An index for measuring functional extension and evenness in trait space

Tao Zhang, Grant M. Domke, Matthew B. Russell, Jeremy W. Lichstein

Research output: Contribution to journalArticlepeer-review


Most existing functional diversity indices focus on a single facet of functional diversity. Although these indices are useful for quantifying specific aspects of functional diversity, they often present some conceptual or practical limitations in estimating functional diversity. Here, we present a new functional extension and evenness (FEE) index that encompasses two important aspects of functional diversity. This new index is based on the straightforward notion that a community has high diversity when its species are distant from each other in trait space. The index quantifies functional diversity by evaluating the overall extension of species traits and the interspecific differences of a species assemblage in trait space. The concept of minimum spanning tree (MST) of points was adopted to obtain the essential distribution properties for a species assembly in trait space. We combined the total length of MST branches (extension) and the variation of branch lengths (evenness) into a raw FEE0 metric and then translated FEE0 to a species richness-independent FEE index using a null model approach. We assessed the properties of FEE and used multiple approaches to evaluate its performance. The results show that the FEE index performs well in quantifying functional diversity and presents the following desired properties: (a) It allows a fair comparison of functional diversity across different species richness levels; (b) it preserves the essence of single-facet indices while overcoming some of their limitations; (c) it standardizes comparisons among communities by taking into consideration the trait space of the shared species pool; and (d) it has the potential to distinguish among different community assembly processes. With these attributes, we suggest that the FEE index is a promising metric to inform biodiversity conservation policy and management, especially in applications at large spatial and/or temporal scales.

Original languageEnglish (US)
JournalEcology and Evolution
StateAccepted/In press - 2021

Bibliographical note

Funding Information:
This research was funded by the US Department of Agriculture, Forest Service‐Northern Research Station (15‐JV‐11242305‐029), Minnesota Agricultural Experiment Station (MIN‐42‐063), and NSF (DEB‐1442280). We are grateful to Silva Lab members at University of Minnesota for discussion of the early results. We thank the two anonymous reviewers for their comments that improved this work.

Publisher Copyright:
© 2021 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd.


  • community assembly processes
  • cumulative distribution function
  • functional diversity index
  • minimum spanning tree
  • null model
  • species pool
  • species richness
  • trait space

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