Asynchrony and functional diversity couple herbivore community dynamics to host plant diversity

Domain: chemistry
Published: 2026-01-15
Editorial outcome: accept

Abstract
Biodiversity loss can destabilize ecosystem functioning. How biodiversity–stability relationships are interlinked across trophic levels remains poorly investigated, however, limiting our ability to predict ecosystem-level consequences of declining biodiversity. Here, we analyze the drivers of multi-year herbivore community stability—as a key connector between primary producers and higher trophic levels—and its coupling with host tree diversity and growth stability along a subtropical tree diversity gradient. Phylogenetic diversity, abundance asynchrony and population stability of herbivores emerge as key intra-community regulators of herbivore temporal stability. These regulators, in turn, are strongly affected by changes in tree species richness through tree functional diversity, tree growth asynchrony, and tree growth population stability. Importantly, accounting for herbivore dietary specialization unveil clear stabilizing effects of tree species richness on the community stability of specialists but not of generalists. For the overall herbivore community, higher tree richness results in less stable abundance dynamics. Our findings suggest that biodiversity loss will propagate bottom-up to affect the stability of communities at higher trophic levels, and particularly destabilize communities of more vulnerable specialists. Global change and plantation management may thus also compromise biodiversity conservation by reducing abundance and species richness stability of higher trophic levels.

Introduction
In times of global change—characterized by an increased severity of biodiversity loss, natural disturbances, and pest outbreaks1–3—maintaining the stability of ecosystem functioning has emerged as a key environmental and societal challenge4. Early theory5 and recent empirical research6,7 have emphasized the central role that biodiversity can play in promoting ecosystem stability. Such stabilizing effects may operate through mechanisms such as averaging effects, negative covariance among species, and insurance effects8 and suggest that the current biodiversity crisis9,10 might have long-term negative consequences for ecosystem stability and ultimately human well-being6,11. The full consequences, however, are difficult to assess. This is because, for one, biodiversity–stability relationships may vary with environmental (e.g., climate) conditions and across spatial and temporal scales7,12,13, and we currently lack insights from many ecosystems.

Moreover, most empirical studies on diversity–stability relationships have focused on individual trophic levels, mostly the primary producer level12,14. We still lack a thorough knowledge on the extent to which diversity–stability relationships across trophic levels are interlinked through food webs15. Since trophic interactions are key drivers of ecosystem functions16, such insights are essential to develop a clear understanding of how biodiversity affects ecosystem stability.

For ecosystem functioning and stability, the interactions between plants and herbivores are of particular relevance as the latter indirectly link producers with higher trophic levels17,18. The temporal stability (i.e., invariability in time at community and population level) of herbivore abundance can substantially alter ecosystems via stability of herbivory19,20. This, for instance, can be well-illustrated by forests where recent climate change effects have led to herbivore outbreaks that, in turn, caused large-scale forest diebacks3. The temporal stability of herbivore communities is also important for the provisioning of food for higher trophic levels and the long-term maintenance of biodiversity21. Herbivore temporal stability is often low in monocultures, whereas herbivore communities are assumed to be more stable in plant mixtures15. Monoculture plantations represent a dominant forest management practice worldwide22, but their low biodiversity may reduce their capacity to buffer against environmental fluctuations and pest outbreaks. The regulatory mechanisms driving the dynamics of trophic interactions between plants and herbivores across temporal scales are, however, not well explored at the community level23.

For tree communities, the temporal stability in growth and productivity has recently received increasing attention14,24. In general, community stability can be partitioned into species asynchrony, i.e., a temporal misalignment of species dynamics, and the stability of species’ populations themselves14. It was shown that tree diversity stabilizes biomass productivity via compensatory effects of tree growth asynchrony (i.e., variability in the extent of temporal growth consistency among tree species) among functionally distinctive species and, to a lesser extent, population growth stability (i.e., the extent of temporal growth consistency within tree species)14. By contrast, the importance of such metrics in herbivore communities for determining stability of abundance and species richness over time remain largely unexplored for forest ecosystems. Even more so, the extent to which herbivore community dynamics are moderated by the diversity and temporal stability in the growth rates of their host plant communities remains unclear. Herbivore community stability might show a tight coupling with community asynchrony and population stability in plant growth as moderators of resource availability. It could be expected that generalist herbivores, which are less restricted in their host use, benefit from growth asynchrony of host species by stabilizing in both abundance and species richness over time25. Stability of dietary specialists, by contrast, might be positively affected by the population stability of their host plants15. However, empirical insights on how diversity and stability are interlinked between herbivores and their host plants are available primarily for grasslands, which are characterized by a strong yearly biomass turnover of herbs and graminoids15. As long-lived organisms, trees are rather constant and predictable resources, and associated herbivores might thus be less affected by changes in the stability of host biomass production. Composition and functional diversity of tree-feeding herbivores might therefore be more directly connected to host plant diversity, determining the availability of different resources26. This pattern could also be captured by phylogenetic relatedness among herbivore species, which can reflect herbivore functional dissimilarity and co-evolutionary processes27.

Here, we analyze the mechanisms underlying herbivore community stability in a species-rich subtropical forest ecosystem, assessing key properties of herbivore communities and the extent to which these are coupled to the diversity and growth stability of their host tree communities. We use time series data (17 replicated sampling seasons spread across six years from 2017 to 2022) of a dominant group of insect herbivores (lepidopteran larvae) and their host tree communities in a large-scale tree biodiversity experiment (BEF-China28). The experiment was conducted at two sites located 4 km apart, and our study included 52 randomly distributed plots reflecting a tree species richness gradient from monocultures to mixtures with up to 24 species. Specifically, we use linear models and path analysis to test the pathways that directly and indirectly (via tree and herbivore community asynchrony and population stability and via herbivore phylogenetic diversity) connect tree species richness and its effects on tree growth to the temporal stability of herbivore abundance and species richness (Fig. 1). Because we are interested in the relative role of direct and indirect effects of tree species richness, we test four alternative path models of decreasing complexity that sequentially dropped direct effects of tree species richness and growth on herbivore stability (Fig. 1c). We hypothesize that (1) the community dynamics of herbivores are coupled to the diversity and growth rate stability of their host tree communities, with metrics related to tree growth asynchrony and population stability explaining the stability of the associated herbivore community (Fig. 1a, b). As tree growth stability, in turn, depends on tree diversity, we further expect that (2) the loss of tree diversity indirectly (via tree growth) destabilizes herbivore communities (Fig. 1c). Finally, we expect that at the level of herbivore communities, (3) community stability is primarily driven by asynchrony among dominant generalist herbivores, with specialist species being much rarer and their stability depending more strongly on population stability.

Results
In total, 17,850 caterpillars were collected over the sampling period of the 6 years. A total of 243 molecular operational taxonomic units (MOTUs) were included in our final analysis after excluding species represented by fewer than five individuals. Of these 243 MOTUs, 140 were classified as generalist herbivores (accounting for 9063 individuals) and 103 as specialists (with 4943 individuals).

Tree diversity indirectly stabilizes herbivore communities
Linear models and subsequent path analysis showed that tree species richness ultimately moderated the community dynamics of herbivores. For the path models, the model variant excluding direct effects of all tree variables on herbivore stability metrics and direct effects of tree species richness on herbivore phylogenetic diversity (measured as mean pairwise phylogenetic distance, MPD) and population stability (model variant 4, Fig. 1c) received the highest support (Table S1). Consequently, the effects of tree species richness were largely indirect by influencing herbivore asynchrony, population stability, and herbivore phylogenetic diversity (or, alternatively, abundance/richness, see supplementary methods and sensitivity analysis results in Figs S1 and S2 and Tables S2–S7) via tree functional diversity as well as asynchrony and population stability of tree growth (Figs. 2, 3 and S3; Tables S8–S10).

Divergent effects of tree diversity on generalists and specialists
Tree species richness was positively related to herbivore abundance and richness stability, particularly for specialist herbivores (Figs. 2c, 3c and 4). These positive effects were mediated by the strong link between tree species richness and tree growth asynchrony, and the positive effects of the latter on herbivore phylogenetic diversity and asynchrony, which in turn promoted herbivore stability. By contrast, the effects of tree species richness on generalist and, subsequently, also on overall herbivore stability were altogether non-significant, because positive effects via tree growth asynchrony (and tree growth population stability) were weaker and counteracted by negative effects via tree functional diversity (Fig. 4) on herbivore population stability (Fig. 2b) and herbivore asynchrony (Fig. 2a). Interestingly, tree functional diversity was negatively related to herbivore asynchrony—with negative effects on overall and specialist patterns (Figs. 3a, c and Tables S8 and S10) and no effects on generalists (Fig. 3b and Table S9)—while tree asynchrony consistently promoted herbivore phylogenetic diversity (Fig. 3 and Tables S8–S10). Average population stability of tree growth was negatively related to population stability of generalist and overall herbivores, whereas there was no relationship for specialists (Fig. 3 and Tables S8–S10). These patterns were consistent across additional sensitivity analyses (see supplementary method and note text), including the use of an alternative stability index, the inclusion of all caterpillar individuals in path models, and path models excluding monocultures.

Herbivore asynchrony and phylogenetic diversity stabilize herbivore communities
Within the herbivore communities, abundance stability was particularly promoted by herbivore asynchrony (Fig. 2d and 3 and Table S11) and, to a lesser extent, by population stability in all cases (Figs. 3 and S3a and Table S11). Herbivore richness stability was strongly related to herbivore abundance stability in all cases (Fig. 3) and furthermore affected by both herbivore phylogenetic diversity (for overall herbivores and specialists; Figs. 2e and 3 and Table S12) and herbivore population stability (including for generalists; Figs. 2f and 3).

Herbivore phylogenetic diversity positively affected herbivore population stability of specialist and overall herbivores (Figs. 3 and S3b and Table S13), and increased herbivore asynchrony for specialists (Figs. S2c and 3c and Table S14).

Methods
Experimental site and design
We collected data from the BEF-China tree diversity experiment28, which is located at Xingangshan, Jiangxi Province, southeast China (29°08′–29°11′N, 117°90′–117°93′E). The experiment comprises two study sites, namely site A and site B, each spanning approximately 20 ha, and established in 2009 (site A) and 2010 (site B). The study sites are characterized by seasonal monsoon climate with a mean annual temperature of 16.7 °C and a mean annual precipitation of 1800 mm51. The region is characterized by highly diverse native subtropical forests, predominantly composed of a blend of broadleaved evergreen and deciduous tree species.

There are 566 study plots (25.8 × 25.8 m) established on the two sites, forming a tree species richness gradient ranging from monocultures to mixtures of up to 24 tree species. Within each plot, 400 saplings were planted, arranged in 20 rows and 20 columns with a mean planting distance of 1.29 m. Altogether, 40 locally common tree species were planted across the two study sites. Tree species compositions at the two sites were largely non-overlapping, with different species pools of 16 broadleaved species at each site (and an overlap of eight tree species in the 24-species mixtures that were planted on both sites). In order to achieve a comprehensive representation of species across diverse levels of biodiversity, compositions of mixtures were randomly assigned based on a “broken-stick” design28. The allocation of tree species to planting positions within each plot was done randomly, and the total number of individuals per plot was evenly distributed among the planted species.

For our study, we selected a subset of 64 plots (32 per site) which followed a broken-stick design and spanned the tree diversity gradient from monocultures to 24 species-mixtures. Per site, we considered sixteen monocultures, as well as eight 2-, four 4-, two 8-, one 16-, and one 24-species mixtures. We excluded twelve plots due to high tree mortality, resulting in a final set of 52 plots included for our analyses.

