A graphical causal model for resolving species identity effects and biodiversity–ecosystem function correlations

Abstract

Identifying and clearly communicating the drivers of ecosystem function is a crucially important goal for both basic and applied ecology. This has proven difficult because the putative causes (e.g., environment, species identity, biodiversity, and functional traits) are numerous and correlated. The problem is exacerbated by a lack of a formal framework for unambiguously relating theoretical language to precise, quantitative expressions of that language. Using a formal framework for the graphical expression of complex causal hypotheses, we developed a causal diagram of the concepts required to comprehensively test whether hypothesized sets of functional traits mediate the relationship between community structure and ecosystem function. We then used causal analysis, simulations, and field data to develop and test analytical strategies for understanding how community structure influences ecosystem functions via functional traits. Formal causal analysis showed that biodiversity-ecosystem function correlations are noncausal associations. Using simulations, we showed how biodiversity correlations and species identity effects can arise from misspecification or incomplete mediation by functional trait composites. We also found that different types of model misspecification result in different patterns of residuals, which may be used to diagnose gaps in functional trait hypotheses. Treating the model misspecifications eliminated associations between species identity or biodiversity and ecosystem function. Finally, we provide an example of the analysis of field data to demonstrate how to use these insights to conduct a research program that has the goal of understanding the mechanistic trait relationships that link community structure to ecosystem function.

Riparian forest restoration as sources of biodiversity and ecosystem functions in anthropogenic landscapes

“…Numerous well-known studies have posited that species identity and biodiversity are surrogates of functional-trait effects on ecosystem functioning (see Hättenschwiler et al, 2018; Szefer et al, 2017). However, according to (Schoolmaster et al, 2020), these surrogates should not be assumed to be “causal” although significant biodiversity–ecosystem function correlations are spurious associations that arise from common-cause confounding in mis-specified trait-based ecosystem function models. Residual effects of species identity, while causally related (i.e., elements of species composition), also indicate incomplete trait information.…”

Exploring the relationship between functional structure and ecosystem multifunctionality requires intraspecific trait variability

“…S1). These results are suggestive, but it is community composition, rather than species richness per se, that should account for differences in ecosystem multifunctionality 11-12 . As Schoolmaster et al .…”

“…These results are suggestive, but it is community composition, rather than species richness per se, that should account for differences in ecosystem multifunctionality [11][12] . As Schoolmaster et al (2020) argue, it is essential to correctly account for the cause -effect links between the components of biodiversity and ecosystem functions since community composition would influence ecosystem functions via plant phenotypic traits at small local spatial scales [13][14] .…”

“…As Schoolmaster et al (2020) argue, it is essential to correctly account for the cause -effect links between the components of biodiversity and ecosystem functions since community composition would influence ecosystem functions via plant phenotypic traits at small local spatial scales [13][14] . Hence, trait-based approaches (response/effect traits) quantifying compositional complexity have been proposed as a way of mediating the relationship between global change and ecosystem functions [12][13][14][15] . In this paper, we will apply trait-based approaches, and call the pattern of trait values expressed in a community that determined both by the identity and relative abundance of the species possessing these traits, as the community functional structure 16 .…”

Increase in leaf organic acids to enhance adaptability of dominant plant species in karst habitats

“…The coordination among different leaf traits, which are shaped by evolution (Firn et al, 2019), allows plant adaptation to diverse habitats (Ahrens et al, 2020;Firn et al, 2019;Gratani & Bombelli, 2000;Moreira & Pearse, 2017;Wright et al, 2004Wright et al, , 2005. Correlations among leaf traits are used to interpret the biodiversity-ecosystem functions (Cronin & Schoolmaster, 2018;Schoolmaster et al, 2020) and can be used to track how plants respond to environmental change (Cronin & Schoolmaster, 2018). For example, plants can decrease SLA to increase water-use efficiency (WUE) in habitats with low water availability (Maxwell et al, 2018) and synthesize OA to abate the adverse effects of excess leaf [Ca] (Kinzel, 1983).…”