Functional diversity (FD) is an important component of biodiversity that quantifies the difference in functional traits between organisms. However, FD studies are often limited by the availability of trait data and FD indices are sensitive to data gaps. The distribution of species abundance and trait data, and its transformation, may further affect the accuracy of indices when data is incomplete. Using an existing approach, we simulated the effects of missing trait data by gradually removing data from a plant, an ant and a bird community dataset (12, 59, and 8 plots containing 62, 297 and 238 species respectively). We ranked plots by FD values calculated from full datasets and then from our increasingly incomplete datasets and compared the ranking between the original and virtually reduced datasets to assess the accuracy of FD indices when used on datasets with increasingly missing data. Finally, we tested the accuracy of FD indices with and without data transformation, and the effect of missing trait data per plot or per the whole pool of species. FD indices became less accurate as the amount of missing data increased, with the loss of accuracy depending on the index. But, where transformation improved the normality of the trait data, FD values from incomplete datasets were more accurate than before transformation. The distribution of data and its transformation are therefore as important as data completeness and can even mitigate the effect of missing data. Since the effect of missing trait values pool-wise or plot-wise depends on the data distribution, the method should be decided case by case. Data distribution and data transformation should be given more careful consideration when designing, analysing and interpreting FD studies, especially where trait data are missing. To this end, we provide the R package "traitor" to facilitate assessments of missing trait data.

Department of Biosciences University of Helsinki Helsinki Finland

Department of Botany Faculty of Science University of South Bohemia České Budějovice Czech Republic Area de Biodiversidad y Conservación Departamento de Ciencias Escuela Superior de Ciencias Experimentales y Tecnología Universidad Rey Juan Carlos C Tulipán s n Móstoles Spain

Department of Botany Faculty of Science University of South Bohemia České Budějovice Czech Republic Institute of Botany Biology Centre CAS Třeboň Czech Republic

Department of Botany Faculty of Science University of South Bohemia České Budějovice Czech Republic Institute of Entomology Biology Centre CAS České Budějovice Czech Republic

Department of Earth Ocean and Ecological Sciences University of Liverpool Liverpool United Kingdom Centre for Invasion Biology Department of Zoology and Entomology University of Pretoria Pretoria South Africa

Department of Soil Science Faculty of Natural Sciences Comenius University Bratislava Slovak Republic Department of Botany Faculty of Science University of South Bohemia České Budějovice Czech Republic

Institute of Botany Biology Centre CAS Třeboň Czech Republic

Institute of Ecology and Earth Sciences University of Tartu Tartu Estonia

Institute of Entomology Biology Centre CAS České Budějovice Czech Republic

Institute of Entomology Biology Centre CAS České Budějovice Czech Republic Department of Zoology Faculty of Science University of South Bohemia České Budějovice Czech Republic

Institute of Soil Biology Biology Centre CAS České Budějovice Czech Republic Department of Ecosystem Biology Faculty of Science University of South Bohemia České Budějovice Czech Republic

School of Biological Sciences University of East Anglia Norwich United Kingdom Department of Zoology University of Cambridge Cambridge United Kingdom

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