PCB3044 Ch.15 Mutualism & Commensalism

Positive Interactions (3)

-Commensalism is also everywhere. -Millions of species form +/0 relationships with organisms that provide habitat. • Lichens grow on trees • Bacteria on human skin • In kelp forests, many species depend on the kelp for habitat and do no harm to the kelp. -Mutualism can arise from a host-parasite interaction

Key Concept 15.2

-Each partner in a mutualistic interaction acts in ways that serve its own ecological and evolutionary interests. Mutualisms are categorized by the type of benefits that result: 1) Trophic mutualism 2) Habitat mutualism 3) Service mutualism

Summary

-In mutualism and commensalism, there may be costs, but neither species is harmed and the benefits outweigh the costs. -Positive interactions are self serving, not altruistic. -Interactions can evolve and change depending on conditions. -Mutualism and commensalism can alter population abundance and distribution, as well as community diversity.

Key concept 15.1

-In positive interactions, neither species is harmed and the benefits are greater than the costs for at least one species.

Positive Interactions (1) ; Mutualism, Commensalism, Symbiosis

-Mutualism: Mutually 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. --• Symbioses can include parasitism (+/-), commensalism (+/0), and mutualism (+/+).

Introduction ; Positive Interactions

-Positive interactions ; are those in which one or both species benefit and neither is harmed

Key Concept 15.3

-Positive interactions affect the abundances and distributions of populations as well as the composition of ecological communities.

Characteristics of Mutualism (1)

-Trophic mutualisms: Mutualist receives energy or nutrients from its partner.(leaf-cutter ants, fungus, mycorrhizae) -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.(service includes pollination, dispersal, defense vs. herbivores, predators, or parasites) -Mutualists are not altruistic- they are in it for their own benefit. -There are costs involved, giving up space/time, but net benefits must exceed net costs for both partners -If environmental conditions change, and benefit is reduced or cost increased for either partner, the outcome may change, particularly for facultative interactions.

Positive Interactions (2) ; Mycorrhizae, Ectomycorrhizae, Arbuscular mycorrihizae

-mycorrhizae ; symbiotic associations between the roots of plants 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

Characteristics of Mutualism(2)

How do mutualists avoid overexploitation? -Cheaters ; are individuals that increase offspring production by overexploiting their mutualistic partner. If this happens, the interaction probably will not persist, or... "Penalties" may be imposed on cheaters

Positive Interactions (4) ; can be.....

Positive interactions can be: 1) obligate (not optional for either species). - Show clear signs of coevolution - Common among mutualistic relationships 2) facultative (not obligatory) - Show few signs of coevolution. - Commensalism is always facultative for the species that does not benefit. -When interactions no longer provide a net benefit, they may cease to occur -Positive interactions may be more common in stressful environments