Humans are modifying both the identities and numbers of species in ecosystems, but the impacts of such changes on ecosystem processes are controversial. Although the organisms living in an ecosystem control its functioning (Lawton and Brown 1993, Vitousek and Hooper 1993, Walker 1991, Chapin III et al 1995), it has not been clear how much of this control is determined by the identities of the species present (Chapin III et al 1995, Givnish 1994), by the number of species present (Vitousek and Hooper 1993, Chapin III el al 1995, McNaughton 1995, Tilman et al 1996), by the number of different functional roles that these species represent (Lawton and Brown 1993, Vitousek and Hooper 1993, Hooper), or by which functional roles are represented (Chapin III et al 1995, Vitousek 1990). The effects of species or functional diversity are expected to increase with the magnitude of the differences among species or functional groups (Tilman et al 1997). These differences are also expected to influence the magnitude of the effects caused by compositional differences. However, the relative effects attributable to diversity versus composition are unclear.

[Modified from: Tilman, D., J. Knops, D. Wedin, P. Reich, M. Ritchie and E. Siemann. 1997. The influence of functional diversity and composition on ecosystem
processes. Science 277:1300-1302.]