Species 1Species 2

+

+

Each population has a positive effect on the other.

Example: hummingbird and a hummingbird pollinated plant:

MUTUALISM (+,+)

0)n(l

0)n(l

1

2

2

1 dN

Nd

dN

Nd

In interspecific mutualism, species exchange goods or services beneficial to the other species:

• food for food • food for services • services for services

To be a true mutualism, giving up the goods or services should represent a cost to the organism.

Definitions:

Obligate mutualism: both species would not persist without the other.

Facultative mutualism: both species can live without the mutualistic species.

Symbiosis: a very close association between two species (“living together”).

parasitic symbiosis

mutualistic symbiosis (obligate or facultative)

Plant-pollinator mutualism(food for services)

One yucca species can only be pollinated by one species of moth.

One species of yucca moth lays its eggs only inside flowers of only one species of yucca.

An obligate mutualism.

Ant-aphid mutualism(food for services)

Ants are protecting aphids from competitors and predators.

Aphids squirt honeydew that is eaten by ants.

A facultative mutualism.

Rhizobium-legume mutualism(food for food)

Plants supply sugars to the rhizobium bacteria and shelter against toxic levels of oxygen.

Rhizobium bacteria fix nitrogen and export it to plants roots.

A facultative-obligate mutualism.

Clownfish-sea anemone mutualism(service-for-service)

Both species protect the other from their respective predators.

The clownfish has a mucus on its skin that protects from the anemone’s sting.

A facultative mutualism.

Termite – intestinal flagellate mutualism(food-for-service)

The flagellate digests wood and transforms it into food for the termite, the termite provides a livable habitat to the flagellate.

An obligate mutualism.Also a symbiosis

Müllerian Mimicry(services for services mutualism)

Unpalatable species that look alike all teach their predators the same message.

Young blue jay is offered the wing of a monarch butterfly.

Blue jay after eating the monarch wing.

+Deters predators of both species

Deters predators of both species

+

Mutualism:All toxic look-alikes benefit each other.

What should the isoclines of a mutualistic relationship look like?

N1

N2

competition

N1

N2

mutualism

N1

N2

K2

K1N1

N2

K1K2

Mutualists always coexist.

Mutualism is facultative: each species alone has a K.

Both species have higher densities at the equilibrium when the mutualist is there than when it is not there.

Negative K’s: neither species can live on its own. Mutualism is obligate

If the species coexist, it is due to the stabilizing effect iof competition with other species.

Both species have a minimal viable population size due to the mutualism.

Spc 1 can exist without spc 2 but not vice versa. Mutualism is obligate only for spc 2, not for spc 1.

Both species coexist or spc 2 goes to extinction and spc 1 to carrying capacity.

There is a spc 1-dependent minimal viable population size only for spc 2.

N1

N2

K1

K2

Theoretical outcomes of two-species mutualism:

Obligate mutualism (neither species can persist on its own):

Coexistence or Extinction, depending on species densities.

Facultative mutualism (both species can persist on their own):

Stable coexistence at densities above each species’ single-speciescarrying capacity.

Facultative-obligate mutualism (only one species can persist without the other):

The obligate mutualist has a chance of going extinct.Both species coexist if the obligate mutualist maintains high

numbers.

+Deters predators of both species

Deters predators of both species

+

Mutualism:All toxic look-alikes benefit each other.

Monarch butterfly(dangerous model)

Viceroy butterfly(harmless mimic)

Batesian Mimicry:A palatable or harmless mimic exploits a non-palatable or

dangerous model

Milkweed

Monarch caterpillar

The sap of milkweeds contains a mild toxin: cardiac glycoside

Viceroy

Coral snake (model) King snake (mimic)

Bumblebee (model)

Robber fly (mimic)

Wasp (model) Beetle mimicMoth mimic Fly mimic

Viceroy butterfly(mimic)

What kind of a species interaction is this?

Monarch butterfly(model)

Viceroy butterfly(mimic)

+Deters predators of both species

Prevents the predators from learning to avoid

-

Exploitation:The mimic exploits the model

Monarch butterfly(model)

Milkweed (nectar producing) Lantana (no nectar)

Epidendrum(no nectar)

Sexual baiting: some orchids mimic female wasps: male wasps will copulate, orchids will get pollinated and the wasp will get nothing.

If cheating in a mutualistic relationship can produce an exploitative relationship,…

and if cheating increases the benefit of the cheater,….

Then why are there so many mutualistic species relationships?

Occasional cheating also occurs within mutualisms:

Yucca-moth mutualism: sometimes moths lay too many eggs into the flower.

Some strains of rhizobium bacteria do not produce excess nitrogen.

Why haven’t these mutualisms disappeared long ago?

Cheating must be sanctioned (so that the cost of cheating exceeds the benefits of cheating.)

Yucca plants abort fruits with too many moth larvae. All larvae die.

Rhizobia strains that do not export nitrogen are oxygen starved by the host plant and die.

Summary:

Mutualism is a mutually beneficial ecological interaction between two species.

In mutualism, species exchange goods or services, at a cost to both parties. However, benefits generally outweigh the costs.

Mutualism is obligate, facultative, or obligate-facultative.

Mutualistic relationships can be precarious: if mutually obligate, one species’ demise leads to the other species’ demise as well.

There is a tendency for cheaters to invade or evolve from within. For a mutualism to be evolutionarily stable, the cheater must be punished. If the cheater cannot be effectively punished, the interaction may evolve into an exploitation.