Sampling, species delimitation and phylogenetic analyses
We collected lepidopteran larvae, a key group of insect herbivores in forest ecosystems, across 17 sampling events spread across six years (from 2017 to 2022). To capture seasonal changes in caterpillar assemblages and gain a rather comprehensive picture of the occurring fauna, we sampled three times per year (spring: April, summer: June, and autumn: September; April in 2020 was missed because of the COVD-19 pandemic). Caterpillars were sampled by beating individual tree branches with a padded stick and subsequent collecting all dislodged individuals that fell on a white sheet (1.5 × 1.5 m), placed directly beneath the branches52. To standardize the sampling effort, we started collecting the trees in the first row of each sampled plot and continued until 80 living trees were sampled. Because of the random planting design allowed us to adequately cover the complete tree species composition per plot. Collected caterpillars were individually preserved in tubes filled with 99.5% ethanol and stored at −20 °C until further processing.

We sequenced the barcode region (i.e., mitochondrial cytochrome c oxidase subunit I (COI), 658 bp) of all caterpillars using universal primer pairs, LCO1490 and HCO219853. Polymerase chain reactions (PCR) were conducted in 96-well plates with a total volume of 30 μl, consisting of 10 μl ddH₂O, 15 μl Premix PrimeSTAR HS (TaKaRa), 1 μl of each primer (10 μM), and 3 μl of genomic DNA template. The thermocycling conditions were as follows: an initial denaturation at 94 °C for 2 min; 29 cycles of 94 °C for 50 s, 50 °C for 50 s, and 72 °C for 1 min; and a final extension at 72 °C for 6 min. All reactions were run on an Eppendorf Mastercycler Gradient. PCR products were examined on a 1% agarose gel, and samples showing clear single bands were purified and sequenced using BigDye v3.1 chemistry on an ABI3730xl DNA Analyzer (Applied Biosystems). Based on the obtained sequences, caterpillars were then assigned to molecular operational taxonomic units (MOTUs), which are widely used in ecological studies and have been proven useful for taxa or regions where taxonomic knowledge is largely incomplete54. We used three methods to generate MOTUs and then chose the most consistent clustering method for further analysis, they are: threshold-based hierarchical clustering with BLASTclust, Automatic Barcode Gap Discovery (ABGD), and Poisson Tree Processes model (PTP). In total, 885 distinct MOTUs were identified using the most consistent method of molecular species delimitation.

As a proxy for herbivore functional diversity, phylogenetic diversity is a useful surrogate27,55. To gain more accurate estimates of phylogenetic diversity, we first used RaxML56 to place our MOTUs onto a comprehensive (11,000 species) synthesis phylogeny of Lepidoptera, which was obtained from https://insectphylo.org/ (Chesters et al.57). We then calculated abundance weighted herbivore phylogenetic diversity (i.e., mean pairwise phylogenetic distance; MPD) for sets of the phylogenetically-placed herbivore MOTUs at community level.

Host specialization estimation
To evaluate host specificity of individual caterpillar species, we calculated the wood volume weighted MPD of host trees utilized by a given caterpillar species. MPD-based indices are commonly employed to quantify host plant specialization of insect herbivores (e.g. 58–61). The weighted host MPD considers both frequency and phylogenetic relatedness of the host plants. The highest level of dietary specialization is indicated by host MPD values of zero, whereas the degree of dietary generality increases with increasing host MPD. Weighted host MPD values were calculated for every caterpillar species, using the ‘mpd’ function as implemented in the picante package62. To do so, we used a maximum likelihood phylogeny covering all caterpillar species included in this study. For each caterpillar species, host plant records across all plots were considered to calculate its degree of dietary specialization.

Before classifying species as either specialists or generalists, we first excluded species with fewer than five individuals. This way, we aimed at eliminating the bias caused by rare species (i.e., that generalists with low abundances are erroneously considered as specialists). Subsequently, we applied hierarchical clustering based on host MPD using partitioning around medoids (PAM)63. This method uses algorithms to group taxa into well-defined ‘hard’ clusters with minimal prior input, thereby enhancing the objectivity of the classification procedure. Clustering analyses were carried out using two clusters (k = 2, i.e., generalists and specialists). The threshold value was then used to separate specialists (host MPD < 0.181, 103 species) from generalists (host MPD > 0.181, 140 species).

There are other and often more complex phylogenetic indices that could be used to reflect herbivore diet breadth (e.g., 64), but we chose the classification as described above to allow for an easier comparison with earlier studies that commonly applied a binary demarcation for dietary breadth as well (e.g., 65).

Tree growth and tree functional diversity
The aboveground wood volume (Vi) of an individual tree i was measured annually between 2016 and 2021. It was determined by the formula:

Vi = Hi × π(BRi)2 (1)

where Hi denotes the height of the tree, and BRi represents its basal radius measured at ground level, with both measurements in meters.

For each designated plot and corresponding year, the total tree volume, denoted as V (m³/ha), was determined by summing the wood volumes of the living trees (Vi) in the central 36 planting positions of the plot. This calculation was standardized based on the plot area.

The absolute growth rates (TGRi) of individual trees were calculated for both monocultures and mixtures within the plot:

TGRi = Vi,t2 − Vi,t1 (2)

where Vi,t1 and Vi,t2 are the tree wood volumes at two consecutive years, t1 and t2, within the sampling period from 2016 to 2021. TGRi was used for the calculation of tree asynchrony and population stability.

We used six functional traits for tree functional diversity calculation, namely specific leaf area (SLA), leaf dry matter content (LDMC), ratio of leaf carbon and nitrogen content (C:N), leaf magnesium content (Mg), leaf tannin content, and leaf lignin content. These functional traits have been recognized to reflect host plant palatability and are often closely related to herbivore performance and degree of herbivory66,67. We measured the functional traits at three to five different tree heights per species in 201868. Tree functional diversity was calculated on plot level as the mean pairwise distance between mean trait values per tree species, weighted by tree wood volume, and expressed as Rao’s Q69 (note that Rao’s Q is 0 in monocultures).

Temporal stability, population stability, and asynchrony calculation
We calculated the temporal stability of herbivores based on the overall abundance and richness of herbivores at plot level and per year (i.e., multiple measures per year were pooled). The temporal stability was calculated as the inverse of the coefficient of variation

stability = μ / σ (3)

where μ and σ are the mean value and standard deviation, respectively, of herbivore abundance and herbivore richness at plot level and across sampling years. The metrics should thus be understood as stability of herbivore communities, where higher values denote a more stable community.

The plot level-average population stability of tree growth and herbivores was calculated as the sum of all species’ temporal stability indices (applying formula 3).

We calculated species asynchrony (hereafter asynchrony) at community level for both trees and herbivores using the species synchrony statistic φ70 as 1 − φ

asynchrony = 1 − (σ2 / (Σn i=1 σi)2) (4)

where σ is the overall community standard deviation across years and σi is the temporal standard deviation of the annual absolute tree growth (for trees) or abundance (for herbivores) of species i in a plot of n species. Therefore, asynchrony will increase if the variance in individual tree species growth or herbivore abundance increase, corresponding to the variance in tree growth or herbivore abundance at community level. The asynchrony ranges from 0 to 1, representing a gradient from complete synchrony to complete asynchrony (note that asynchrony value is 0 in monocultures).

Modeling and statistical analysis
We used linear models to check the effects of tree species richness, functional diversity, population stability, and herbivore phylogenetic diversity on herbivore asynchrony and population stability for all herbivores, generalist herbivores, and specialist herbivores, respectively. Moreover, we also included herbivore asynchrony and population stability as additional predictors and checked their effects on temporal stability of herbivore abundance and richness. We did not include tree asynchrony as a predictor in the linear models due to the strong correlation with tree species richness (Pearson’s r = 0.77, P < 0.001). We included the interactions between site and tree species richness, site and tree functional diversity, site and tree population stability as further predictors. We streamlined the linear models through a stepwise process guided by the Akaike Information Criterion, corrected for small sample sizes (AICc). Subsequently, we selected subset models characterized by the lowest AICc values. In addition, we used single linear models to directly test the relationships between tree species richness, abundance, and richness stability of herbivores (separately for all herbivores, generalists, and specialists).

To improve normality and variance homogeneity of the model residuals, population stability, abundance stability, and richness stability as response variables were log-transformed. Likewise, tree species richness as a predictor was log-transformed in all analyses. To enable direct comparisons of model estimates, all continuous predictors were standardized (mean = 0, standard deviation = 1) prior to the analyses.

We used path analyses to disentangle direct and indirect drivers of the plant community (i.e., tree species richness, functional diversity, asynchrony, and population stability) on herbivore stability (abundance stability and richness stability) through herbivore phylogenetic diversity, asynchrony, and population stability. In the absence of herbivore trait measurements, this allowed us to test the potential effects of herbivore functional diversity on herbivore dynamics (keeping the potential links similar to trees).

We constructed our initial model according to the current knowledge on mechanisms driving biodiversity-stability relationships for plants, insects, and ecosystem functions14,29,48. We assumed that tree species richness influences tree functional diversity, tree asynchrony and tree population stability (Fig. 1). Moreover, we assumed that all tree-based predictors can directly affect herbivore phylogenetic diversity, herbivore asynchrony, herbivore population stability and herbivore stability (Fig. 1). The herbivore phylogenetic diversity were separately included in the models to reveal the potential different roles of herbivore community metrics in community stability. For both trees and herbivores, we hypothesized that herbivore phylogenetic diversity can directly affect asynchrony and population stability. We considered covariances between asynchrony and population stability in all models.

To test for effects of feeding specialization on herbivore-host tree interactions, three separate models were fitted based on the initial model structure described above: one for all herbivores, one for generalists, and one for specialists. Moreover, to assess the level of support for different ways in which host tree-based metrics influence herbivore community dynamics, we compared four alternative model structures that successively removed interactions between different levels of host tree and herbivore community data (Fig. 1): First, we considered the initial model with all potential links included (Model 1). Second, we assumed that effects are entirely mediated by stability components, except for effects such as microclimate buffering that may remain in tree species richness. Therefore, we removed the direct connections between trees and herbivore abundance and richness stability, except for effects of tree species richness (Model 2). Third, we assumed that there are no such effects of tree species richness, so we additionally excluded the direct pathways between tree species richness and herbivore stability metrics (Model 3). Fourth, we assumed that tree species richness only has indirect effects via tree stability components and functional composition. We hence removed the links between tree species richness and herbivore phylogenetic diversity and population stability (Model 4). The same model structure selected for overall herbivores was applied to generalists and specialists to ensure comparability across groups. Model fit statistics were summarized (Tables S1 and S15). The best-fitting model was selected based on the lowest AIC value71. We used bootstrapped P values based on 1000 bootstrap draws to ensure robust results72. The effect sizes of tree species richness on herbivore richness stability were then summarized. We additionally calculated effect sizes, including non-significant pathways, and assessed their variability using 1000 bootstrap draws (see supplementary for the results).

We ran additional sensitivity analyses for the path models in which we assessed the role of the herbivore diversity metric and the potential methodological influence of monoculture plots, respectively (see Supplementary Methods and note).

