Eco Ch 15 Mutualism and Commensalism

Positive interactions

one or both species benefit, and neither
is harmed

Mutualism

beneficial interaction
between individuals of two species (+/+
relationship)

Commensalism

Individuals of one species
benefit; individuals of the other species do not
benefit but are not harmed (+/0 relationship).

Symbiosis

Two species live in close
physiological contact with each other.

mycorrhizae

symbiotic associations between the roots
and various fungi

Ectomycorrhizae

The fungus grows between
root cells and forms a mantle around the root.

Arbuscular mycorrhizae

The fungus
penetrates the cell walls of some root cells,
forming a branched network called an
arbuscule.

Coral bleaching

occurs when coral
polyps expel algae
that live inside their
tissues

obligate

(required for either species).
- Show clear signs of coevolution
- Common among mutualistic
relationships

facultative

(not obligatory; optional)
- Show few signs of coevolution.
- Commensalism is always facultative for
the species that does not benefit.

Trophic mutualisms

Mutualist receives
energy or nutrients from its partner

Habitat mutualisms

One partner provides the
other with shelter, living space, or favorable
habitat.

Service mutualisms

One partner performs
an ecological service for the other.

Cheaters

individuals that increase
offspring production by overexploiting their
mutualistic partner