Rethinking Ecological Measures Of Functional Diversity

TL;DR

This paper critically reviews existing functional diversity metrics, revealing significant flaws and emphasizing the need for new measures that better capture ecological realities and support biodiversity conservation.

Contribution

It systematically evaluates fifteen functional diversity metrics against essential criteria and ecosystem scenarios, highlighting their shortcomings and guiding future metric development.

Findings

All metrics fail in at least one ecosystem scenario.

Most metrics violate set and distance monotonicity principles.

Metrics often do not decrease with species loss or increase with redundancy.

Abstract

Understanding functional diversity, the range and variability of species' roles and actions within their communities, is key to predicting and preserving the functions that sustain both nature and human well-being. In this paper, we provide a comprehensive review of the literature on functional diversity measurement. We begin by consolidating essential criteria that effective measures of functional diversity should meet. We then evaluate fifteen widely used functional diversity metrics against these criteria and assess their performance across six synthetic ecosystem scenarios where optimal behavior is known. Surprisingly, our analysis reveals that none of the widely used metrics fully satisfy all the established requirements, and all fail in at least one ecosystem scenario. In particular, we find that almost all metrics flagrantly violate set monotonicity and distance monotonicity, requirements that adding a novel species should increase diversity, and that the magnitude of that increase should grow with trait dissimilarity. We also find that metrics fail to decline when rare, functionally extreme species are lost, and even increase when a perfectly redundant species is added. These critical flaws leave them blind to the very biodiversity loss that functional diversity measures are intended to detect. Our findings underscore the urgent need to develop a new generation of functional diversity metrics that more accurately reflect ecological realities.