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.