FEEr: An R package for calculating functional extension and evenness index

Since the 1990s, functional diversity has attracted growing attention from biodiversity scientists. In recent years, various functional diversity indices have been proposed; but many of them presented conceptual and/or practical limitations. We recently developed a functional diversity index, Functional Extension and Evenness (FEE), to overcome some of the limitations. In this paper, we describe the content and major features of a new R package, FEEr, which serves as a tool for calculating the FEE index. The FEEr package includes one major core function to accomplish the FEE calculation and several support functions to provide important intermediate results relevant to the FEE calculation. To demonstrate the usage of this package, we present an abbreviated case study of quantifying the taxonomic and functional diversities of national forest inventory plots in the state of Tennessee, United States. In this application, we compare the correlations between species richness and FEE values in these plots. We also briefly summarize how species richness and FEE values vary among forest types and ecoprovinces in Tennessee. The results indicate that species richness and the FEE index reflect complementary aspects of the biodiversity profile in Tennessee's forests. The functional diversity package outlined in this work provides a comprehensive foundation for subsequent analyses to address the needs of biodiversity conservation.