Abstract
Species richness is a frequently used measure of biodiversity. The compilation of a complete species list is an often unattainable goal. Estimators of species richness have been developed to overcome this problem. While the use of these estimators is becoming increasingly popular, working with the observed number of species is still common practice.To assess whether patterns of beetle communities based on observed numbers may be compared among each other, we compared patterns from observed and estimated numbers of species for beetle communities in the canopy of the Leipzig floodplain forest. These patterns were species richness and the number of shared species among three tree species and two canopy strata.We tested the applicability of the asymptotic Chao1 estimator and the estimate provided by the nonasymptotic rarefaction–extrapolation method for all tree species and both upper canopy and lower canopy. In the majority of cases, the ranking patterns of species richness for host tree species and strata were the same for the observed and estimated number of species. The ranking patterns of the number of species shared among host tree species and strata, however, were significantly different between observed and estimated values.Our results indicate that the observed number of species under‐represents species richness and the number of shared species. However, ranking comparisons of published patterns based on the number of observed species may be acceptable for species richness but likely not reliable for the number of shared species. Further studies are needed to corroborate this conclusion. We encourage to use estimators and to provide open access to data to allow comparative assessments.
Highlights
Our results indicate that the observed number of species under-represents species richness and the number of shared species
Since Erwin's study on global species richness based on the data he acquired on beetles in tropical forest canopies (1982), beetles have been used as study organisms when it comes to studying species richness
Species richness is often represented as the observed number of species, even though species abundance relations are mostly skewed toward few species with many individuals and many species represented by increasingly fewer individuals (Follner & Henle, 2001)
Summary
Species richness is one of the most important measures of biodiversity. Investigations in forest canopies pose particular challenges (Barker & Pinard, 2011) that make obtaining reliable measures of species richness more complicated than in other ecosystems. 2.5 | Observed and estimated number of shared species across studied strata and tree species and for F. excelsior and T. cordata:. Species observed and estimated to be shared between the different strata and tree species were compared and visualized by Venn diagrams using the R-package “VennDiagram” (Chen, 2018). For both estimators and the observed value, species richness was higher in T. cordata than in the other tree species. The tested subsets are the sampled canopy strata and tree species, respectively in the parameter “estimator”) on species richness (Table 4). Using Venn diagrams, we compared the observed and estimated numbers of shared beetle species between the tree species (Q. robur, F. excelsior, T. cordata) and the canopy strata (Figure 3). The number of unique beetle species was highest for T. cordata and lowest in Q. robur for the observed and estimated values (Figure 3a,b). We rejected the null hypothesis and concluded that the observed distribution differed significantly from the expected distribution (Table 5)
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