Example: hummingbird and a hummingbird pollinated plant:
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Example: hummingbird and a hummingbird pollinated plant:. +. Species 2. Species 1. +. MUTUALISM (+,+). Each population has a positive effect on the other. In interspecific mutualism, species exchange goods or services beneficial to the other species: food for food food for services

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Example: hummingbird and a hummingbird pollinated plant:


Species 2

Species 1



Each population has a positive effect on the other.

  • Definitions: services beneficial to the other species:

    • 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 services beneficial to the other species:

(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 moth.

(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 moth.

(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 moth.


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 moth.


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

M moth.üllerian Mimicry

(services for services mutualism)

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

Deters predators of both species moth.



Deters predators of both species


All toxic look-alikes benefit each other.







N2 like?








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.

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.

N2 like?




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.

Theoretical outcomes of two-species mutualism: like?

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-species

carrying capacity.

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

The obligate mutualisthas a chance of going extinct.

Both species coexist if the obligate mutualist maintains high numbers.

Deters predators of both species like?



Deters predators of both species


All toxic look-alikes benefit each other.

Batesian Mimicry: like?

A palatable or harmless mimic exploits a non-palatable or dangerous model

Monarch butterfly

(dangerous model)

Viceroy butterfly

(harmless mimic)

Monarch caterpillar like?

The sap of milkweeds contains a mild toxin: cardiac glycoside


Viceroy like?

Coral snake (model) like?

King snake (mimic)

Robber fly (mimic) like?

Bumblebee (model)

Wasp (model) like?

Moth mimic

Beetle mimic

Fly mimic

Monarch butterfly like?


Viceroy butterfly


What kind of a species

interaction is this?

Prevents the predators from learning to avoid like?


Monarch butterfly


Viceroy butterfly



Deters predators of both species


The mimic exploits the model

Lantana (no nectar) like?

Milkweed (nectar producing)


(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?

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

Some strains of rhizobium bacteria do not produce excess nitrogen.

Occasional cheating also occurs within mutualisms:

  • Why haven’t these mutualisms disappeared long ago?

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

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

  • Summary: the flower.

  • 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.