Why are biodiversity—ecosystem functioning relationships so elusive? Trophic interactions may amplify ecosystem function variability

Abstract

The relationship between biodiversity and ecosystem functions (BEFs) has attracted great interest. Studies on BEF have so far focused on the average trend of ecosystem function as species diversity increases. A tantalizing but rarely addressed question is why large variations in ecosystem functions are often observed across systems with similar species diversity, likely obscuring observed BEFs. Here we use a multi‐trophic food web model in combination with empirical data to examine the relationships between species richness and the variation in ecosystem functions (VEFs) including biomass, metabolism, decomposition, and primary and secondary production. We then probe the mechanisms underlying these relationships, focusing on the role of trophic interactions. While our results reinforce the previously documented positive BEF relationships, we found that ecosystem functions exhibit significant variation within each level of species richness and the magnitude of this variation displays a hump‐shaped relationship with species richness. Our analyses demonstrate that VEFs is reduced when consumer diversity increases through elevated nonlinearity in trophic interactions, and/or when the diversity of basal species such as producers and decomposers decreases. This explanation is supported by a 34‐year empirical food web time series from the Gulf of Riga ecosystem. Our work suggests that biodiversity loss may not only result in ecosystem function decline, but also reduce the predictability of functions by generating greater function variability among ecosystems. It thus helps to reconcile the debate on the generality of positive BEF relationships and to disentangle the drivers of ecosystem stability. The role of trophic interactions and the variation in their strengths mediated by functional responses in shaping ecosystem function variation warrants further investigations and better incorporation into biodiversity—ecosystem functioning research.

Variation in diversity–function relationships can be explained by species interactions

“…These and other questions will feed current and future research to further our understanding of BEF relationships. The findings by Wu et al (2022) provide a step forward towards a more mechanistic understanding of variation in BEF relationships, and call for further investigation in this field.…”

“…Importantly though, the results presented below are corroborated with an independent 34‐year (1981–2014) food web dataset from the Gulf of Riga ecosystem in the Baltic Sea (Kortsch et al, 2021). Wu et al (2022) find a consistent positive effect of increasing species richness on ecosystem functioning, although the rates of increase (i.e. steepness of the BEF curve) differ across individual functions.…”

“…In addition, most studies are based on small‐scale experimental manipulations of biodiversity, and so the extent to which the nature and architecture of species interactions determine functionality in ecosystems remains unclear. The study by Wu et al (2022) integrates the complexity of natural multitrophic communities into the framework of BEF. More specifically, it significantly contributes to BEF research by (i) providing a mechanistic understanding of VEF, (ii) adopting a food web approach and (iii) considering real‐world, uncontrolled communities.…”

“…The unimodal relationship between VEF and species richness suggests that there is something else beyond the number of species that influences VEF. Wu et al (2022) investigate the role of trophic interactions in shaping such variation in ecosystem function. By looking at taxa composition across trophic levels (characterized by the proportion of primary producers, decomposers and consumers), the analysis reveals changes in the proportion of trophic‐level species along the gradient of species richness.…”

“…The unimodal relationship between VEF and species richness suggests that there is something else beyond the number of species that influences VEF. Wu et al (2022) investigate the role of trophic interactions in shaping such variation in ecosystem function.…”