Mutualism and Commensalism

Learning goals:
* Be able to describe the basics of mutualism, including the types, biology, and ecological impacts
* Learn the natural history of common mutualisms: mycorrhizae, lichens, coral

Facilitative interactions are very widespread in nature
Mutualism - mutually beneficial interaction between individuals of two species (+/+)
Commensalism – individuals of one species benefit, while individuals of the other species do not benefit and are not harmed (+/0)

Facilitative interactions can be symbiotic (both species living in close association) or non-symbiotic
The benefits of positive interactions can take many forms.
Sometimes there is a cost to one or both partners, but the net effect of the interaction is positive because for each species, the benefits are greater than the costs.

Mutualism and commensalism are NOT altruistic interactions involving concious decsions to help each other out- they are exploitative interactions that may have costs, but the net effect is a benefit in terms of increased survival, growth, or reproduction
if cost < benefit, then it’s worth it in evolutionary terms

Basic types of mutualisms:
Trophic mutualism - one species helps another species obtain energy
Habitat mutualism - favorable habitat or shelter is provided by one partner, and some additional benefit occurs for the other partner; habitat commensalism occurs if only one partner benefits, and the other is unaffected
Service mutualism – service provided to one partner, including pollination, seed dispersal, defense

Mutualisms are widespread
e.g. mycorrhizae- symbiotic mutualism between a fungus and a plant root; plant provides the fungus with energy (C), fungus enhances soil resource uptake, protects against pathogens and soil toxins (e.g. heavy metals)
Extremely widespread: occurs in around 80% of angiosperms (flowering plants) species and all of the gymnosperms (conifers)- sometimes an obligate relationship
Two major categories:
Ectomycorrhizae- fungus forms a “sheath” around the root (mantle) & grows in between cortical cells- occur most commonly in woody species
Arbuscular mycorrhizae (endomycorrhizae) - fungus penetrates cells of root; exchange of energy and nutrients occurs through arbuscules
Other widespread mutualisms include trophic mutualisms associated with digestion of cellulose, lichens, pollination, seed dispersal

Mutualisms can be obligate (required) or facultative (optional)
Obligate mutualisms are usually highly coevolved, with specialized behaviors

Facilitation in one environment may be competition or parasitism in another = “context” dependent

What keeps mutualisms or commensalisms from becoming parasitisms?
If one organism exploits the other such that the benefits < cost, the relationship shifts to a +/-
Some organisms may exert penalties on cheaters of mutualism

Ecological consequences of mutualisms and commensalisms include enhancement of population growth by increasing survival, growth, and reproduction