All analyses were conducted in R 4.2.2 (www.r-project.org) with the packages picante62, vegan73, FD74, and lavaan75.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This study investigates the multitrophic relationships linking tree species richness, functional diversity, tree temporal dynamics, herbivore functional diversity, herbivore temporal dynamics and herbivore stability. It is based on a multi-year dataset from a large forest tree species experiment in southeastern China (BEF-China). Over six years, lepidopteran caterpillars were sampled from a total of 52 experimental plots, covering a tree species gradient from one to 24 tree species. Based on lepidopteran data and repeated estimates of tree growth rate / volume, several indices describing functional diversity / temporal dynamics / stability were determined and related using a series of linear models and path analyses. To better understand underlying patterns, lepidopterans were also separated into food specialists and generalists and analyses were repeated for specialist and generalist subsets. Three hypotheses are tested: (1) temporal dynamics of herbivores are coupled to characteristics and dynamics of tree communities; (2) reduced tree diversity indirectly reduces stability of herbivore communities through reduced stability of tree growth; (3) at the herbivore trophic level, asynchrony drives stability of generalists, while population stability drives stability of specialists. The study finds that the temporal dynamics and stability of herbivores are strongly related to the characteristics and dynamics of tree communities. The relationships between herbivore stability and tree community characteristics and dynamics are mediated by herbivore phylogenetic diversity, asynchrony and population stability, with different relationships between generalist and specialist communities. Higher tree diversity was associated with higher herbivore stability, particularly for specialist herbivores, but this relationship was not mediated by tree growth stability. The authors conclude that lower tree species richness - a common outcome of global change - thus puts herbivore communities at risk.\n\nThe study addresses a highly relevant topic and interesting questions based on a robust and interesting dataset. The manuscript is well prepared, demonstrates the relevance of the topic by thoroughly embedding it in the scientific literature, and manages to present a complex topic in a relatively understandable way, making it interesting to the wide readership of Nature Communications. I do have some concerns, mainly related to the design and implementation of the statistical analyses, which I think should be addressed. Some may be based on gaps and ambiguities in the methods sections, which should be revised in some places. Also, although the manuscript is very well written, I would encourage the authors to streamline the discussion, which is currently quite long, e.g. by linking it more closely to the three study hypotheses.\n\nMain concerns:\n- The study examines a complex, multi-trophic system based on a set of indices describing the tree community (richness, functional diversity, temporal dynamics/stability) and the herbivore community (\"functional\" diversity, temporal dynamics/stability). However, it excludes a basic aspect of the herbivore community that I believe is crucial and could explain some of the reported relationships: species richness and abundance of herbivores. Since richness is a fundamental aspect of the study, it should not be reported at only one trophic level. In the presentation of the path analyses, I would suggest including these two parameters at the base of the herbivore community characteristics (adding a third level in Fig. 3). Cf. for example Blüthgen et al. ref. 19 in the present study.\n- The separation of generalist and specialist herbivores is a crucial step in this study. Here it is based on the dataset itself, i.e. species (MOTUs) observed to interact with only a few plants are assigned to the specialist taxa (based on a mean pairwise distance approach). I wonder how the overall abundance of a species in the dataset will affect its classification as generalist/specialist. I would expect the 'specialist' category to include not only specialist species, but also rare generalists (which could not be observed on many plants due to small sample sizes). Showing how total abundance and mean pairwise distance are related would be a first step in addressing this potential confounding factor. If the expected relationship is found, then the results should be reconsidered in this light. For example, I would expect herbivore asynchrony of rare species to be more closely related to herbivore richness stability than that of common species (in the latter case, there is a larger buffer for losing species when variability is higher). In general, an indication of how many individuals are included in each category (generalists, specialists) is missing and should be added. Since the overall dynamics are strongly related to the dynamics of generalists, I assume that generalists form the majority of communities (see also L164ff, L245).\n- The dataset is collected within a tree species richness experiment, which includes many plots with only one tree species (24 out of 52). Two of the tree community indices (tree functional diversity, tree asynchrony) are by definition bounded at 0 for these 24 plots, resulting in a set of three highly correlated variables (tree species richness, tree functional diversity, tree asynchrony). To ensure that the observed relationships are not an artefact of these highly constrained variables, I suggest including sensitivity analyses excluding these single-species plots.\n- The dataset contains a number of strong outliers (see e.g. Fig. 2). I.e. plots with extremely low specialist herbivore asynchrony (A_N11, A_O27), extremely high specialist herbivore abundance stability (A_T15), extremely high specialist herbivore richness stability (B_V19) and very low tree population stability (B_M29). I suggest including sensitivity analyses excluding these plots.\n\nI have several additional comments that may help to improve the manuscript:\nL65f: I would restrict \"frequency and severity\" to natural disturbances and pest outbreaks. \"Frequency of biodiversity loss\" is somewhat unclear.\nL118: I suggest to quickly introduce the spatial spread of sampling sites here as well. Also, I suggest: \"... sampling seasons spread across six years ...\"\nL132: average population stability of herbivores, right?\nL143: I would add the link to Fig. 4\nL143ff: Significant positive effects are only found for specialists. The effects for generalists/overall communities are never statistically significant (judged from Fig. 2). So either attribute all effects except for specialist effects as \"no effects\" or also mention that parts of the relations were negative (although not significant). cf. also L183, where the \"no effect\" is interpreted as a negative effect.\nL149/Figure 4: Could it be an idea to include some more details in Figure 4, i.e. to further distinguish indirect effects through tree FD and tree asynchrony? E.g. by including different fill colours.\nL176: This first sentence is a bit unclear. Herbivore diversity was not in the models (only phylogenetic diversity). And why not include the important link to the trees here as well?\nL217: I suggest to use a different wording than \"dietary niches\". Niches are a species trait, which is not affected by the offer of food plants. Maybe change to \"food spectrum\"?\nL225: The \"weaker\" effect has a larger effect size, though (Fig. 3). It is just the significance which is weaker. I suggest using the term \"clearer\".\nL231-234: I suggest removing this paragraph, as it is merely a repetition of results.\nL283ff: It should be made clear that the effect which is hard to explain is the direct effect. As made clear later (L294), the net effect is positive. I think this is crucial here and should be made clear much earlier in this discussion.\nL292: I think the argument needs to be explained a bit more, as the mentioned link between abundance and richness is based on absolute numbers, not on stability.\nL346: I would mention the 17 sampling events once more here.\nL369: Here, the abbreviation MPD is used for a different concept than just a few lines earlier (L362). This confused me a lot, as I was relating to this second definition when trying to understand Fig. 3. The abbreviation MPD should only be used for one concept.\nL383: I would add the number of MOTUs that go into each of the categories.\nL392: Were these measurements done every year from 2016 to 2021? L408 suggests differently, this should be made clear here. If it is every second year, does it mean that three time points were included? (2016, 2018, 2020)\nL401f: Does this mean that individual variation was not included in plots with > 1 species? Unclear formulation.\nL417: I think it is important to mention here that Rao's Q is 0 by definition if tree species richness is 1.\nL421ff: The temporal level at which these calculations were done is not clear. Was it at the level of sampling events? (n = 17) Or of years? (n = 6) L427 suggests that it is years. If so, it should be made clear from the start that data were pooled per year and plot.\nL432: How was species-level stability of trees determined? I could not find this detail. In the data file that was attached for review, I assume the column \"pop_tree\" to reflect tree population stability. It takes negative values, which I cannot understand based on the formula (3). Please clarify.\nL440: Again, please clarify at which temporal resolution this calculation was done.\nL442: Why was tree growth asynchrony not determined at individual level, but at species level? From how I understand, this data should be available at individual level. It would have the advantage of decoupling tree species richness 1 from asynchrony 0.\nL446: I think it is important to mention here that tree growth asynchrony is 0 by definition if tree species richness is 1.\nL458: Why was the interaction site x mean MPD not included?\nL463: \"... species richness, abundance stability and richness stability ...\"\nL471: Are the log transformations introduced above also included in the path analyses? If yes, how are they accounted for when aggregating path coefficients (i.e., Fig. 4)?\nL489: Can you specify how the non-significant pathways that were excluded were selected?\nL706/Figure 2: There are several inconsistencies in this figure. y axes for panels b, d and e, as well as for panels c and f are not the same, although they show the same variables. The y axes labels in panels a and h are missing zero. Also, it is not totally clear whether the linear regression lines are based on the models introduced in L465ff or whether this is a separate set of models (only including one predictor).\nL718: The definition of MPD is missing from the legend. Also, what the numbers in the yellow circles are showing is not mentioned (variance explained?).\nSI L20 / Fig. S2: is 870 the number of MOTUs that went into the analyses? If yes, why is the number different from the 885 MOTUs introduced at L136?\nSI L54 / Table S4: In the overall models, the second predictor variable should be 'herbivore asynchrony', not 'tree asynchrony'. Please check model output tables again, I did not do a systematic check.", "reviewer_2": "I enjoyed reading the manuscript “Asynchrony and functional diversity couple herbivore and host community dynamics”. The authors have tackled a vitally important topic—how community patterns and stability at one trophic level contribute to stability at a higher trophic level—and brought to bear an incredibly impressive dataset from a very large experiment. Moreover, the work is novel, and the study is solidly conducted. I have just three suggestions, and they are focused on improving the framing and analyses. I’d like to thank the authors again for an important contribution to the literature. My suggestions:\nFirst, the study seeks to address the novel and important question of how one trophic level (trees) influence the stability of a higher trophic level (lep herbivores), but much of the text and analyses are focused on how different metrics and aspects of stability relate to each other within just the herbivore community. While it is interesting to explore the relationships between different metrics of herbivore stability at different scales, it felt both a little circular and tangential to the stated goal of examining how tree community dynamics influence herbivore community stability. It felt circular because, unless I’m missing something, it has to be true that the stability of an herbivore community will be determined by the stability and asynchrony of its constituent populations. I suggest either (1) refocusing the manuscript around the effect of the plant community on herbivore stability (and reducing text on the links between different metrics of herbivore stability) or (2) making exploring relationships between different herbivore metrics an explicit secondary goal of the manuscript. The first seems preferrable to me, but either could work.\nSecond, the manuscript is framed around examining the effects of how plant community dynamics influence herbivore community stability, but the authors have relatively weak data on community dynamics. Their data are change in tree biomass over only 6 years. Most of this change seems to be tree growth from saplings to larger trees rather than turnover, which is what most ecologists would consider to be community dynamics. It seems extremely unrealistic to calculate tree population stability or tree asynchrony over just six years. Really what the authors are calculating is variation in tree growth rates, I think, though it’s a bit hard to tell from the methods. This could be very relevant for insect herbivores because insect herbivores often perform best when plants grow rapidly, so there is something interesting and important going on here. I just think the authors need to tone down claims of linking tree community dynamics to herbivore community patterns. Instead, the authors measured individual growth patterns and variation in growth patterns. Framing things in terms of community dynamics also obscures the most powerful aspect of the study: the tree species richness treatments. This was a randomized species richness experiment, giving the authors have incredible power to link tree richness to herbivore community patterns and stability. Much of that, however, is lost in the analyses and discussion of tree community dynamics (differences which are downstream from their tree species richness treatments). I suggest focusing the framing and analyses and discussion around the experimental treatment of tree species richness and toning down claims of linking tree community dynamics to herbivore patterns.