Discussion
We used an ecological adaptation of the Price equation to partition components of compositional change into rates of change in gains, losses, and persistent species and the biomass change associated with each over time. Using data from 59 global grasslands we show that high compositional turnover under ambient conditions also affects turnover in community aboveground biomass, while aggregate plot-level biomass remains stable over time. In contrast, the addition of multiple limiting nutrients resulted in greater species loss and reduced gains compared to controls, which both contribute to a net decline in richness. Under fertilization, species loss was associated with a decline in biomass over time and the species that were gained were associated with overall biomass gains. Species that persisted over time were also associated with biomass gained, jointly leading to overall biomass increases with nutrient addition, on average.
Some of the most important components of biodiversity change are not obvious when considering just changes in species numbers (i.e. species richness) because these aggregate measures often obscure functional contributions resulting from change in species composition (Joneset al. 2017; Hillebrand et al. 2018). In addition, compositional change (i.e. species turnover) can be uncoupled from changes in species richness (Hillebrand et al. 2018; Bloweset al. 2019), whether richness is changing or not (Harpoleet al. 2016; Hautier et al. 2018; Seabloom et al.2020) in global grasslands. In this study, we observed substantial turnover of species and biomass over time but no change in overall richness and biomass in ambient conditions (Figure 2, Figure 3). In contrast, in fertilised conditions, biomass associated with species gained and persistent species outweighed the biomass lost by species losses (Figure 3, Figure 4) even though species losses increased and species gains decreased over time. Under fertlized conditions, we found much variation in the biomass change associated with persistent species, resulting in little change in this biomass component over time overall (Figure 3). However, we did find a substantial difference in the biomass associated with persistent species in fertilized plots compared to control plots, contributing to overall biomass gains (Figure 4). Our findings help elucidate how the components of community change contribute to biomass production under fertilization over time. The strength and direction of biodiversity change depends on the balance of species losses, species gains, and species that persist over time (Dornelas et al. 2019), and as we show here, so do changes in ecosystem functioning. Focusing on aggregate measures of biodiversity change alone can lead to underestimation of change and its impacts on the functioning of ecosystems.
Our findings show that changes in community composition affect measures of ecosystem function in different ways, and these effects are context dependent (Figure 5, Figure S8). Variation in site-level responses to nutrient addition can be explained by the direction and magnitude of compositional and biomass change associated with species losses, gains, and changes in persistent species abundances overtime, all of which varied considerably by site in our study. In communities where richness increased overall we found there were fewer species losses, and greater species gains over time (Figure 5). In communities where overall biomass declined, we found that biomass associated with persistent species had reduced over time (Figure 5). Over the last decades, research has advanced understanding of the relationship between species richness and ecosystem function (Tilman et al. 2014), but our results highlight the importance of discerning how different components of composition change affect functioning.
Rates of change in the metrics investigated here were uncorrelated, supporting the idea that drivers of change can act relatively independently on diversity, composition, and function (Helsen et al. 2014). This indicates that increasing biomass associated with fertilization may contribute to diversity loss, and changes in composition can in turn have varying effects on biomass (Harpoleet al. 2016; Leibold et al. 2017). Our results support the idea that diversity and functioning changes need to be considered concomitantly (Ladouceur et al. 2020) to better understand how these relationships shift under global change processes and pressures. Our results show that the effect of compositional change on ecosystem functioning is dependent on the magnitude and functional contribution of species entering, persisting, and exiting communities. Which species thrive under nutrient addition and which are excluded from fertilized communities, is in part determined by species identities, their traits, and the matching of traits to the environment (Lind et al. 2013; Seabloom et al. 2015; Morgan et al. 2016). Because species contribute to ecosystem function to different extents (Isbell et al. 2013; Hautier et al. 2018), considering various compositional changes simultaneously and in relation to their individual contributions to function provides a more comprehensive understanding of the effects of global change pressures on ecological communities and ecosystems.
Grassland productivity is often limited by multiple nutrients (Fayet al. 2015; Harpole et al. 2016), and species richness and productivity are controlled by a complex network of processes (Graceet al. 2016). Here we quantified how the effect of NPK on species losses, gains, and changes in persistent species abundances across time contributes to variation in overall community responses to fertilization in terms of richness and biomass (Figure 5). Because the plots used in this analyses were unfenced, we expect that herbivory reduced biomass (Borer et al. 2014b, 2020; Hodapp et al. 2018; Ebelinget al. 2021), possibly explaining some variation in the effect of NPK on aboveground biomass in many sites. Further work could investigate composition and biomass relationships under fertilization and with herbivory exclosures. Additionally, some variation in site-level responses may be due to water limitation, and may account for some cases where nutrient induced species-loss does not affect biomass (Figure S8). Opportunities also exist for future work to explore additional mechanisms driving patterns within and across sites (Figure S8) (Avolioet al. 2021), spatial scales (Chase et al. 2019; Barryet al. 2021; Seabloom et al. 2021), and according to species’ identities and characteristics (Crawford et al. 2021). The risk of a species being lost from a plot decreases with its abundance in both space and time, and varies across lifespans and functional forms (Wilfahrt et al. 2021). The degree to which these species’ characteristics influence the magnitude of community level species loss and gains and their associated effects on function are beyond the scope of this investigation. However, because our temporal approach provides estimates of rates of functional change over time, a similar approach could possibly be adapted to functions that are not additive, such as stability (e.g., estimates of temporal variability within an assemblage).
In sum, we partition measures of species richness and a measure of ecosystem functioning (live biomass) to better understand the underlying mechanisms of community change under pressure from a key driver of global environmental change, nutrient enrichment. Our results demonstrate that the components of compositional change are key to understanding the relationship between diversity and ecosystem functioning, particularly in ecological systems that are experiencing ongoing anthropogenic change. By partitioning the roles of individual species, this work provides a more detailed understanding of the relationships between biodiversity change and ecosystem function in natural systems and how global change drivers can affect them.