Problems in using Beals' index to detect species trends in incomplete floristic monitoring data (Reply to Bruelheide et al. (2020))

Abstract

Bruelheide et al. (Diversity and Distributions, 26, 2020, 782) explored repeated habitat mapping data to identify floristic changes over time on the basis of two surveys. Because of the incompleteness of the data, they utilized the Beals' index based on the aggregated data from both surveys as a statistical tool for the analysis. The aim of this note is to illustrate problems of this approach, which in particular is shown to produce a systematic underestimation of species decrease (and—potentially less relevant in practice—increase). A specific set of model cases will be introduced to show the effects of unjustified usage of the Beals' index in this specific form.