\nThird, the authors use stepwise model selection, but major problems with stepwise model selection have been known for a long time. For example, see Whittingham et al. 2006. Why do we still use stepwise modelling in ecology and behaviour? Journal of Animal Ecology. A better approach would be to use AIC to compare a set of candidate models only once. Of course, this may be unwieldy given how many potential models the authors have. This, however, relates to my last two comments. Following my first comment would reduce the number of predictors related to herbivore community metrics. Following my second comment would reduce the number of plant community dynamic predictors. This would simplify the number of potential models down to a more reasonable number of models to compare. Or just fit the full model and focus on the estimated effect sizes.\nFinally, the manuscript was well written but a bit hard to read because of the frequent mention of many subtly diversity/stability terms for different scales: abundance stability, richness stability, population stability, and more. Moreover, there are many different facets of stability for any one quantity, and different metrics for each facet. Through the introduction, results, and discussion, it is a little unclear what specifically the authors mean by stability. I suggest simplifying all this, reducing jargon, focusing on the most important aspects of stability for this study, and precisely defining each term early in the manuscript.\nMinor comments:\nGiven the authors’ goal of examining stability/variability in trophic interactions and herbivory, two it might be helpful to refer to two recent papers that examine variability in plant-herbivore interactions: Herbivory Variability Network et al. 2023. Science.; Wetzel et al. 2023. Annual Review of Ecology, Evolution, and Systematics.\nThe authors use mu / sigma as a metric for stability (inverse of CV). This adjusts variability by the mean, but I worry that rare species (with low abundances) will have unrealistically low stability (high variability). This metric will lead to very low values for species that are typically rare because their mean is so low, and it might have nothing to do with stability per se. It might be helpful to also analyze the data with sigma as the measure of stability to see how that changes results and interpretation. Alternatively, a recent extension of the CV addresses some of these issues and generally makes for a more robust metric: Kvalseth 2016 Journal of Applied Statistics.\nI want to end by thanking the authors for their important work. I look forward to seeing it in print!", "reviewer_3": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "RESPONSE TO REVIEWER 1\nReviewer #1 (Remarks to the Author):\nThis study investigates the multitrophic relationships linking tree species richness, functional diversity, tree temporal dynamics, herbivore functional diversity, herbivore temporal dynamics and herbivore stability. It is based on a multi-year dataset from a large forest tree species experiment in southeastern China (BEF-China). Over six years, lepidopteran caterpillars were sampled from a total of 52 experimental plots, covering a tree species gradient from one to 24 tree species. Based on lepidopteran data and repeated estimates of tree growth rate / volume, several indices describing functional diversity / temporal dynamics / stability were determined and related using a series of linear models and path analyses. To better understand underlying patterns, lepidopterans were also separated into food specialists and generalists and analyses were repeated for specialist and generalist subsets. Three hypotheses are tested: (1) temporal dynamics of herbivores are coupled to characteristics and dynamics of tree communities; (2) reduced tree diversity indirectly reduces stability of herbivore communities through reduced stability of tree growth; (3) at the herbivore trophic level, asynchrony drives stability of generalists, while population stability drives stability of specialists. The study finds that the temporal dynamics and stability of herbivores are strongly related to the characteristics and dynamics of tree communities. The relationships between herbivore stability and tree community characteristics and dynamics are mediated by herbivore phylogenetic diversity, asynchrony and population stability, with different relationships between generalist and specialist communities. Higher tree diversity was associated with higher herbivore stability, particularly for specialist herbivores, but this relationship was not mediated by tree growth stability. The authors conclude that lower tree species richness - a common outcome of global change - thus puts herbivore communities at risk.\nThe study addresses a highly relevant topic and interesting questions based on a robust and interesting dataset. The manuscript is well prepared, demonstrates the relevance of the topic by thoroughly embedding it in the scientific literature, and manages to present a complex topic in a relatively understandable way, making it interesting to the wide readership of Nature Communications. I do have some concerns, mainly related to the design and implementation of the statistical analyses, which I think should be addressed. Some may be based on gaps and ambiguities in the methods sections, which should be revised in some places. Also, although the manuscript is very well written, I would encourage the authors to streamline the discussion, which is currently quite long, e.g. by linking it more closely to the three study hypotheses.\nMain concerns:\n- The study examines a complex, multi-trophic system based on a set of indices describing the tree community (richness, functional diversity, temporal dynamics/stability) and the herbivore community (\"functional\" diversity, temporal dynamics/stability). However, it excludes a basic aspect of the herbivore community that I believe is crucial and could explain some of the reported relationships: species richness and abundance of herbivores. Since richness is a fundamental aspect of the study, it should not be reported at only one trophic level. In the presentation of the path analyses, I would suggest including these two parameters at the base of the herbivore community characteristics (adding a third level in Fig. 3). Cf. for example Blüthgen et al. ref. 19 in the present study.\nResponse: We agree that considering species richness and abundance of herbivores can help to put our results into a broader context. We therefore now consider herbivore abundance and richness in our models to test their potential roles. To avoid overly complex models (as strongly recommended by Reviewer 2 to streamline the models), we decided after careful consideration to replace herbivore MPD with herbivore abundance or herbivore species richness in alternative models, rather than trying to add these predictors on top of the current model structure. Especially since herbivore MPD and herbivore richness were strongly correlated (Pearson’s r = 0.73, p < 0.001) and also abundance was positively associated with herbivore MPD (r = 0.37, p = 0.006), we considered treating these metrics as largely alternative measures of herbivore diversity to be the most appropriate approach. This is confirmed by the results, which showed that the main pathways connecting host trees and herbivores as well as connections within the herbivore community remained relevant in all three alternative models, although the use of the different measures also revealed interesting new details, (e.g. in contrast to the findings for phylogentic diversity, direct effects of tree species richness on herbivore abundance and richness remained; Fig. S3, S4; Table S6-11). The corresponding details have been added to the Methods (L503-L505) and Results sections (L184-L188), and the implications of the comparison among the three alternative models are now discussed in the Discussion section (L243-L247).\n“To evaluate the role of herbivore abundance and richness in the bottom-up process, we substituted herbivore MPD with herbivore abundance and richness in separate alternative models.”\n“Replacing herbivore phylogenetic diversity by herbivore abundance or richness showed overall similar relationships, although direct effects of tree species richness on herbivore abundance and richness remained after accounting for effects of tree functional diversity, tree growth asynchrony and tree population stability (Figs S3, S4; Table S6-11).”\n“When herbivore MPD was replaced by herbivore abundance and richness (Fig. S3, S4), the driving role of tree species richness behind these effects became more evident, emphasizing the consequences that biodiversity loss at the host level can have for higher trophic-level diversity and, ultimately, stability.”\n2\n- The separation of generalist and specialist herbivores is a crucial step in this study. Here it is based on the dataset itself, i.e. species (MOTUs) observed to interact with only a few plants are assigned to the specialist taxa (based on a mean pairwise distance approach). I wonder how the overall abundance of a species in the dataset will affect its classification as generalist/specialist. I would expect the 'specialist' category to include not only specialist species, but also rare generalists (which could not be observed on many plants due to small sample sizes). Showing how total abundance and mean pairwise distance are related would be a first step in addressing this potential confounding factor. If the expected relationship is found, then the results should be reconsidered in this light. For example, I would expect herbivore asynchrony of rare species to be more closely related to herbivore richness stability than that of common species (in the latter case, there is a larger buffer for losing species when variability is higher). In general, an indication of how many individuals are included in each category (generalists, specialists) is missing and should be added. Since the overall dynamics are strongly related to the dynamics of generalists, I assume that generalists form the majority of communities (see also L164ff, L245).\nResponse: We agree that rare species can be difficult to evaluate with respect to their host specialization. To avoid potential effects of rare generalists, we therefore now excluded species with fewer than five individuals from our analyses and recalculated all stability and asynchrony metrics based on this more robust data set. Additionally, we examined the correlation between mean abundance and MPD of host specialization for all herbivores (r = -0.094) and specialists (r = -0.160), and both correlations were weak. In summary, now we are confident that the classification of herbivores into generalists and specialists is not significantly confounded by species abundance, and that effects of rare generalists are not strongly influencing our final results. We have rephased the methods to make readers aware of our approach (L387-L396).\n“Before classifying species as either specialists or generalists, we first excluded species with fewer than five individuals. This way, we aimed at eliminating the bias caused by rare species (i.e., that generalists with low abundances are erroneously considered as specialists). Subsequently, we applied hierarchical clustering alongside using two partitioning methods: K-means and partitioning around medoids (PAM)53. This method uses algorithms to group taxa into well-defined 'hard' clusters with minimal prior input, thereby enhancing the objectivity of the classification procedure. Clustering analyses were carried out using two clusters (k = 2, i.e., generalists and specialists). The threshold value was then used to separate specialists (MPD < 0.181, 103 species) from generalists (MPD > 0.181, 140 species).”\n- The dataset is collected within a tree species richness experiment, which includes many plots with only one tree species (24 out of 52). Two of the tree community indices (tree functional diversity, tree asynchrony) are by definition bounded at 0 for these 24 plots, resulting in a set of three highly correlated variables (tree species richness, tree functional diversity, tree asynchrony). To ensure that the observed relationships are not an artefact of these highly constrained variables, I suggest including sensitivity analyses excluding these single-species plots.\n“The exclusion of monocultures from our analyses expectedly led to weaker effects for some of the observed relationships (e.g. effects of tree species richness on tree growth population stability, relationship between herbivore abundance and richness stabilities; Fig. S8; Table S23-25). However, the main pathways connecting tree species richness with herbivore stability via tree functional diversity, tree growth asynchrony and subsequently herbivore asynchrony and population stability remained strong and significant (Fig. S87).”\nand (L287-L289): “Our observation that effects of tree population stability were reduced even more when removing monocultures from the analyses confirms this assumption.”\n- The dataset contains a number of strong outliers (see e.g. Fig. 2). I.e. plots with extremely low specialist herbivore asynchrony (A_N11, A_O27), extremely high specialist herbivore abundance stability (A_T15), extremely high specialist herbivore richness stability (B_V19) and very low tree population stability (B_M29). I suggest including sensitivity analyses excluding these plots.\nResponse: Thank you for pointing out these outliers, which helped us to identify potential inconsistencies in our data set. These outliers were apparently caused by the inclusion of extremely rare species. As we now excluded rare species and recalculated all metrics, the data of the above-mentioned plots no longer showed outlier values, obviating the need to conduct additional sensitivity analyses in this case.\nI have several additional comments that may help to improve the manuscript:\nL65f: I would restrict \"frequency and severity\" to natural disturbances and pest outbreaks. \"Frequency of biodiversity loss\" is somewhat unclear.\nResponse: We deleted “frequency” and kept “biodiversity loss”.\nL118: I suggest to quickly introduce the spatial spread of sampling sites here as well. Also, I suggest: \"... sampling seasons spread across six years ...\"\nResponse: We now added (L123-L125): “The experiment was conducted at two sites located 4 km apart, comprising randomly distributed study plots with a tree species richness gradient from monoculture to 24 mixtures.”\nL132: average population stability of herbivores, right?\n“, while rarer specialist stability depends more strongly on the average population stability of herbivores.”\nL143: I would add the link to Fig. 4\nResponse: Added as suggested.\nL143ff: Significant positive effects are only found for specialists. The effects for generalists/overall communities are never statistically significant (judged from Fig. 2). So either attribute all effects except for specialist effects as \"no effects\" or also mention that parts of the relations were negative (although not significant). cf. also L183, where the \"no effect\" is interpreted as a negative effect.