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

-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 + +

Commensalism neutral +

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 mossTillandsia 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

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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...

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 acaciasbull’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

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)

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

Unoccupied (treatmnt) Occupied(control)

N (sample size = number of stems) 66 72Total regrowth wet weight (grams) 2,900 41,750Total number of leaves 3,460 7,786Total number swollen thorns 2,596 7,483

Table 2. Incidence of plant-eating insects on shoots of Acacia cornigeraUnoccupied (treatmnt) Occupied

(control)N (no. of plant shoots examined) 1,109 1,241Daytime% of shoots with insects 38.5 2.7Mean no. insects per shoot 0.881 0.039

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+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 N’s

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