Variability in population and community biomass in a grassland community affected by environmental productivity and diversity

Temporal variability of the biomass of individual populations and total community biomass were studied in a species rich meadow (35-40 species 0.25 m(-2) plot). Communities were experimentally subjected to fertilization and removal of the dominant Molinia caerulea in a factorial design. The typical dominance structure, with few dominants and many subordinate species, developed in all the treatments. Variability was measured by the coefficient of variation between years, from the fifth to the eighth year of the experiment (to avoid the initial response to the manipulations). Dominant removal increased the diversity measured by the reciprocal of the Simpson index, while fertilization decreased both species number and diversity and also caused a shift in community composition. The variability of both the total community and of individual species was higher in the fertilized plots. The coefficient of variation decreased with species mean biomass in all the plots. In non-fertilized plots, the dominant species had lower variability than total biomass. The biomass values of individual species fluctuated in a concordant manner over the years (i.e. were positively correlated). All of these factors will decrease the strength of the expected portfolio effect. It is argued that the effect of environmental productivity on variability is more pronounced than the effect of diversity.