Download 61BL3313 Population and Community Ecology

61BL3313
Population and Community Ecology
Lecture 08 Commensalism and mutualism
Spring 2013
Dr Ed Harris
Announcements
- Tutorial today
- comments / questions?
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This time: Commensals and mutualists
-Commensalism
-Mutualism
-Mutualism types
-Some examples
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This time: Commensals and mutualists
Species A
Species B
Competition
-
-
Predation
-
+
Herbivory
-
+
Parasitism
-
+
Disease
-
+
Mutualism
+
+
neutral
+
Commensalism
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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
Spanish moss
Tillandsia usneoides
Commensalism between cattle (as food beaters) and cattle
egrets (3 white birds, 2 sitting on cows) in Jamaica
Commensalism between cattle (as food beaters)
and cattle egrets in Jamaica
Cactus wren
-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
Trophic mutualism comprised of Rhizobium
(bacteria are red, false-color image in right figure)
in soybean root nodules (left figure)
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
Defensive mutualism: ants and acacias
bull’s horn acacia
(Acacia trees & Pseudomyrmex)
Newly developing bull’s 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)
Older, hollowed-out bull’s horns of Acacia cornigera,
next to main trunk
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
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)
Small grove of Acacia cornigera
trees in Costa Rica, showing
ground cleared around base of
trees by a single colony of
Pseudomyrmex ants
Ant-acacia system, Costa Rica
Ground cleared by ants around Acacia tree
in Costa Rica
Dan Janzen (1966) tested ecological impact of ants on plants
*Co-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)
Table 1. Total wet weight of suckers regenerated, and leaf crop as response to
acacia shrub stems
Occupied
Unoccupied (treatmnt)
(control)
72
N (sample size = number of stems) 66
41,750
Total regrowth wet weight (grams) 2,900
7,786
3,460
Total number of leaves
7,483
2,596
Total number swollen thorns
Table 2. Incidence of plant-eating insects on shoots of Acacia cornigera
Occupied
Unoccupied (treatmnt)
(control)
1,241
1,109
N (no. of plant shoots examined)
Daytime
2.7
38.5
% of shoots with insects
0.039
0.881
Mean no. insects per shoot
Janzen’s 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 don’t fall and water is limiting to plant
growth for up to half a year
-Mutualism has evolved here in a stressful environment for
plants
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.
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
Melastome fruits (see arrow) eaten by, and seeds
dispersed by, Cocos Finch, Pinaroloxias inornata
Coevolution important in mutualisms
Define coevolution as reciprocal evolutionary
adaptations involving both partners of ecologically
interacting species (often difficult to document in
nature)
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
Coevolution important in mutualisms
Dodo bird’s 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)
Simplistic, but useful model of mutualism based on
expansion of logistic model
dN1/dt = r1N1[(X1-N1+
dN2/dt = r2N2[(X2-N2+
12N2)/X1]
21N1)/X2]
All variables same as in logistic model, except
is mutualistic per capita effect of species 1 on
species 2, and 12 is effect of species 2 on
species 1; these alphas increase N’s
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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
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)
Conclusions
Mutualism extremely common, widespread in nature
Human agriculture is mutualistic in nature
Many mutualisms have co-evolved
Mutualism ranges from facultative to obligate
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