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Lecture 13: Mutualism

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  • What commensalism is
  • What kinds there are


  • What mutualism is
  • What organisms exhibit mutualism
  • How mutualisms may have come to be

How do commensalism and mutualism fit into conceptual model of species interactions?

Species A
Species B
Competition (-) (-)
(-) (+)
(-) (+)
(-) (+)
(-) (+)


Four kinds of commensalism

  1. Trophic
  2. Transport
  3. Structural/support
  4. Shelter

Two broad classes of mutualism

  1. Facultative
  2. Obligate

Usually one partner is going to get energy out of the deal

a) Trophic –

b) Defensive –

c) Dispersive –

Mutualisms should evolve if benefits outweigh the costs

Plant mutualisms

1. Chloroplasts

Theory of Serial


Endo= inside

Symbiosis= 2 different organisms

living in close association

Proposed by Lynn Margulis

Plants & Fungi

Mycorrhizae = fungus roots

VAM = vesicular-arbuscular mycorrhizae

Aka. endomycorrhizae  and Ectomycorrhizae

Why have mycorrhizae?

Plant benefits: water & nutrients

Fungus benefits: food (energy) from exudates

Mycorrhizae and Water:

Mycorrhizae and nutrients

Nancy Johnson’s Experiments

Fertilized soils have different mycorrhizal species. How does this affect plants?

4 treatments:

  • No nutrients
  • + P
  • + N
  • + N + P

H: increased soil fertility selects for less mutualistic mycorrhizae species


fertility, plant exudate

less exudate “hungry” fungi

If correct, then what should be expected?

Defensive mutualism – Ants and Acacia

What do plants get out of the relationship?

Dan Janzen’s experiments

Coral (again!)

Zooxanthellae + coral + crustaceans

Cool human application

Mutualism between indigenous African tribe and a woodpecker

Homo sapiens + Indicator indicator

With honey guide 3.2 hours to find nest. Without 8.9 hours.


  1. Commensalism and mutualism are similar in the sort of possible benefits
  2. Plants exhibit many kinds of mutualisms, almost always they must give up energy (energy is often one benefit gained by a mutualist even without plants)
  3. Environment affects the kinds of mutualisms that can develop
  4. Costs must be less than benefits for mutualism to evolve.
Copyright 2008, by the Contributing Authors. Cite/attribute Resource . factpetersen. (2008, February 28). Lecture 13: Mutualism. Retrieved January 07, 2011, from Free Online Course Materials — USU OpenCourseWare Web site: This work is licensed under a Creative Commons License Creative Commons License