Ecosystem functioning has long been known to strongly depend on abiotic factors, such as precipitation and soil nutrients, and biotic factors, such as species identity and dominance. During the past quarter century, experimental, theoretical, and observational studies have found that ecosystem functioning also strongly depends on species richness. Changes in numbers of plant species over time at a particular place can impact ecosystem functioning as much as changes in species composition, global change drivers, nutrients, disturbance, or herbivory. Despite these advances in understanding, studies continue to either assume that species influence ecosystem processes in proportion to their relative abundances, which ignores richness effects, or that rare species impact ecosystem functioning as much as abundant species, which ignores dominance effects. New models are uniquely able to relax both of these assumptions, assessing rather than assuming the functional importance of rare species.
There is now growing evidence that rare species have disproportionately large impacts on many ecosystem processes. This is not to say that ecosystem functioning depends as much on rare species as on dominant species, but rather that the effects of rare species are inaccurately predicted by the mass ratio hypothesis. More studies are needed that quantitatively assess the extent to which rare species impact ecosystem functioning, rather than simply assuming their effects are negligible or substantial. Future studies will also need to separate the effects of species dominance from species identity and account for changes in the identity of dominant species across spatially heterogeneous, temporally fluctuating, and globally shifting environmental conditions.