\nResponse: As suggested, we now discuss all effects of tree richness on overall and generalist herbivores as “nonsignificant negative effects”, including the results from L184.\nL162-167:\n“By contrast, the effects of tree species richness on generalist and, subsequently, also on overall herbivore stability were altogether non-significant, because positive effects via tree growth asynchrony (as well as tree growth population stability) were weaker and counteracted by negative effects via tree functional diversity (Fig. 4) on herbivore population stability (Fig. 2b) and herbivore asynchrony (Fig. 2a).”\nand L208-L211:\n“…this altogether resulted in less stable abundance dynamics at higher tree species richness via tree functional diversity, even though the direct relationship between tree species richness and herbivore stability was not significant in our linear models.”\nL149/Figure 4: Could it be an idea to include some more details in Figure 4, i.e. to further distinguish indirect effects through tree FD and tree asynchrony? E.g. by including different fill colours.\nResponse: Thank you for this helpful suggestion. We now distinguished the indirect effects through tree FD, tree asynchrony and tree population stability, and updated Figure 4 with different colors.\nL176: This first sentence is a bit unclear. Herbivore diversity was not in the models (only phylogenetic diversity). And why not include the important link to the trees here as well?\nResponse: We clarified and rephrased the first sentence of the discussion, focusing more on the important link between herbivore dynamics and their host tree richness (as also suggested by Reviewer 2). It now reads (L199-201):\n“Our study unravels how the temporal stability of abundance and species richness of herbivore communities are linked to changes in the species richness of their host tree communities.”\nL217: I suggest to use a different wording than \"dietary niches\". Niches are a species trait, which is not affected by the offer of food plants. Maybe change to \"food spectrum\"?\nResponse: Changed accordingly.\nL225: The \"weaker\" effect has a larger effect size, though (Fig. 3). It is just the significance which is weaker. I suggest using the term \"clearer\".\nResponse: Thank you for your suggestion. We now changed it accordingly.\nResponse: We removed the paragraph according to your suggestion, which helped to streamline the discussion.\nL283ff: It should be made clear that the effect which is hard to explain is the direct effect. As made clear later (L294), the net effect is positive. I think this is crucial here and should be made clear much earlier in this discussion.\nResponse: We deleted this part of the discussion as in the updated results, this direct effect was not observed anymore. We took care to highlight the overall net effects clearly in our discussion.\nL292: I think the argument needs to be explained a bit more, as the mentioned link between abundance and richness is based on absolute numbers, not on stability.\nResponse: To streamline the discussion, we now have deleted the argument. Based on the suggestions of Reviewer 2, we focused the discussion more on the interlinkages between host trees and herbivores.\nL346: I would mention the 17 sampling events once more here.\nResponse: Thanks, we mentioned it again as suggested.\nL369: Here, the abbreviation MPD is used for a different concept than just a few lines earlier (L362). This confused me a lot, as I was relating to this second definition when trying to understand Fig. 3. The abbreviation MPD should only be used for one concept.\nResponse: Now we redefined the abbreviation MPD to “the weighted phylogenetic mean pairwise distance” and make it clear in the text what exactly it refers to (caterpillar communities or host tree use).\nL383: I would add the number of MOTUs that go into each of the categories.\nResponse: Now we added the numbers of MOTUs for generalists and specialists (see L142-L143; L394-L396).\n“140 were classified as generalist herbivores (accounting for 9063 individuals) and 103 as specialists (with 4943 individuals).\"\n7\n“The threshold value was then used to separate specialists (MPD < 0.181, 103 species) from generalists (MPD > 0.181, 140 species).”\nL392: Were these measurements done every year from 2016 to 2021? L408 suggests differently, this should be made clear here. If it is every second year, does it mean that three time points were included? (2016, 2018, 2020)\nResponse: Yes, these values were measured every year. The ‘two years’ mentioned in L408 is including previous year and the current year. We now rephased it to make it clear. See L418-L419.\n“, where V and V are the tree wood volumes at two consecutive years, t and t , within the sampling period from 2016 to 2021.”\nt1 t2 1 2\nL401f: Does this mean that individual variation was not included in plots with > 1 species? Unclear formulation.\nResponse: Of course, the individual variation of mixtures was also included. We have rephased the sentence to make this clear (see L413-414).\n\"…The absolute growth rates (TGR) of individual trees in a plot were calculated as…“\nL417: I think it is important to mention here that Rao's Q is 0 by definition if tree species richness is 1.\nResponse: Now we mention it here to make readers aware of it (see L427-428).\n“, weighted by tree wood volume, and expressed as Rao's Q (note that Rao's Q is 0 in monocultures).”\nL421ff: The temporal level at which these calculations were done is not clear. Was it at the level of sampling events? (n = 17) Or of years? (n = 6) L427 suggests that it is years. If so, it should be made clear from the start that data were pooled per year and plot.\nResponse: Thanks. Now we highlight this at the beginning of the paragraph, clarifying that all data were analyzed per year (i.e. pooling measures within years) (see L431-L433).\n“We calculated the temporal stability of herbivores based on the overall abundance and richness of herbivores at plot level and per year (i.e., multiple measures per year were pooled).”\nL432: How was species-level stability of trees determined? I could not find this detail. In the data file that was attached for review, I assume the column \"pop_tree\" to reflect tree population stability. It takes negative values, which I cannot understand based on the formula (3). Please clarify.\nResponse: Negative values by themselves are not a surprising result given that tree mortality – albeit rarely – can lead to negative growth. However, we reevaluated the way we calculate population stability following your comment and realized that using negative weights to calculate weighted average population stability is not meaningful and can greatly inflate the effects of negative growth. We therefore switched to using unweighted population stability for both trees and herbivores (see L449-L450), which also reduced the occurrence of negative values to only two plots.\n“The plot level-average population stability of tree growth and herbivores were calculated as the sum of all species’ temporal stability indices”\nL440: Again, please clarify at which temporal resolution this calculation was done.\nResponse: We rephrased the sentence to make it clear that all calculations were done on data aggregated per year, as we were interested in annual variability and because tree growth data were measured annually.\nL442: Why was tree growth asynchrony not determined at individual level, but at species level? From how I understand, this data should be available at individual level. It would have the advantage of decoupling tree species richness 1 from asynchrony 0.\nResponse: We really like the idea, but we finally decided against implementing it for a combination of reasons. First, our study builds on previous work that deconstructs community stability in its species asynchrony and population stability components (e.g., Schnabel et al. 2021 Sci Adv\nhttps://www.science.org/doi/full/10.1126/sciadv.abk1643; Thibaut & Connolly 2013 Ecol Lett 2013\nhttps://onlinelibrary.wiley.com/doi/full/10.1111/ele.12019). Moving away from this approach would reduce the comparability of our findings. Second, switching to tree-based analyses might be easily possible for trees, but would not be as straight forward for herbivores, where we would need to focus on herbivore abundances per tree, which can be rather low. This leads to a strong sensitivity to very small changes in abundance. Third, using tree-based analyses would introduce a spatial variability component, which would steer the study in a very different direction and deserves to be investigated properly. We would also loose the diversity aspect of the stability analyses, which was our main focus and seemed to make a lot of sense given that our experiment manipulates diversity specifically. Finally, we knew from our sensitivity analyses that monoculture plots did not drastically alter our findings (L193-L196), hence did not see a strong need to decouple richness 1 from asynchrony 0.\n“However, the main pathways connecting tree species richness with herbivore stability via tree functional diversity, tree growth asynchrony and, subsequently, herbivore asynchrony and population stability remained strong and significant (Fig. S8).”\nL446: I think it is important to mention here that tree growth asynchrony is 0 by definition if tree species richness is 1.\nResponse: Now we mention this accordingly to make readers aware (see L461-L463).\n9\n“The asynchrony ranges from 0 to 1, representing a gradient from complete synchrony to complete asynchrony (note that asynchrony value is 0 in monocultures).”\nL458: Why was the interaction site x mean MPD not included?\nResponse: We have now included the interaction between site and mean MPD in our linear models. The results indicate that this interaction does not significantly influence the overall outcomes in either model (V or V ). Specifically, only one model (overall abundance stability, Table S12 for V , Table S19 for V )\n1 2 1 2\nshows a significant effect of tree species richness, but this does not substantially alter the effects of other variables. Although tree richness is retained in the final model with a significant effect, in the other two models (Table S13, S15 for V ; Table S20, S22 for V ) where the interaction is included, it remains non-significant, and the effects of other variables remain unchanged.\nResponse: Changed accordingly.\nL471: Are the log transformations introduced above also included in the path analyses? If yes, how are they accounted for when aggregating path coefficients (i.e., Fig. 4)?\nResponse: Yes, we also included them in the path analyses. The path coefficients were also based on the models with log-transformed data. Now we highlight this information in the figure captions to make readers aware of this.\nL489: Can you specify how the non-significant pathways that were excluded were selected?\nResponse: Following the suggestions of Reviewer 2, we have changed our modeling procedure and now do not select pathways within individual models, but rather compare multiple alternative model variants in which certain links have been dropped based on pre-defined assumptions (see L495-L506).\n“We constructed our initial model according to the current knowledge on mechanism driving biodiversity-stability relationships for plants, insects and ecosystem functions11,25,39. We assumed that tree species richness influences tree functional diversity, tree asynchrony and tree population stability (Fig. S3). Moreover, we assumed that all tree-based predictors can directly affect herbivore metrics, including herbivore MPD/abundance/richness, herbivore asynchrony, herbivore population stability and herbivore stability (Fig. S3). For both trees and herbivores, we hypothesized that functional diversity/MPD can directly affect asynchrony and population stability. To evaluate the role of herbivore abundance and richness in the bottom-up process, we substituted herbivore MPD with herbivore abundance and richness in separate alternative models. We considered covariances between asynchrony and population stability in all models.”\nResponse: Thank you for spotting these inconsistencies. We have adapted y axes of all panels of Figure 2 as suggested. For the linear models, we firstly considered multiple predictors. To directly test the relationships between tree species richness and abundance and richness stability of herbivores (separately for all herbivores, generalists and specialists), we then used single linear models, which are only including one predictor (i.e., tree species richness). We have further clarified it in the methods (see L478-L481).\n“In addition, we used single linear models to directly test the relationships between tree species richness, abundance and richness stability of herbivores (separately for all herbivores, generalists and specialists).”\nL718: The definition of MPD is missing from the legend. Also, what the numbers in the yellow circles are showing is not mentioned (variance explained?).\nResponse: We now provide the definition of MPD and the meaning of the yellow circles (i.e., explained variance) in the figure legend.\nSI L20 / Fig. S2: is 870 the number of MOTUs that went into the analyses? If yes, why is the number different from the 885 MOTUs introduced at L136?\nResponse: We now optimized the classification of generalists and specialists, so the original figure is not relevant anymore.\nSI L54 / Table S4: In the overall models, the second predictor variable should be 'herbivore asynchrony', not 'tree asynchrony'. Please check model output tables again, I did not do a systematic check.\nResponse: Thank you. We corrected it and checked the full text to avoid the similar errors.\nRESPONSE TO REVIEWER 2\nReviewer #2 (Remarks to the Author):\nI enjoyed reading the manuscript “Asynchrony and functional diversity couple herbivore and host community dynamics”. The authors have tackled a vitally important topic—how community patterns and stability at one trophic level contribute to stability at a higher trophic level—and brought to bear an\n11\nincredibly impressive dataset from a very large experiment. Moreover, the work is novel, and the study is solidly conducted. I have just three suggestions, and they are focused on improving the framing and analyses. I’d like to thank the authors again for an important contribution to the literature. My suggestions:\nFirst, the study seeks to address the novel and important question of how one trophic level (trees) influence the stability of a higher trophic level (lep herbivores), but much of the text and analyses are focused on how different metrics and aspects of stability relate to each other within just the herbivore community. While it is interesting to explore the relationships between different metrics of herbivore stability at different scales, it felt both a little circular and tangential to the stated goal of examining how tree community dynamics influence herbivore community stability. It felt circular because, unless I’m missing something, it has to be true that the stability of an herbivore community will be determined by the stability and asynchrony of its constituent populations. I suggest either (1) refocusing the manuscript around the effect of the plant community on herbivore stability (and reducing text on the links between different metrics of herbivore stability) or (2) making exploring relationships between different herbivore metrics an explicit secondary goal of the manuscript. The first seems preferrable to me, but either could work.