Is the Humped Relationship between species Richness and Biomass an Artefact due to Plot Size?

Abstract

If plants vary in size, then the use of small quadrats of fixed size will show maximum species richness at intermediate biomass. Fewer plants are found in plots of fixed size both if plants increase in size (and biomass therefore increases), or if vegetation becomes sparse (biomass decreases).The supposed response of species richness to biomass is therefore produced as an artefact of varying plant number. Grime derived a humped relationship between species richness and biomass from studies using relatively small quadrats of 50 cm x 50 cm, or 1/4 m2 (AlMufti et al. 1977; Grime 1979), and similar patterns have been found in many later studies on terrestrial vegetation (e.g. Moore & Keddy 1989; Wisheu & Keddy 1989; Wheeler & Shaw 1991; Garcia et al. 1993; Rosenzweig & Abramsky 1993; Abrams 1995). The ecological reason usually proposed to explain the humped relationship is that stress reduces diversity at both extremes (harsh environments at low biomass and intensive competition at high biomass, Grime 1979; Abrams 1995), although other explanations have been proposed (Rosenzweig & Abramsky 1993). In this paper I show how a humped response can be produced without assuming any biological interactions.