Practice question

The sign notation (+,0) expresses the defining outcome of commensalism. The first species gains food, shelter, transport, or another fitness advantage, whereas no detectable change occurs in the second species. Examples can include epiphytes using trees as physical support or remoras obtaining transport and food scraps from large marine animals, provided the host's fitness remains essentially unchanged. (+,+) describes mutualism, (+,-) describes exploitation such as predation or parasitism, and (-,0) describes amensalism. Because ecological effects depend on environmental conditions and measurement scale, an apparent zero may later prove slightly positive or negative, but the formal commensal category remains positive-neutral. From an evolutionary perspective, traits persist when their net effects improve inclusive or direct fitness under prevailing conditions. The ecological terminology therefore summarizes both an immediate mechanism and its likely consequences across generations. A careful interpretation retains the assumptions of the underlying model and avoids extending it beyond available evidence. Within those assumptions, the keyed concept gives the most consistent account of the biological pattern and its expected outcome.