Practice question

A lichen is a stable symbiotic consortium dominated by a fungus and a photosynthetic partner, usually a green alga or cyanobacterium. The photobiont supplies carbohydrates produced by photosynthesis, while the fungus provides structure, water retention, mineral access, and protection from environmental stress. In cyanobacterial lichens, nitrogen fixation may add another resource. These reciprocal contributions justify the traditional classification as mutualism, although modern research shows that benefit balance varies and additional yeasts or bacteria may participate. Parasitism would require a net cost to one partner, competition reciprocal harm, and commensalism no measurable benefit to one member. Lichens therefore illustrate integrated function arising from unlike organisms. At population level, this mechanism can influence abundance, coexistence, and evolutionary selection. Separating immediate individual effects from longer-term community outcomes gives the selected concept a clearer ecological meaning and avoids relying only on memorized terminology. The relevant evidence concerns process rather than wording alone. Linking the described pattern to energetic returns, fitness consequences, or receiver responses makes the inference biologically coherent and distinguishes it from the competing alternatives.