\nResponse: Thank you very much for your thoughtful review and constructive feedback. We are pleased that you recognize the contribution of our study to the field. In response to your suggestion, we have revised the manuscript to place greater emphasis on the effects of the host tree communities on herbivore stability, aligning more closely with the primary goal of the study. For instance, we have added and clarified text in sections (e.g., L199-L203) to highlight these relationships. Additionally, we have reduced the focus on the links between different metrics of herbivore stability (e.g. by reconsidering the panels shown in Fig. 2 and reducing the discussing on the topic in L279-L299), but did not completely remove the exploration of within-herbivore community metrics, as we consider this part an important contribution to shedding light on the “black box” of trophic interactions. Specifically, it allows us to show the pathways via which host tree effects ultimately affect herbivore stability. We believe the changes make the manuscript more cohesive and aligned with the stated objectives.\nSecond, the manuscript is framed around examining the effects of how plant community dynamics influence herbivore community stability, but the authors have relatively weak data on community dynamics. Their data are change in tree biomass over only 6 years. Most of this change seems to be tree growth from saplings to larger trees rather than turnover, which is what most ecologists would consider to be community dynamics. It seems extremely unrealistic to calculate tree population stability or tree asynchrony over just six years. Really what the authors are calculating is variation in tree growth rates, I think, though it’s a bit hard to tell from the methods. This could be very relevant for insect herbivores because insect herbivores often perform best when plants grow rapidly, so there is something interesting and important going on here. I just think the authors need to tone down claims of linking tree community dynamics to herbivore community patterns. Instead, the authors measured individual growth patterns and variation in growth patterns. Framing things in terms of community dynamics also obscures the most powerful aspect of the study: the tree species richness treatments. This was a randomized species richness experiment, giving the authors have incredible power to link tree richness to herbivore community patterns and stability. Much of that, however, is lost in the analyses and discussion of tree community dynamics (differences which are downstream from their tree species richness treatments). I suggest focusing the framing and analyses and\n12\ndiscussion around the experimental treatment of tree species richness and toning down claims of linking tree community dynamics to herbivore patterns.\nResponse: Thank you for your insightful suggestions. We have addressed your concerns, toning down the claims regarding community dynamics of host trees, and instead focusing more on the (indirect) linkages between tree species richness and herbivore stability. Specifically, 1) We revised the manuscript title to emphasize the importance of tree species richness as the central focus of the study. 2) We rewrote the discussion of tree community metrics and their influence on herbivore patterns, instead highlighting tree species richness as the primary driver and taking care throughout the manuscript that our results are not mistaken for long-term, decadal plant-herbivore community dynamics (e.g. see L156, L229-L261). 3) We clarified throughout the manuscript that our data primarily reflect variation in tree growth rates rather than longer-term community dynamics (e.g. changed tree dynamics to tree growth variability) in the traditional sense, ensuring that our framing aligns with the scope and nature of the data. These revisions help to better align the manuscript with its strongest aspects, particularly the experimental design and the role of tree species richness in driving herbivore community patterns and stability. We believe these changes enhance the clarity and focus of the study. We kept the terminology of asynchrony and population stability for trees, however, because these are standard terms easily understandable to readers familiar with temporal community analyses and because these terms have also been used in similar temporal context in other studies (e.g. Schnabel et al. 2021 Sci Adv\nhttps://www.science.org/doi/full/10.1126/sciadv.abk1643), ensuring that our results can be understood in the context of related studies.\nThird, the authors use stepwise model selection, but major problems with stepwise model selection have been known for a long time. For example, see Whittingham et al. 2006. Why do we still use stepwise modelling in ecology and behaviour? Journal of Animal Ecology. A better approach would be to use AIC to compare a set of candidate models only once. Of course, this may be unwieldy given how many potential models the authors have. This, however, relates to my last two comments. Following my first comment would reduce the number of predictors related to herbivore community metrics. Following my second comment would reduce the number of plant community dynamic predictors. This would simplify the number of potential models down to a more reasonable number of models to compare. Or just fit the full model and focus on the estimated effect sizes.\nResponse: Thank you for highlighting the issues with stepwise model selection and providing a constructive alternative. We have revised our model selection procedure in line with your suggestions. Specifically, we proposed four candidate model structures based on our hypotheses and prior knowledge. We then selected the best model structure by comparing model fit statistics (e.g., AIC, see Table S1, S2).\nL509-L525:\n“Moreover, to assess the level of support for different ways in which host tree-based metrics influence herbivore community dynamics, we compared four alternative model structures that successively removed interactions between different levels of host tree and herbivore community data (Fig. S1): First, we considered the initial model with all potential links included (Model 1). Second, we assumed that effects are entirely mediated by stability components, except for effects such as microclimate buffering that may remain in tree species richness. Therefore, we removed the direct connections between trees and herbivore\n13\nabundance and richness stability, except for effects of tree species richness. Third, we assumed that there are no such effects of tree species richness, so we additionally excluded the direct pathways between tree species richness and herbivore stability metrics. Fourth, we assumed that tree species richness only has indirect effects via tree stability components and functional composition. We hence removed the links between tree species richness and herbivore MPD/abundance/richness and population stability. Model fit statistics were summarized (Table S1, S2). The best-fitting model was selected based on the lowest AIC value62. We used bootstrapped P values based on 1000 bootstrap draws to ensure robust results63.\n.”\nFinally, the manuscript was well written but a bit hard to read because of the frequent mention of many subtly diversity/stability terms for different scales: abundance stability, richness stability, population stability, and more. Moreover, there are many different facets of stability for any one quantity, and different metrics for each facet. Through the introduction, results, and discussion, it is a little unclear what specifically the authors mean by stability. I suggest simplifying all this, reducing jargon, focusing on the most important aspects of stability for this study, and precisely defining each term early in the manuscript.\nResponse: Thank you for this valuable suggestion. We agree that reducing jargon will make it easier for readers to understand the core message of our study. In response, we have streamlined the terminology throughout the manuscript, focusing on the most important aspects of stability relevant to this study and reducing the use of overly technical terms. For instance, temporal dynamics, among other adjustments. Additionally, we have provided precise definitions of key terms, such as abundance stability, richness stability, and population stability, early in the manuscript to ensure clarity and consistency, so that readers can better follow the necessary distinctions we make among different components of the host tree and herbivore community data (e.g. L82-L83, L95-L96). We hope that the revised text strikes a balance between avoiding overly complex jargon and nevertheless providing well-defined definitions of necessary key terms. We believe these changes will significantly improve the readability and accessibility of our work.\nMinor comments:\nGiven the authors’ goal of examining stability/variability in trophic interactions and herbivory, two it might be helpful to refer to two recent papers that examine variability in plant-herbivore interactions: Herbivory Variability Network et al. 2023. Science.; Wetzel et al. 2023. Annual Review of Ecology, Evolution, and Systematics.\nResponse: Thanks for your suggestion. We now included the suggested papers in our manuscript (see L82-84).\n“The temporal stability (i.e. invariability in time at community and population level) of herbivore abundance can substantially alter ecosystems via herbivory17,18.”\nThe authors use mu / sigma as a metric for stability (inverse of CV). This adjusts variability by the mean, but I worry that rare species (with low abundances) will have unrealistically low stability (high variability). This metric will lead to very low values for species that are typically rare because their mean is so low, and it might have nothing to do with stability per se. It might be helpful to also analyze the data with sigma as the measure of stability to see how that changes results and interpretation. Alternatively, a recent extension of the CV addresses some of these issues and generally makes for a more robust metric: Kvalseth 2016 Journal of Applied Statistics.\nResponse: We have now additionally checked the results based on the metric proposed by Kvalseth (2016) in our analyses and found that the results are largely consistent with those based on the inverse of CV. This is probably due to the fact that we now excluded rare species (n < 5) from our dataset, thereby minimizing the potential influence of low-abundance species on stability metrics. This approach strengthens the robustness of our conclusions by ensuring that our results are not disproportionately driven by species with inherently low stability due to low abundance. Thank you for suggesting this improvement, which has enhanced the reliability of our findings. See L443-L447 in methods, L186-L188 in results, and Fig. S5-7, Table S16-22.\n“Moreover, to further eliminate the potential bias caused by rare species (e.g., unrealistically low stability), we also utilized an alternative stability metric from Kvålseth et al.58\n√𝛔𝟐+𝛍𝟐\nstability (V ) = ”\n2 𝛔𝟐\n“In addition, our findings remained highly consistent when an alternative stability metric (V ) was used in\n2\nour models (Fig. S2d-f, S5-7; Table S16-22).\nI want to end by thanking the authors for their important work. I look forward to seeing it in print!\nResponse: Thanks again for your constructive comments to help us to further improve the manuscript!\nRESPONSE TO REVIEWER 3\nReviewer #3 (Remarks to the Author):\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.\nResponse: Thank you for supporting the review process. We have thoroughly considered all comments, which greatly enhanced the clarity and robustness of our manuscript.\nAsynchrony and functional diversity couple herbivore community\ndynamics to host plant diversity\nMing-Qiang Wang, Georg Albert, Carlo L. Seifert, Douglas Chesters, Helge Bruelheide, Jing-Ting Chen,\nAndréa Davrinche, Sylvia Haider, Shan Li, Yi Li, Goddert von Oheimb, Tobias Proß, Keping Ma,\nXiaojuan Liu, Arong Luo, Andreas Schuldt*, Chao-Dong Zhu*\nRESPONSE TO REVIEWER 1\nReviewer #1 (Remarks to the Author):\nThank you for this revised version, which I find to have clearly improved and has taken into account all major\ncomments made by the reviewers on the earlier version of this manuscript.\nI have few somewhat larger (but not major) points and a set of minor points that could still be addressed.\nResponse: We sincerely thank the reviewer for the additional suggestions, which helped us to further\nimprove the clarity and quality of our work. Below, we address each of the points in detail.\nLarger points:\n- Based on comments of both reviewers on the previous version, additional analyses were necessary\n(herbivore richness, herbivore abundance, version 1 and 2 of stability). Thank you for including these!\nClearly, this did not help to reduce the complexity of the manuscript. While I fully support that these\nadditional analyses were made and I think they contribute to the robustness of the results, I would only\nmention these additional analyses in the main text where really necessary and otherwise relate to them in\nthe methods / supplementary materials. E.g. L184ff, L243ff could be moved to increase clarity.\nResponse: We agree that streamlining the presentation of the additional analyses improves clarity.\nFollowing your suggestion, we have now kept references to these analyses in the main text only where\nthey are essential for interpreting the results (see L155-L200). Especially, in all other cases, we moved the\nmethods and results of the sensitivity analyses, as well as the corresponding figures and tables, into the\nsupplementary information to keep the clarity of the main text.\n- It is great that very rare species are excluded for the definition of generalists and specialists, which goes\nback to a comment I made on the first version. What is however not clear to me now is why all these species\nwere also excluded from the overall analyses. If I see it correctly (L141ff), due to this criterion of 5+\nindividuals per species, 3844 individuals were excluded from the analyses (more than 20%), which is a lot.