Results
Nutrient addition and herbivore exclusion had no significant interactive
effects on any variables measured except for spatial asynchrony (Fig.
1). That is, herbivore exclusion decreased spatial asynchrony under
ambient conditions but not under fertilized conditions. Although
nutrient addition and herbivore exclusion had no significant interactive
effects on gamma stability, herbivore exclusion reduced gamma stability
by 11 % on average compared with that of the control under the ambient
conditions. Additionally, under the ambient conditions, the effects of
herbivores on spatial asynchrony and gamma stability tended to be more
apparent at sites where herbivore index was higher (Fig. S5). Nutrient
addition alone decreased alpha diversity and evenness, but it did not
affect beta diversity. Nutrient addition increased temporal community
dissimilarity by increasing temporal balanced variation and decreasing
temporal abundance gradients, whereas it decreased spatial abundance
gradients. Moreover, nutrient addition decreased alpha stability and
marginally decreased gamma stability. Similarly, herbivore exclusion
decreased alpha diversity and evenness, but it did not affect beta
diversity. Herbivore exclusion increased spatial community dissimilarity
by increasing spatial balanced variation, and it also marginally
increased temporal balanced variation. Herbivore exclusion had no
significant effects on alpha and gamma stability, but marginally reduced
spatial asynchrony (Fig. 1; see Table S3 for test statistics).
The SEM clarified the direct and indirect effects of nutrient addition,
herbivore exclusion, and their interaction on stability at the local and
larger spatial scales (Fig. 2). Nutrient addition and herbivore
exclusion alone influenced gamma stability through different pathways.
Nutrient addition decreased gamma stability by decreasing alpha
stability, but not spatial asynchrony. The negative effect of nutrient
addition on alpha stability was mediated by decreased alpha diversity,
evenness, and increased temporal community dissimilarity. Moreover,
temporal community dissimilarity was the primary factor by which
nutrient addition decreased alpha and gamma stability, contributing ca.
50 % of the decline in stability. In contrast, herbivore exclusion
weakly decreased gamma stability by decreasing spatial asynchrony,
although it also had a weak negative effect on alpha stability by
decreasing alpha diversity (Fig. 2). A decrease in spatial asynchrony
with herbivore exclusion was counteracted by its positive interaction
with nutrient addition. Spatial asynchrony was positively correlated
with beta diversity but not with spatial community dissimilarity. The
effects of herbivore exclusion and the interactive effects of nutrient
addition and herbivore exclusion on spatial asynchrony were not mediated
by beta diversity or spatial community dissimilarity. Results were
generally robust to the inclusion of environmental factors (see
supplementary text and Fig. S6).
The variation in temporal community dissimilarity was mainly driven by
temporal balanced variation, which was negatively correlated with alpha
stability (slope and 95 % confidence intervals (CI): -0.68, CI =
[-1.12, -0.24]; Fig. 3A). In contrast, temporal abundance gradients
only accounted for a small portion of temporal community dissimilarity
and were unrelated to alpha stability (-0.50, CI = [-1.78, 0.77]).
The variation in spatial community dissimilarity was mainly driven by
spatial balanced variation, which was unrelated with spatial asynchrony
(-0.02, CI = [-0.24, 0.21]). In contrast, spatial abundance
gradients only accounted for a small portion of spatial community
dissimilarity but were negatively correlated with spatial asynchrony
(-0.86, CI = [-1.71, -0.01]).