Lecture 5: Chapter 15 – Mutualism and Commensalism (Positive interactions)
Concept 15.1 – In positive interactions, at least one species benefits
• Positive interactions are very important when facing global change
• Positive interactions are those in which one or both species involved benefit (+/0) (+/+)
o Now even (+/-) is considered a positive interaction if the net outcome is largely +ve
o The interaction may have minor costs to one or both individuals, but the overall NET
outcome must be beneficial
▪ E.g. In algal-coral mutualism the algae has to provide carbs and the coral
provides nutrients and habitat, both costs that the other benefits from, BUT
those benefits outweigh the costs for both
▪ E.g. milkweed uses 37% of its energy from photosynthesis to produce nectar to
o It is a positive interaction so long as the benefits are greater than the costs for both
o If environmental conditions change, and benefit is reduced or cost increased for either
partner, the outcome may change, particularly for facultative interactions.
• Benefits of positive interactions can be in the form of shelter, food, transportation, etc.
• These positive interactions are also called Facilitation
• Symbiosis – when two species live in close physiological contact with each other
o E.g. mutualism, parasitism, commensalism
• Mutualism (+/+) – interaction is beneficial to both individuals involved
• Mycorrhizae = Mutualistic symbiosis b/w plant roots and fungi
o Plant roots give steady supply of nutrients/food to fungi
o Fungi provides a vastly increased surface area to plant and can protect plant
o This in turn effects the distribution of plants by allowing them to live in other places
o Two types:
▪ Ectomycorrhizae - Fungi grow between root cells forming a mantle around the
▪ Arbuscular mycorrhizae - Fungi penetrate the cell walls of root cells forming a
branched network called an arbuscule
• Another example of mutualistic symbiosis is bacteria in gut
• Commensalism (+/0) – interaction is beneficial to one individual involved while the other is
unaffected (neither benefited or harmed)
• Many interactions where one individual provides habitat for another = Commensalistic
o E.g. lichens on trees, bacteria on skin
• Many different interaction such as parasitism can result/evolve into positive interactions
o E.g. bacteria infects organism, but coevolve and eventually depend on each other
o (+/-) (+/0) or (+/+) • Despite this, many mutualisms and commensalisms are facultative and show few-no signs of
• In deserts, the shade of adult plants creates cooler, moister conditions. Seeds of many plants
can only germinate in this shade. The adult is called a nurse plant
o One species of nurse plant may protect the seedlings of many other species.
o Desert ironwood serves as a nurse plant for 165 different species.
o The nurse plant and beneficiary species may evolve little in response to one another.
• Interactions are categorized on outcome: positive (benefits > costs), negative, neutral
o Costs and benefits often vary in space and time
o E.g. soil temperature determines whether wetland plants are commensals or
▪ Wetland soils can be anoxic. Cattails aerate soils by passively transporting
oxygen through continuous air spaces in the leaves, stems, and roots
▪ Some of this oxygen becomes available to other plants
▪ In an experiment, cattails (Typha) and a forget-me-not (Myosotis) that lacks
continuous air spaces, were grown at different temperatures.
▪ At low temperatures, soil oxygen increased when cattails were present, but not
at the higher temperatures.
▪ At low temperatures, growth of Myosotis increased when Typha was present
(Typha had a positive effect on Myosotis).
▪ At the higher temperatures, presence of Typha decreased growth of Myosotis
(Typha had a negative effect on Myosotis).
• Studies to assess the importance of positive interactions:
o Performance of a target species with neighbors present is compared to performance
when neighbors are removed.
▪ A group of ecologists looked at effects of neighboring plants on 115 target
species in 11 different regions.
▪ Performance was measured as change in biomass or leaf number.
▪ Relative neighbor effect (RNE) = target species’ performance with neighbors