Trait Contribution Evenness—A functional redundancy metric for community-aggregated traits in microbial communities

Abstract:

The concept of functional redundancy has received considerable attention in both the macroecology and microbial ecology literature; however, variability in trait data (e.g., ecosystem function, metabolic pathways, phenotype, etc.) analyzed as well as differences in study system scale (i.e., population, community, and ecosystem), has resulted in various measures of functional redundancy. Here, we propose an approach to quantify functional redundancy that is based on traditional measures of community diversity. We call this approach trait contribution evenness (TCE). With TCE, functional redundancy is measured as how evenly distributed a trait is among taxa in a community. Resilience of a trait to taxa extinctions is often viewed as an ecological consequence of highly redundant traits. As such, we find this measure appealing as TCE measures the apportioned contribution by different taxa to an ecosystem function TCE differs from other functional redundancy metrics which typically consider taxonomic and functional richness. We use in silico extinction simulations to show that TCE functional redundancy is closely related to the resilience of a trait to extinctions. Finally, to illustrate the applicability of TCE, we analyzed the functional redundancy of eight nitrogen-transforming pathways using 2,631 metagenome-assembled genomes from 47 TARA Oceans sites. We found that the NH₄⁺ assimilation pathway was the most functionally redundant (0.6 to 0.7) while nitrification had the lowest functional redundancy (0 to 0.1). Here, TCE functional redundancy addresses shortfalls of other functional redundancy measurements by providing a generalizable, quantitative, and comparable functional redundancy measurement.