Q.Lichen is considered as a good example of obligate mutualism. Explain.

Biotic factors — the interactions among living organisms — are sorted into positive interactions (at least one species benefits, neither is harmed) and negative interactions (at least one species is harmed). Mutualism benefits both partners in an obligate association (Rhizobium–legume root nodules fixing nitrogen; Anabaena in Azolla and in Cycas's coralloid roots; Nostoc in Anthoceros; fig–wasp pollination; lichen as an alga–fungus partnership; mycorrhiza as a root–fungal-hyphae partnership); a looser non-obligate version, proto-cooperation, benefits both without either depending on it (soil bacteria/fungi and nearby plants). Commensalism benefits one partner (the commensal) while the other (the host) is unaffected — epiphytic orchids/lianas using a tree purely for physical support, drawing their own water/nutrients from the atmosphere via velamen-covered aerial roots. Predation is an interaction where a predator captures, kills and eats a prey; several plants (Drosera, Nepenthes, Dionaea, Utricularia, Sarracenia) are themselves insectivorous predators, while plants defend against being preyed upon with chemicals (Calotropis's cardiac glycosides, tobacco's nicotine, coffee's caffeine, Cinchona's quinine) or physical structures (thorns, spines, latex). Parasitism benefits a smaller parasite at the expense of a larger host, via haustoria that tap the host's vascular tissue; parasites are holoparasites (fully dependent, e.g. Cuscuta, Balanophora, Orobanche, Rafflesia) or hemiparasites/partial parasites (still photosynthesise, draw only water/minerals, e.g. Viscum, Loranthus, Santalum). Competition harms both participants and is intraspecific (same species, most severe, identical resource needs) or interspecific (different species, e.g. grassland grasses competing during drought, or Utricularia …