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.