Phytoplankton Diversity and Productivity
Sampling effort matters! It matters so much that spurious patterns can arise if it is not properly controlled for. Pedro Cermeño, Tamara Rodriguez, Maria Dornelas et al. report that phytoplankton species richness is not correlated with productivity in a recent feature paper in Marine Ecology Progress Series. This finding contradicts previous claims that species richness peaks at intermediate productivity, a pattern that is increasingly regarded as a zombie idea. This result arises from using multiple approaches to equalize sampling effort across different productivities.
Previously, scientists equalized effort by sampling similar volumes of water for phytoplankton, but that can result in differences of orders of magnitude in the number of individuals sampled. When the number of individuals sampled is equalized across a productivity gradient, either by subsampling or by extrapolating to estimate the number of species not seen, species richness becomes uncorrelated with productivity. The main implications of this study are that sampling effort must be equalized to allow comparisons, and that in general phytoplankton samples need to be much larger than commonly asserted in order to more accurately estimate species richness.