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Lecture 5

BIOL 2050 Lecture 5: Lecture 5 - Mutualism and Commensalism (Positive interactions)

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York University
BIOL 2050
Christopher Lortie

Lecture 5: Chapter 15 – Mutualism and Commensalism (Positive interactions) (a) 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 attract insects o It is a positive interaction so long as the benefits are greater than the costs for both individuals 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 root ▪ 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 symbiosis o E.g. lichens on trees, bacteria on skin • Many different interaction such as parasitism can result/evolve into positive interactions (mutualism/commensalism) 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 coevolution • 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 competitors ▪ 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
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