Practice question

The idiosyncratic hypothesis treats ecosystem performance as dependent on which species are present and how they interact, not as a smooth function of species number. Adding a species may increase productivity through complementarity, reduce it through competition, or produce little change if its role overlaps with existing organisms. Consequently, function can rise, fall, or fluctuate as richness changes, generating an unpredictable relationship. This contrasts with the redundancy model, in which several species share functional roles and initial losses have little effect; the rivet model, in which each loss progressively weakens function; and the keystone model, in which one species has a disproportionately large effect. “Unpredictable” does not mean ecologically uncaused. The response can result from species identity, trophic links, facilitation, inhibitory effects, environmental conditions, and the order in which species enter or disappear. Once these mechanisms are known, particular outcomes may be explained, but richness alone is insufficient to forecast them. The idiosyncratic framework therefore warns against assuming that every added species contributes equally or that all communities with the same richness provide the same ecosystem function.