Introduction
Human pressures are fundamentally changing the global environment in terms of species diversity and the functioning of ecosystems (Moreno-Mateos et al. 2017; Chaplin-Kramer et al. 2019). There are elevated extinction rates globally, but this is often not reflected in measures of species richness and diversity at local scales (Dornelas et al. 2014; Blowes et al. 2019). Instead, compositional change in species is predominant (Hillebrand et al.2018; Blowes et al. 2019), as there is a mixture of winners and losers in ecological communities under anthropogenic pressures (Dornelaset al. 2019). Biodiversity is known to positively influence ecosystems in terms of important functions such as biomass production, nutrient absorption, and carbon sequestration (Cardinale et al.2013; Hooper et al. 2016), and species loss is known to negatively affect these measures of ecosystem function (Smith & Knapp 2003; Isbell et al. 2013; Genung et al. 2020). However, aggregate community measures of biodiversity and functioning, while somewhat interdependent, can also respond independently to external processes and pressures (Grace et al. 2016; Ladouceur et al. 2020). It is not well understood how compositional change resulting from global change pressures or disturbance affects measures of ecosystem function.
A major source of global biodiversity change is the increased inputs of biologically limiting nutrients to the environment from anthropogenic activities (Ackerman et al. 2019; McCann et al. 2021). In plant communities, fertilization can act independently on multiple resource-limited processes, which may interact with or counteract one another (Harpole & Tilman 2007). More fundamentally, alterations in nutrient supplies change the conditions of species coexistence via tradeoffs in competition for limiting resources, which can result in dramatic, long-term shifts in species richness and composition (Harpoleet al. 2016; Midolo et al. 2019; Seabloom et al.2020). Resulting changes in biodiversity might further alter key ecosystem functions and services such as the production of biomass, carbon sequestration, and nutrient cycling (Hooper et al. 2005). Live aboveground biomass is a particularly important measure of ecosystem function, as plant biomass is an important source of energy for most life on land (Yang et al. 2020). However, the relationship between biodiversity and aboveground biomass under global change pressures such as nutrient enrichment varies in direction and strength across contexts, systems, and sites (Harpole et al.2016). Understanding how biodiversity, composition, and aboveground biomass change are interrelated is essential for anticipating the impacts of global change pressures such as nutrient deposition on ecosystems and their functions.
Global change drivers such as nutrient addition can alter community assembly processes, community composition, and ecosystem functioning concurrently (Bannar-Martin et al. 2017; Leibold & Chase 2017; Leibold et al. 2017). In some cases, small changes in species richness mask large compositional changes (Spaak et al. 2017; Hillebrand et al. 2018). Changes in competition and coexistence resulting from nutrient inputs can affect compositional turnover, or community change, including gains of novel species, losses of existing species, and changes in abundance of species that persist. Because the functional contributions of novel species may not offset the functional contributions of species that are lost, the processes controlling species diversity and those controlling ecosystem functions may be decoupled. Differences in community change following fertilization could also help explain findings of little change in overall community function despite substantial loss of diversity (Fay et al. 2015; Harpole et al. 2016).
Here, we apply an adaptation of the Price equation (Price 1970, 1972; Fox & Kerr 2012) to separate the functional contributions of individual species that are lost, gained, or persist under ambient and fertilized conditions to better understand the role of these community assembly processes on the functioning of ecosystems ((Bannar-Martin et al.2017)). The Price equation was originally developed for use in evolutionary biology (Price 1970, 1972), but has potential to be widely adapted and applied in many contexts to compare two samples and quantify what is unique in each, versus shared between the two (Lehtonen et al. 2020). In ecology, this approach can help elucidate the biological relationships that underpin the variation between aggregate changes in species richness, composition, and additive measures of ecosystem functioning, and has been adapted for this use in many ways (Winfreeet al. 2015; Genung et al. 2020; Lefcheck et al.2021; Ulrich et al. 2021). We use a novel application of this approach based on (Fox & Kerr 2012; Bannar-Martin et al. 2017), to link temporal changes in biodiversity to an additive measure of ecosystem functioning (i.e., aboveground biomass) using a long-term dataset with global reach (Figure 1). By following compositional changes in experimental plots through time, we separate species richness change to quantify the cumulative number of species lost, gained, and persisting, as well as the associated change in aboveground biomass attributed to each (Figure 1).
We quantify how community compositional change induced by nutrient addition contributes to altered ecosystem function (aboveground biomass) using data from sites within the Nutrient Network, a globally distributed nutrient addition experiment, replicated across grassland sites (NutNet;http://www.nutnet.org) (Boreret al. 2014a). Specifically, we synthesize results from 59 experimental sites across six continents comparing control plots and plots that were fertilized with a combination of nitrogen (N), phosphorus (P), potassium (K) and micronutrients (hereafter the NPK treatment). We leverage long-term data to determine rates of change over time for each component.
Based on previous work that documented reduced richness with fertilization (Borer et al. 2014b; Harpole et al. 2016), we expect that overall, the rate of species lost will exceed that of species gained following nutrient addition. However, whether a loss in richness will be associated with change in function likely depends on the functional contributions of species lost, gained, or persisting in the community. On the one hand, a weak response of persistent species or the loss of relatively high-functioning species could be associated with minimal changes or even reductions in biomass (Fay et al. 2015; Harpole et al. 2016). On the other hand, if functional change associated with persisting and gained species exceeds that of lost species in response to nutrient addition, biomass may increase even if more species are lost than gained. Determining which components of community change are associated with changes in function would advance understanding of how global change affects interdependent dimensions of natural systems.