Organisms And Populations

A lichen is the classic textbook example of mutualism, an interaction in which both interacting species are benefited and the relationship is obligatory for survival. The fungal partner provides shelter, anchorage, water and mineral absorption, while the photosynthetic alga (or cyanobacterium) manufactures carbohydrates that nourish the fungus. Because both partners gain (+ , +), the interaction is mutualism rather than any one-sided relationship. Commensalism is incorrect because in it only one species benefits while the other is neither harmed nor helped (+ , 0), as in an orchid growing on a tree branch. Parasitism is wrong since one organism benefits at the expense of the host (+ , -). Amensalism describes a situation where one species is harmed and the other is unaffected (- , 0). As described in NCERT Class 12, Chapter 13 (Organisms and Populations), mycorrhizae and the Rhizobium-legume association are other mutualistic examples. Plausibility check: since the alga cannot easily survive the desiccation of bare rock alone and the fungus cannot photosynthesise, the bidirectional dependence confirms a (+ , +) relationship, validating mutualism.