomly selected sample of Acacia
  cornigera trees to remove Pseudomyrmex ants
• -Kept ants from re-colonizing experimental trees
  using tanglefoot (sticky goo) at base of trees
• -Monitored plant growth of cut, re-growing
  suckers (stems), and ant activity at experimental
  (defaunated) versus control trees (containing
  normal densities of ants)

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Janzens conclusions

• -Ants play active role in protecting plants from
  herbivory by insects (and other animals)
• -Both ants and acacias involved in co-evolved,
  obligate relationship (each depends on other
  species, in specialized, one-to-one relationship)
• -Value of ants to plants is particularly great in
  tropical dry forests, where rains dont fall and
  water is limiting to plant growth for up to half
  a year
• -Mutualism has evolved here in a stressful
  environment for plants

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Protective mutualisms Nutritive mutualisms
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Other facultative mutualisms with extrafloral
nectary plants Ipomoea (Morning glory), various
legumes (Mung Beans etc), Cotton and other
mallows, lots of tropical trees like Balsa.
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Dispersive mutualism Flowers of Penstemon sp. in
the Sonoran Desert pollinated by the rufous
hummingbird
Penstemon sp. being pollinated by a
bee Pollination is an extraordinarily important
mutualism
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Melastome fruits (see arrow) eaten by, and seeds
dispersed by, Cocos Finch, Pinaroloxias inornata
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Coevolution important in mutualisms

• Define coevolution as reciprocal evolutionary
  adaptations involving both partners of
  ecologically interacting species (often difficult
  to document in nature)

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Coevolution important in mutualisms

• Coevolution well documented in a few cases
• In Ant-Acacia system, both participants have
  traits that are unique to the interaction, and
  that facilitate the mutualism
• Unique Acacia traits include Beltian bodies,
  hollow thorns
• Ant traits include high running speed, stinging
  ferocity, 24-hour activity patrolling plant,
  attacks on plants

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Coevolution important in mutualisms

• Dodo birds extinction on Island of Mauritius
  jeopardized survival of its coevolved tree,
  Calvaria major, indicating obligate relationship
  of tree to bird (bird evolved to abrade seed in
  gut, helping germination)

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Simplistic, but useful model of mutualism based
on expansion of logistic model

• dN1/dt r1N1(X1-N1?12N2)/X1
• dN2/dt r2N2(X2-N2?21N1)/X2
• All variables same as in logistic model, except
  ?21 is mutualistic per capita effect of species 1
  on species 2, and ?12 is effect of species 2 on
  species 1 these alphas increase Ns

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Aspects of mutualism not included in model?

• Benefit of mutualism increases with decreased
  resource availability
• Examples
• -Nitrogen-fixing Alders in nutrient-stressed bogs
  
• -Many legumes in tropics dominate in
  nitrogen-poor soils
• -Plants with mycorrhizal fungi prevalent in
  phosphorus- poor soils
• -Corals prevalent in nutrient-poor
  (carbon-limited) tropical water
• -Termites cattle use microbial mutualists to
  digest cellulose (plant cell walls wood,
  difficult-to-digest)
• Lesson theory of mutualism needs to incorporate
  resource-use dynamics

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Another aspect of mutualism not in model

• Mutualism often found in stressed habitats (In
  favorable environments, by contrast, species can
  make it on their own, without expending energy on
  behalf of mutualist)
• Examples
• -Ant-acacia mutualism in tropical deciduous
  forests (seasonally water-stressed soils)
• -Other nectary and domatia mediated mutualisms
  common on white sand (low nutrient) tropical
  soils.
• -Lichens (association of fungus with alga) live
  in physically, and nutrient-stressed environments
  (e.g., arctic tundra, dry soils, water-stressed
  tree canopies, rocks)
• Lesson theory of mutualism needs to incorporate
  life-history characteristics, and negative
  feedback mitigating against mutualism at higher
  population densities

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Applied ecology humans have developed extensive
mutualisms with plants animals that provide us
with food and other resources. In turn, we
provide nutrients, water, and protection from
herbivores.
Blue Mountain Coffee, sun-grown, in Jamaica
(coffee bushes in foreground, and across hills in
distance)
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Conclusions

• Mutualism extremely common, widespread in nature
• Human agriculture is mutualistic in nature
• Many mutualisms have co-evolved
• Mutualism ranges from facultative to obligate

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Conclusions

• Natural history of mutualism indicates a variety
  of factors that will make population growth
  models more realistic consumer-resource
  dynamics, tradeoffs, habitat stress
• Commensalism also widespread, not well understood