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Abstract The relationship between biodiversity and ecosystem function (BEF) captivates ecologists, but the factors responsible for the direction of this relationship remain unclear. While higher ecosystem functioning at higher biodiversity levels (‘positive BEF’) is not universal in nature, negative BEF relationships seem puzzlingly rare. Here, we develop a dynamical consumer‐resource model inspired by microbial decomposer communities in pitcher plant leaves to investigate BEF. We manipulate microbial diversity via controlled colonization and measure their function as total ammonia production. We test how niche partitioning among bacteria and other ecological processes influence BEF in the leaves. We find that a negative BEF can emerge from reciprocal interspecific inhibition in ammonia production causing a negative complementarity effect, or from competitive hierarchies causing a negative selection effect. Absent these factors, a positive BEF was the typical outcome. Our findings provide a potential explanation for the rarity of negative BEF in empirical data. 

Biodiversity–ecosystem function (BEF) relationships have been a major topic since the 1990s, because of sustainability and extinction crises, although BEF relationships are not easily correlated with habitat, taxa, or diversity. We proposed and tested two predictors for BEF relationship shapes: (i) individual species abundance (indicating coexistence and competitive abilities) and functional performance, and (ii) specific functions underlying a broader function. Darlingtonia californica is a carnivorous plant with modified leaves housing bacteria that contribute to insect degradation and nitrogen provision. We isolated 14 distinct bacterial strains from D. californica leaf fluid, grew the isolates in monocultures and mixed cultures, and evaluated each using insect degradation assays and protein, ammonia, nitrate, chitinase, and protease quantification assays. We asked three questions related to our two predictors of BEF shapes: First, can function of a bacterial community be predicted based on functions of individual species? Second, are there correlations between bacterial species function and abundance change? Finally, does degradative function in bacterial communities correlate with nitrogen-related processes in the context of this key limiting nutrient? We found that increased degradative function correlated with decreased species abundance changes, highly functioning mixed cultures could result from both highly or moderately functioning isolates, and degradative function did not rely on tested nitrogen-related functions. BEF relationships present a trade-off between degradative function and bacterial abundance changes, but not with the specific nitrogen assays. Although species with strong functional contributions can also be good competitors, they may also be poor competitors or play complex roles in community assembly. [Formula: see text] Copyright © 2026 The Author(s). This is an open access article distributed under the CC BY 4.0 International license .