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USGS2007

Does species diversity limit productivity in natural grassland communities?

Theoretical analyses and experimental studies of synthesized assemblages indicate that under particular circumstances species diversity can enhance community productivity through niche complementarity. It remains unclear whether this process has important effects in mature natural ecosystems where competitive feedbacks and complex environmental influences affect diversity-productivity relationships. In this study, we evaluated diversity-productivity relationships while statistically controlling for environmental influences in 12 natural grassland ecosystems. Because diversity-productivity relationships are conspicuously nonlinear, we developed a nonlinear structural equation modeling (SEM) methodology to separate the effects of diversity on productivity from the effects of productivity on diversity. Meta-analysis was used to summarize the SEM findings across studies. While competitive effects were readily detected, enhancement of production by diversity was not. These results suggest that the influence of small-scale diversity on productivity in mature natural systems is a weak force, both in absolute terms and relative to the effects of other controls on productivity. ?? 2007 Blackwell Publishing Ltd/CNRS.

Ecology Letters↗

USGS1999

The relationship between species density and community biomass in grazed and ungrazed coastal meadows

Previous studies have indicated that the relationship between community biomass and species density can be represented by a multivariate model in which abiotic variables influence species density both through effects on biomass and through effects on the species pool. In this paper, we use data from grazed and ungrazed coastal meadows in Finland to evaluate and extend this general conceptual model of the factors controlling species density. Structural equation analysis was used to evaluate a model for all meadows and then to perform a multigroup analysis to determine how grazed and ungrazed meadows differ. By itself, biomass could explain only 12% of the variation in species density while the multivariate model was able to explain 47% using five types of predictor variables: site, soil, flooding, grazing, and biomass. Analyses found that flooding explained the greatest amount of variability in species density, primarily through negative effects on the species pool. Grazing was also found to have a strong effect on species density and results suggest that its negative influence may be largely through reductions in the species pool in grazed meadows. The most important difference found between grazed and ungrazed meadows was that species density had a strong negative relationship to biomass in the ungrazed meadows but no significant relationship in the grazed ones. Thus, it appears that the influence of competition on species density was much greater in ungrazed meadows compared to grazed ones.

Oikos↗