Discussion
Based on a globally coordinated experiment in grasslands, our analysis
demonstrates that (1) nutrient addition and herbivore exclusion mainly
had additive effects, with nutrient addition consistently reducing
stability at the local and larger spatial scales, while herbivore
exclusion weakly reduced stability at both scales; (2) nutrient addition
reduced stability primarily by increasing temporal community
dissimilarity and decreasing species richness and evenness. In contrast,
herbivore exclusion reduced gamma stability mainly by reducing spatial
asynchrony, but also weakly by decreasing local species richness.
Temporal and spatial community dissimilarity was mainly attributed to
balanced variation (i.e. change in relative abundance among species but
not total abundance in communities across time or space), pointing at
the importance of turnover driven by species replacement in determining
grassland stability.
In contrast to our hypothesis 1, our analysis provides weak support for
interactive effects of nutrients and herbivores on stability across
spatial scales and other plant community properties (additive for plant
diversity and stability but a synergistic effect for spatial
asynchrony). Previous analyses using different subsets of NetNut data
looking at the joint effects of nutrient and herbivores on species
richness and aboveground biomass also found weak interactive effects
(Borer et al. 2014b, 2020). Several non-exclusive processes may
explain the lack of the interactive effects of nutrients and herbivores
found here. First, relatively low grazing intensity at many sites.
Indeed, we found that the effects of herbivores on spatial asynchrony
and gamma stability tended to be more apparent at sites with high
grazing intensity under the ambient conditions (Fig. S5). Second,
duration of the current experiment may not be long enough to capture
changes in plant communities (Chen et al. 2019, 2020). Boreret al . (2020) found that the interactive effects of nutrient
addition and herbivore exclusion on aboveground biomass became stronger
using 8-10 years post-treatment data compared with those using 2-4 years
post-treatment data. Third, the effects of nutrients and herbivores on
plant communities may act at different spatial scales, where nutrient
addition influences local communities and herbivores modulate spatial
heterogeneities (see next paragraph). Consistent with previous analyses
(Zhang et al. 2019; Hautier et al. 2020), our results show
that the negative effects of nutrient addition alone cascaded to larger
spatial scales. Herbivore exclusion alone had weak negative effects on
stability at the two scales investigated. Again, this may be due to low
grazing intensity at many sites (Table S1).
Confirming our hypothesis 2, nutrients and herbivores impacted gamma
stability via different pathways. Nutrient addition reduced alpha and
gamma stability probably because nutrient addition intensified
interspecific competition within local communities, causing a decline in
alpha diversity, a decrease in evenness, and an increase in temporal
community dissimilarity. Moreover, increased temporal community
dissimilarity contributed to decreased alpha and gamma stability as much
as the combined effects of decreasing alpha diversity and evenness. This
confirms previous results showing a stronger contribution of temporal
community dissimilarity to alpha stability compared to alpha diversity
(Koerner et al. 2016). In contrast, herbivore exclusion weakly
decreased gamma stability primarily by decreasing spatial asynchrony,
and weakly by decreasing alpha diversity. It suggests that the effects
of herbivores may be more apparent at the larger spatial scale probably
by impacting vegetation heterogeneity, particularly in aboveground
biomass (Glenn et al. 1992; Howison et al. 2017). Our
results suggest that maintaining stability from nutrient addition and
herbivore exclusion in grasslands needs to focus on different processes
across spatial scales.
Additionally, nutrient addition decreased alpha and gamma stability via
increasing temporal balanced variation, while its negative effect on
temporal abundance gradients does not translate into changes in
stability. In contrast, the effects of herbivore exclusion on spatial
asynchrony and gamma stability were not related to spatial community
dissimilarity, which is consistent with previous analyses (Wilcoxet al. 2017; Zhang et al. 2019). Community dissimilarity
across time is commonly used as an index of compositional stability
(Hillebrand et al. 2018; Hillebrand & Kunze 2020; White et
al. 2020) and higher compositional stability has been suggested to lead
to higher biomass stability (Allan et al. 2011). Thus, the
relationship between compositional stability and biomass stability may
depend on the spatial scale considered. This necessitates looking at
multidimensional stability (Donohue et al. 2013). As cover is
usually easier to measure in the field and less destructive compared
with biomass harvesting, many researchers evaluate stability based on
total cover (i.e. mean of total cover through time divided by its
standard deviation; e.g. Post 2013; Beck et al. 2015; Blüthgenet al. 2016; Wilcox et al. 2017). Our results suggest that
using cover data to calculate stability may fail to capture changes
induced by balanced variation. For instance, two communities (in
different years or places) can differ markedly in biomass due to species
replacement even when their total cover remains the same (Fig. S2E). As
a result, a cover-based metric of stability may overestimate ecosystem
stability relative to its biomass-based counterpart (Fig. S7). That
said, community dissimilarity (and its partitioning) serves as a useful
index to predict biomass stability, but spatial scales need to be
considered.
In this study, we regarded aggregated local communities within
treatments across blocks as “larger spatial scale” following previous
analyses (Chalcraft et al. 2008; Wilcox et al. 2017; Zhanget al. 2019; Hautier et al. 2020). However, subplots
belonging to different treatments within blocks are closer to each other
compared with subplots belonging to the same treatments across blocks
(i.e. larger spatial scale), thus dispersal may be stronger for subplots
within blocks than the larger spatial scale used here and dispersal may
bias the results. We argue that dispersal may not influence our results
for two reasons. First, blocks typically spread within a relatively
small area at each site (around 1000 m2), thus all
subplots within sites may be connected by dispersal (Zhang et al.2019). Second, we found that subplots belonging to different treatments
within blocks had relatively higher spatial community dissimilarity
(0.61 average across sites) than those belonging to the same treatments
across blocks (0.55, 0.58, 0.53, 0.57 for control, fence, NPK, and
NPK+fence). This suggests that treatments are the dominant filter for
plant community assembly.
Our results—based on 34 grasslands across four continents—advance
our knowledge in that (1) nutrients and herbivores mainly have additive
effects on stability in grasslands; (2) nutrients and herbivores impact
stability across multiple spatial scales through different pathways,
wherein turnover driven by species replacement is more important than
species richness in determining grassland stability. Our results point
to the need to reduce nitrogen deposition while preserving or
reintroducing herbivores to ensure the stable provisioning of grassland
biomass. More importantly, our results highlight that maintaining
grassland stability in the face of increasing nutrient addition and
herbivore extirpation requires a multi-scale framework to disentangle
the influences of processes operating at different scales to guide
conservation and management practices.