Could we obtain better estimates of plot species richness from multiple-observer plant censuses?

Question Could we better estimate plot species richness by asking several botanists to survey the same plots and using non-parametric estimators of richness? Location Two French deciduous forests. Methods Using replicated, independent censuses made by 11 professional botanists on the same eight 100-m(2) forest plots, the relative performance of different richness estimators (Lincoln-Petersen, Jackknife 1&2, Chao 1&2, Bootstrap, Chao Mth, Darroch) and the variation in their performance with the number of botanists involved (teams with two to eight botanists) were investigated. The sensitivity of these estimators to the presence of misidentifications in the data was also assessed. Results When misidentifications are removed, Chao Mth estimators converged fastest to true richness, but none of the tested estimators correctly accounted for differences in exhaustiveness between the teams. Finally, all estimators were highly sensitive to misidentifications. Conclusions Richness estimators are of little help in the presence of misidentifications and are ineffective at removing between-team discrepancies, thus strongly limiting their usefulness in practice. Methods are presented to show how surveys can be designed to remove misidentifications and limit between-team discrepancies. A sensible sampling design for 100-m(2) plots in temperate forests would involve triplets of botanists and correcting data with the Chao N1. Pairs of botanists would already significantly improve the richness estimates, but such estimates would still be biased low. However, further research is needed to design new richness estimators that are more robust to observer effects.