\nCould it be an option to keep these additional individuals in the overall analyses? Or at least lower the\nthreshold of 5? I feel that simply excluding rare species, which apparently make up for a large part of the\ncommunity, could strongly bias the results.\nResponse: Thank you for raising this important point and for revisiting your earlier comment. Following\nthe previous reviewer comments, we applied the ≥5 individuals threshold across all analyses to ensure\ncomparability among generalist, specialist, and overall herbivore datasets, and to reduce the influence of\npotentially vagrant individuals. However, we agree that it is important to show whether and how the\n1\nexclusion of rare species from the overall analyses may influence the results. Therefore, we have now re-\nrun the overall analyses including all individuals (i.e., without the ≥5 threshold) as an additional sensitivity\nanalysis and present the results in the Supplementary Materials (new Fig. S7 and Tables S23-24). The main\nconclusions remain qualitatively unchanged (e.g. negative effects of tree FD on herbivore stability via\nherbivore asynchrony and positive effects of tree species richness on herbivore stability through tree\nasynchrony and herbivore MPD), suggesting that the overall patterns are robust to the exclusion of\nextremely rare species (see L187-L191).\nFig. S7 Sensitivity analysis for effects of tree diversity on herbivore community stability via bottom-up\nregulation including all herbivore individuals. Potential effects of tree species richness, tree functional\ndiversity (FD), species asynchrony and population stability on community stability of herbivore abundance\nand richness through mean herbivore abundance weighted phylogenetic diversity (MPD), species\nasynchrony and population stability for a overall herbivores using stability based on the inverse of the\ncoefficient of variation (χ2 =5.65 , DF = 11, P =0.890 ), b overall herbivores using stability following Kvålseth\net al. (χ2 =11.19 , DF = 11, P =0.468) based on path model results (see Tables S23-34 for full results). Blue\narrow indicate positive effects, red arrows show negative effects (p ≤ 0.1), grey arrows show non-significant\npathways (p > 0.1). Arrow width was scaled by the standardized path coefficients. The proportion of variance\n(R2) are shown in yellow circles. Note that tree species richness, population stability, abundance stability and\nrichness stability of herbivores were log-transformed. Significance levels: p < 0.1 (.), p < 0.05 (*), p < 0.01\n(**), p < 0.001 (***).\n- I really like the additional information that is now available from Figure 4. One effect that gets really clear\nnow is that higher tree FD is negative for herbivore stability. Although this fact is touched in the discussion\n(L247ff, L255ff), I think it should be discussed in some more depth. Few more word on the explanation and\nalso on how it relates to other studies would be necessary. And, going in a similar direction, the net effect of\ntree diversity on herbivore stability is only clearly positive for specialists (Fig. 4). I feel the net negative\nrelation for overall herbivore stability could be acknowledged more. Statements such as L265ff or 273ff do\nnot reflect the results, neither do some conclusions (e.g. L308ff). I think such statements and conclusions\nshould be toned down throughout.\nResponse: Thank you for highlighting these important points. We agree that the negative relationship\n2\nbetween tree functional diversity and herbivore stability, as well as the net negative relation for overall\nherbivore stability, deserves more explicit discussion. We have now expanded the relevant discussion\nsections (see L287-L297) to provide a more in-depth interpretation of the negative tree FD effect, including\npotential ecological mechanisms and how our findings compare with previous studies:\n“…Therefore, the consistent negative effects of tree functional diversity on both overall and generalist\nherbivore stability suggest that increased functional differentiation among trees creates greater\nheterogeneity in resource traits, which in turn destabilizes herbivore populations when they depend on\nspecific host functional types. Similar findings have been reported in other multitrophic studies, where high\nfunctional diversity of basal resources increased variability in consumer communities13,15. Importantly, while\ntree species richness had a stabilizing effect on specialist herbivores, the net effect of tree diversity on overall\nherbivore stability was weak or even negative, highlighting that positive biodiversity–stability relationships\nmay not uniformly extend across all consumer guilds.…”\nWe also explicitly acknowledge in the revised text that the net effect of tree diversity on herbivore\nstability is only clearly positive for specialists, while it is neutral to negative for overall herbivores.\nCorrespondingly, we have toned down and reworded statements in the Results, Discussion, and\nConclusions (see L293-L297) to more accurately reflect the patterns shown in Fig. 4.\nMinor points:\nL55: I suggest “… tree functional diversity, tree growth asynchrony, and tree growth population stability.”\nResponse: Changed accordingly.\nL84: I suggest “… ecosystems via stability of herbivory.”\nResponse: Changed as suggested.\nL88: I suggest: “Herbivore temporal stability is often low in monocultures …”. Generally, sticking to either\n“stability” or “variability” throughout would increase clarity.\nResponse: Changed as suggested, we now used “stability” throughout the full text.\nL94: Here it would be good to introduce the concept of “growth variability/stability”, which is in other places\nused to summarise growth asynchrony and growth population stability.\nResponse: Thank you for the helpful suggestion. We have now clarified the concept of “growth\nvariability/stability” in the revised sentence, highlighting its role in summarising both growth asynchrony\nand population-level growth stability (see L99-L102):\n“For tree communities, the temporal stability in growth and productivity has recently received increasing\nattention14,24. In general, community stability can be partitioned into species asynchrony, i.e. a temporal\nmisalignment of species dynamics, and the stability of species’ populations themselves14”\nL97: I would replace “species-level” by “population” to keep the wording constant\nResponse: Changed as suggested.\nstick to the same wording throughout, this really helps the readers to follow the text.\nResponse: We now used “stability” in the full text for clarity.\nL127: herbivore communities (i.e. herbivore phylogenetic diversity) is missing from the sentence.\nResponse: We now provided relevant information to make reader to follow the text (see L136-L140):\n“Specifically, we used linear models and path analysis to test the pathways that directly and indirectly (via\ntree and herbivore community asynchrony and population stability, and via herbivore phylogenetic diversity)\nconnect tree species richness and its effects on tree growth to the temporal stability of herbivore abundance\nand species richness”\nL128: I feel that Fig. S1 is quite important and really helps to understand the study when being confronted\nwith it for the first time. I would propose to include it as a main figure. Also, in Fig. S1, I think it is a bit\nunfortunate that the arrow with the label (1) refers to Model 2, the arrow with the label (2) to Model 3, … It\nwould increase clarity when numbers match.\nResponse: We appreciate this suggestion. We have now integrated the information from former Fig. S1\ninto Fig. 1 of the main text. Moreover, we also reduced the numbering of arrows to improve clarity.\nherbivore community stability. Herbivore abundance asynchrony among species and community stability\nare coupled with the diversity and temporal growth rate stability of their host tree communities (Hypothesis\n1), with herbivore community stability ultimately a destabilized by low tree species richness and b stabilized\nby high tree species richness (Hypothesis 2). Specifically, in a monocultures and species-poor mixtures, the\npronounced fluctuations in tree growth may lead to reduced asynchrony among herbivore species, thereby\ndestabilizing the herbivore community. Conversely, in b more diverse mixtures, complementary dynamics\nresulting from the asynchrony among tree species could contribute to the stabilization of plant communities.\nThis stabilization effect extends to herbivore communities, promoting greater asynchrony among herbivore\nspecies and ultimately enhancing the stability of the herbivore community (Hypothesis 3). These\nrelationships were investigated with c path models. Structure based on theoretical expectations and\ncorrelations among herbivore- and tree-based variables: mean phylogenetic distance (MPD), species\nasynchrony, population stability of herbivores; tree functional diversity (FD), species asynchrony, population\nstability of trees. Arrows indicate expected causal relationships. Blue lines are covariances retained in the\npath models. We assessed with four alternative models how direct vs. indirect effects of host tree-based\nmetrics influence herbivore community dynamics. Model 1: both direct and indirect pathways from trees to\nherbivore stability; Model 2: restricting tree growth effects to indirect pathways via herbivore population\nstability and asynchrony, except for tree species richness; Model 3: assuming even species richness acts only\nindirectly; Model 4: assuming that tree diversity influences herbivores solely through effects on tree\nfunctional diversity, asynchrony, and population stability. Model 4 was selected for our final analyses.\nL149: MPD is mentioned for the first time, the abbreviation should be introduced. To increase clarity, it\ncould help to stick to “herbivore phylogenetic diversity”. This term it is used in other places and would also\nhelp to reduce confusion between herbivore MPD and host MPD.\nResponse: Thank you for pointing this out. We have revised the sentence to introduce the abbreviation\n“MPD” upon its first mention. Moreover, we now used “herbivore phylogenetic diversity” to reduce\nconfusion between herbivore MPD and host MPD.\nL150: What I do not understand about model selection is how you proceeded with the submodels\n(generalists, specialists). Do these always follow the main model?\nResponse: Yes. We have clarified that the model structure selected for overall herbivores was also applied\nto generalists and specialists to ensure comparability and interpretability across groups (see L562-564):\n“The same model structure selected for overall herbivores was applied to generalists and specialists to ensure\ncomparability across groups.”\nL150: When reading the text for the first time, it might be very unclear what herbivore abundance/richness\nare here all about. As suggested before, I would focus on these alternative models solely or mainly in the\nmethods section.\nResponse: Thanks for the suggestion. We have streamlined the content in the main text and refer readers\nto the Methods section for details on our alternative models that we used as sensitivity analyses (See\nSupplementary L26-L33):\n“To evaluate the role of herbivore abundance and richness in the bottom-up process, we substituted\nherbivore MPD with herbivore abundance or richness in separate alternative models. While we assumed that\n5\nherbivore MPD best captures diversity effects because it combines abundance-weighted differences in not\nonly species richness, but phylogenetic (and thereby functional) differentiation among species2, models using\nherbivore abundance or richness may be helpful in evaluating the contribution of overall herbivore density\nand the mere number of species, respectively.”\nL156: I would remove “short-term”. Again, this is new terminology that was not used before.\nResponse: Now it was removed as suggested.\nL167: I suggest “… tree functional diversity was negatively related to herbivore asynchrony”, giving the\ndirection of the relation helps.\nResponse: Changed as suggested.\nL241ff: This sentence is somewhat circular with phylogenetic distance being mentioned twice.\nResponse: Thank you for pointing this out. We have revised the sentence to remove the circular phrasing\nand to clarify the proposed mechanism linking temporal resource variability to herbivore phylogenetic\ndiversity (see L259-L263):\n“Tree growth asynchrony promoted herbivore phylogenetic diversity of both generalists and specialists,\nsuggesting that temporal stability in the availability of food resources might provide more niche\nopportunities that favor the coexistence of more distantly related herbivore species and, in consequence,\nfunctionally more diversified consumer communities with more distant phylogenetic relationships37”\nL267: H2 explicitly mentions tree growth as a mediator, but it is not touched here.\nResponse: We now have adapted H2 and discussion to make this clearer (see L148-L149; L301-L304):\n“… the loss of tree diversity indirectly (via tree growth) destabilizes herbivore communities (Fig. 1).”\n“The resulting overall direct and indirect effects via tree growth of tree species richness on specialist\nherbivore community stability were clearly positive, partly supporting our expectation (H2) and underlining\nthe crucial role that tree species richness may play in preventing outbreaks of often specialized insect pests44”\nL276: I suggest “…temporal dynamics of higher trophic levels…”\nResponse: Changed as suggested.\nL283ff: Can you elaborate more on this explanation? I have difficulties to follow, sorry.\nResponse: Thank you for your comment. We have now clarified this explanation to better communicate\nthe underlying mechanism. Specifically, we explain that in mixed-species plots, community-level\nabundance stability is primarily driven by asynchronous fluctuations among host-specific herbivores,\nrather than by the stability of individual populations. This interpretation is consistent with our results,\nwhere asynchrony was the main driver of stability for both generalists and specialists, despite our\nexpectation (H ) that population stability would play a stronger role in the latter (see L319-327):\n3\n6\n“This deviation may be explained by the fact that many of our study plots were mixtures of different tree\nspecies, which hosted distinct herbivore assemblages. In such mixed-species plots, mean plot-level herbivore\nabundance (calculated across all herbivore species) does not necessarily rely on stable populations of each\nspecies. Instead, it can be maintained when fluctuations in abundance are asynchronous among species—\neven among specialists—because multiple host tree species support different herbivore populations. These\ncompensatory dynamics buffer total herbivore abundance over time, reducing the apparent influence of\npopulation stability on plot-level stability25”\nL362: The abbreviation was just introduced before, so no need to define it again\nResponse: Done.