Species Richness and Biomass: Dissection of the Hump-Shaped Relationships

The relationship between species richness and biomass is and has been one of the most controversial subjects in ecology. This paper examines these relationships from the perspectives of habitat heterogeneity and environmental gradients measured within and cross five microhabitat types. Species richness, aboveground biomass, and their relationships were compared in two ways: with samples aggregated by microhabitat type, and with analysis of all microhabitat types. Within microhabitat types, in which the environment was relatively homogeneous, we found the species richness-biomass relationship to be positive, negative, or non-existent. The relationship is positive when biomass is low (in open and half-shrub microhabitats) and negative when it exceeds a certain level (e.g., under shrubs). When the sample area encompassed different microhabitat types, a "hump-shaped" relationship was detected. The mechanisms underlying these observed patterns are discussed from the perspectives of environmental gradients and competition.