This PowerPoint explores three major types of relationships (symbiosis) present in most ecosystems: parasitic, mutualistic, and commensal.
The file has some good images of each type of relationship with accompanying description of the ways the species interact with one another. Students are able to define parasitism, mutualism, and commensalism by themselves (which is a great bonus to any lesson).
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Science Education by Joshua Wickline is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 United States License