Rarefaction does not eliminate the species richness‐biomass relationship in calcareous blackland prairies
2001
Forbes, Sarah P. | Schauwecker, Timothy | Weiher, Evan
Relationships between species richness and biomass are common, but the causes remain controversial. It has been suggested that a hump‐shaped relationship may be an artifact of few individuals in both low and high biomass samples. We used rarefaction to standardize species richness to a constant number of individuals in 99 quadrat samples from relict calcareous prairies. Animal ecologists have long used rarefaction to standardize species richness to a constant number of individuals. The expected species richness of each quadrat was determined for 10, 25, 50, 100 and 150 individual plant ramets using the EcoSim program which resampled the data in each quadrat 1000 times. Because we had few samples with biomass over 300 g/m², our data showed a positive relationship between species richness and biomass. The relationship was largely unaffected by rarefaction. Even at the most extreme case of rarefying to 10 individual ramets, the relationship was clear. Monte Carlo covariance tests did not change the shape of the richness‐biomass relationship and significant correlations between observed species richness and rarified species richness suggest that the number of individuals had negligible effects on richness patterns. Because the data had no strong declining phase, these results may not apply to the declining phase of a humped relationship. The results are discussed with reference to other methods of standardizing species richness. Most importantly, we suggest that expressing species richness per number of individuals is inappropriate and may lead to incorrect conclusions.
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