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Lecture 14

Lecture 14: "Mutualism & Commensalism"

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Western University
Biology 2483A
Hugh Henry

Ecology Lecture No 14 MutualismCommensalism thThursday October 25 2012 Introduction Ant farmers nourish protect and eat the fungal species they grow forming a relationship that benefits both Each species cannot survive without each other In the 1990s a parasitic fungus Escovopsis was discovered that attacks the fungal gardens of leafcutter ants Ants respond to Escovopsis by increasing garden weeding rate and also enlist the help of other species the ants carry a bacterium that makes chemicals that inhibit Escovopsis and also secrete compounds that promote cultivated fungal growth The bacteria also benefit as a third mutualist as it live in crypts on the ants exoskeleton and obtains a food source from the ants glandular secretions Positive Interactions Positive interactions also known as facilitation occur when neither species is harmed and the benefits of the interaction are greater than the costs for at least one species Mutualism involves a mutually beneficial interaction between individuals of two speciesCommensalism is where individuals of one species benefit while individuals of the other species do not benefit and are not harmed 0 Symbiosis describes any relationship in which the two species live in close physiological contact with each other includes parasitism commensalism and mutualism Sometimes there is a cost to one or both partners in facilitation but the net effect is positive For each species the benefits are greater than the costs Interactions Of MutualismCommensalism Most plants form mycorrhizae symbiotic associations between plant roots and various fungi Here the fungi increase the surface area for the plant to take up water and soil nutrients There are two forms of this phenomenon Ectomycorrhizae where the fungus grows between root cells and forms a mantle around the root and arbuscular mychrrhizae where the fungus grows into the soil extending away from the root and also penetrating some root cells Corals form a mutualism with symbiotic algae The coral provides the alga with a home nutrients nitrogen and phosphorus and access to sunlight while the alga provides the coral with carbohydrates produced by photosynthesis Herbivores such as cattle and sheep depend on bacteria and protists that live in their guts to help metabolize cellulose Woodeating insects also have gut protists that can digest cellulose Commensalism often occurs with one species providing habitat to another Lichens that grow on trees or bacteria on your skin are examples of commensalism relationshipsMutualism can arise from a hostparasite interaction In a strain of Amoeba proteus that was infected by a bacterium the bacterium initially caused the host to be smaller grow slowly and often killed it
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