Grain-dependent relationships between plant productivity and invertebrate species richness and biomass in calcareous grasslands

The relationships among productivity, species richness and consumer biomass are of fundamental importance for understanding determinants of biodiversity. These relationships may depend on grain size. We examined the relationships between productivity (above-ground phytomass) and plant species richness and between productivity and species richness and biomass of gastropods and grasshoppers using sampling units of different sizes (0.5, 2.75 and 23 m(2)) in nutrient-poor, calcareous grasslands in north-western Switzerland in two successive years. Species richness of forbs had a unimodal relationship with productivity in sampling units of 0.5 m(2) and was negatively correlated with productivity at the other two plot sizes in one year. In the other year, forb species richness tended to decrease with productivity in sampling units of 23 m(2). No similar relationship was found for grasses. Gastropod biomass had a unimodal relationship with productivity at 0.5 m(2) in the first year. Grasshopper species richness was correlated with forb species richness at plot sizes of 2.75 and 23 m(2). This study demonstrates that patterns detected between productivity and diversity and between productivity and biomass of consumers depend on the grain size used in the investigation and vary among years.