\nL363/387: The exclusion of species with <5 individuals is mentioned twice.\nResponse: Thank you for pointing this out. We have removed the redundant sentence and now mention\nthe exclusion of species with <5 individuals only once (see L432-433):\n“Before classifying species as either specialists or generalists, we first excluded species with fewer than five\nindividuals.”\nL370f: I would mention at which level weighted MPD were calculated to make clear what this was later used\nfor.\nResponse: We now specified the level at which we calculated weighted MPD to promote clarity (see L414-416):\n“We then calculated abundance weighted mean pairwise phylogenetic distance (MPD) for sets of the\nphylogenetically-placed herbivore MOTUs at community level.”\nL380, 381, etc: I would clearly write “host MPD” or “herbivore MPD” in all instances throughout the\nmanuscript. This would really help to reduce confusion, although it got already much clearer compared to\nthe first version of the manuscript.\nResponse: Thank you for highlighting this important point. To improve clarity and consistency, we have\nrevised the manuscript to explicitly distinguish between “host MPD” and “herbivore MPD” in all relevant\ninstances.\nL390: I think here it is missing that this clustering was based on host MPD.\nResponse: Thanks. We have clarified this clustering was based on host MPD.\nL413ff: To be consistent with equ. 3, I suggest to use TGR, V and V .\ni i,t1 i,t2\nResponse: Changed for consistence.\nL419: Here it is missing what was done with TGR in the end.\nResponse: We provided an additional sentence to explain the further calculations based on TGR (see L464-465):\n“TGR was used for the calculation of tree asynchrony and population stability.”\ni\n1 2\nV was also used for the wood volume.\ni\nResponse: Thank you for this helpful comment. We agree that the overlapping abbreviations could cause\nconfusion. We now call it stability (the inverse of the coefficient of variation) in the main text, and refer to\nstability following Kvålseth et al. for the alternative measure in supplementary materials.\nL450: I suggest “… (applying formula 3 or 4).”\nResponse: Changed as suggested.\nL491/500: I would remove abundance and richness here, at is only explained later.\nResponse: We have removed abundance and richness here for clarity.\nL518/520/523: Add the model terms also for Model 2, Model 3 and Model 4 (equivalent to L514).\nResponse: Added as suggested.\nFig. 2: It is still not fully clear to me how these pairwise comparisons that are shown were chosen. Why, for\nexample, it the relation between Herbivore richness stability and Tree species richness missing? Also, how\ncome there is a non-linear relation in Fig. 2d given that these are all linear models (L479ff).\nResponse: For the selection of pairwise comparisons, we aimed to highlight those relationships that also\nemerged as important pathways in the SEM and/or were particularly important for the discussion of the\nresults. We did not add the relationship between tree species richness and herbivore richness stability\nsince the latter was strongly related to herbivore abundance stability and the relationships therefore\nsimilar to the ones in panel c. We have now corrected Fig. 2d to display the appropriate linear relationship\nconsistent with our linear models (and have also updated the corresponding supplementary figure).\nResponse: changed accordingly.\nL795: Explanation of the significance level abbreviations missing from the legend.\nResponse: Thank you for the comment. We have now added an explanation of the significance level\nabbreviations (see L846-L847):\n“Significance levels: p < 0.1 (.), p < 0.05 (*), p < 0.01 (**), p < 0.001 (***).”\nL805: I guess the effect of herbivore abundance stability on herbivore richness stability was also included?\nIt is missing from the list of variables.\nResponse: Yes. We now listed herbivore abundance stability as variables for herbivore richness stability.\nL810: Did the calculations also include non-significant relationships? I think they should. I am just asking\nbecause effects via tree population stability are missing from the specialist effect sizes (but they might\nsimply be very small and thus not visible).\nResponse: We did not include non-significant pathways in the original calculation, as most of these\npathways showed very high uncertainty (large standard errors). Displaying them in the main figure alone\ncould therefore have been misleading for the interpretation of the overall effects. To ensure transparency,\nwe now provide summaries of both approaches: (i) effect sizes based only on significant pathways (main\ntext), and (ii) effect sizes including non-significant pathways (Supplementary Fig. S9). To better capture the\n9\nuncertainty of non-significant path ways, we decided to bootstrap these effects. The corresponding details\nhave been added to the Methods (see L566-569) and Supplementary Results (see L60-63):\nIn Methods:\n“The effect sizes of tree species richness on herbivore richness stability were then summarized. We\nadditionally calculated effect sizes including non-significant pathways and assessed their variability using\n1,000 bootstrap draws.”\nIn Supplementary Results:\n“When non-significant pathways were included, effects of tree asynchrony on all herbivores and of tree\npopulation stability on generalists were more prominent, but characterized by high uncertainty (Fig. S9X).”\nFig. S9 Bootstrapped effects of tree species richness on herbivore richness stability including all pathways\n(significant and non-significant). Bars show summed effects of tree species richness on the abundance and\nrichness stability of all, generalist and specialist herbivores, respectively. Error bars indicate 95% bootstrap\nconfidence intervals. Effect sizes were calculated by summing indirect effects of tree species richness via tree\nfunctional diversity, tree asynchrony, tree population stability, herbivore phylogenetic diversity, herbivore\nasynchrony, herbivore population stability, and herbivore abundance stability. The different colors show\neffects of tree species on herbivore stability via tree functional diversity, tree asynchrony, and tree\npopulation stability, respectively. Effect sizes were calculated as the product of standardized path coefficients connecting\neach predictor with herbivore components, summed over the individual predictors of each component for\npositive and negative effects on herbivore stability metrics, respectively. Note that tree species richness,\npopulation stability, abundance stability and richness stability of herbivores were log-transformed. Stability\nmeasures are based on the inverse of the coefficient of variation (eqn. 3).\nL811: Just to be sure, log-transformation was done for both predictor and response variables, right?\nthe Methods (see L524-L525).\nFig. 4: I like the inclusion of the different pathways. What is now somewhat missing is the overall effect,\nwhich was represented in the old version. It could still be included. See Figure 3b in Neff et al. (2021)\n‘Changes in Plant-Herbivore Network Structure and Robustness along Land-Use Intensity Gradients in\nGrasslands and Forests’. Science Advances. https://doi.org/10.1126/sciadv.abf3985 for a possible example.\nResponse: We now have included again the different pathways and overall effect in the figure.\nFig. 4 Effects of tree species richness on herbivore richness stability. Bars show summed effects of tree\nspecies richness on the abundance and richness stability of all, generalist and specialist herbivores,\nrespectively. Effect sizes were calculated by summing indirect effects of tree species richness via tree\nfunctional diversity, tree asynchrony, tree population stability, herbivore phylogenetic diversity, herbivore\nasynchrony, herbivore population stability, and herbivore abundance stability. The different colors show\neffects of tree species on herbivore stability via tree functional diversity, tree asynchrony, and tree\npopulation stability, respectively. Effect sizes were calculated as the product of standardized path\ncoefficients connecting each predictor with herbivore components, summed over the individual predictors\nof each component for positive and negative effects on herbivore stability metrics, respectively. Black T-\nshaped lines indicate the total effects of tree species richness on herbivore stability metrics. Note that tree\nspecies richness, population stability, abundance stability and richness stability of herbivores were log-\ntransformed. Stability measures are based on the inverse of the coefficient of variation (eqn. 3). See\nSupplementary Results and Fig. S9 for effects including non-significant pathways.\nReviewer #4 (Remarks to the Author):\nOverall statement:\nThis manuscript reports interesting results that connect tree diversity and herbivore community dynamics; it\nis novel in the intra- and inter-community dynamics across trophic levels. Their results show bottom-up\ndynamics (loss of tree biodiversity) as important destabilizing factors for specialist herbivores, with\nimplications for global change and biodiversity conservation. The authors make a nice contribution to the\nlong-term and active biodiversity-stability theory debate in ecology. I applaud the substantial effort of the\nteam in this high-investment research (temporally and in caterpillar replication and identification). The paper\nis quite dense and could use quite a bit of editing to clearly demonstrate how the results contribute to large\necological concepts, rather than get lost in the details of the path analyses; in part, this is due to the authors’\nchoice to have overall linear models as well as path analysis to understand the patterns (which ends up\nbeing difficult to bring together into a cohesive story). A few constructive criticisms:\nResponse: Thank you for the constructive suggestions regarding the clarity and cohesion of our narrative.\nIn the revised manuscript, we have streamlined the presentation of the results by reducing unnecessary\ndetail from the main text, focusing more explicitly on how our findings contribute to broader ecological\nconcepts.\n-Authors should mention (at least in intro) how context-dependency also plays a role in the debate around\nbiodiversity-stability theory\nResponse: We agree with the reviewer that context-dependency is a key factor contributing to the\ncomplexity and ongoing debate surrounding biodiversity–stability relationships. We have now addressed\nthis in the Introduction by highlighting how environmental conditions such as climate can modulate the\nstrength and direction of biodiversity–stability effects (see L74-L77):\n“… This is because, for one, biodiversity–stability relationships may vary with environmental (e.g. climate)\nconditions and across spatial and temporal scales7,12,13, but we currently lack insights from many\necosystems.”\n-The results seem to pack too many details in and the main messages get a little lost.\nResponse: we have streamlined the presentation of the results to strengthen the core message. In\nparticular, we have moved all sensitivity analysis results to the Supplementary Materials, which helps to\nimprove clarity and keep the focus on the main findings.\n-The authors need to make a better ecological argument in the discussion for how, why, and at what level\n(population, within vs. between communities, etc.) the asynchrony in tree growth affects the herbivore\ncommunities. In general, the argument for why the stability matters (rather than mean differences, etc.)\nneeds to be better justified, for the tree and herbivore metrics. This will help the paper feel less\ndisconnected and abstract, and more connected to the implications discussed in the introduction and\ndiscussion sections.\nResponse: We thank the reviewer for pointing this out. We have now expanded our main text of how and\nwhy tree growth asynchrony affects herbivore communities, and at what level these effects are likely to\n12\noperate. Moreover, we have provided a clearer ecological argument for why stability, rather than mean\ndifferences alone, is critical to consider by incorporating a more mechanistic explanation (see L230-L242):\n“Tree growth asynchrony could stabilize herbivore communities by buffering resource availability at the\npopulation level, reducing competition within communities32. Moreover, our study highlights the added value\nof expanding biodiversity and ecosystem functioning relationships by explicitly considering stability\nrelationships. While high average biomass production may reflect favorable conditions in some years, it may\nmask substantial interannual fluctuations in growth31. The repercussions on herbivore diversity and stability\nwe observed can thus have important ecological consequences—for example with respect to predicting the\nprobability of pest outbreaks following climate extreme events and subsequent tree growth variation32—that\nwould not be evident when ignoring such a temporal perspective. Understanding what promotes stability\nallows us to assess how ecosystems buffer disturbances, maintain trophic interactions, and preserve long-\nterm biodiversity under changing environmental conditions.”\n-The plantation/monoculture areas seem poorly integrated into the intro, results, and discussion. There are\na few mentions of the monocultures and it seems like a missed opportunity to connect to human\nmanagement meaningfully.\nResponse: Thanks for the useful comments! Our analyses are not so much monoculture vs. mixture\ncomparisons but analyses along a tree species richness gradient. We have clarified this in the Introduction\nand in Fig. 1. Nevertheless, we agree that the implications of these analyses for management of\nmonoculture plantations are important as well and we have added relevant text in the Introduction (see\nL94-L96) and Conclusions (see L354-L358).\nIn Introduction:\n“Monoculture plantations represent a dominant forest management practice worldwide22, but their low\nbiodiversity may reduce their capacity to buffer against environmental fluctuations and pest outbreaks.”\nIn Conclusions:\n“Our findings suggest that monoculture plantations, despite their widespread use in forest management,\nmay be more prone to herbivore instability due to low tree diversity. In contrast, mixed-species plantings\nappear to enhance stability, highlighting the value of biodiversity-oriented management for more resilient\nforest ecosystems.”\n-I have a few questions about the “all” category for herbivore types, because on the figures points seem to\nbe a subset (not including the specialists). Please explain/clarify.\nResponse: Thank you for this helpful comment. The category “all” refers to the entire herbivore\ncommunity (i.e. generalists + specialists). Because generalist herbivores represent the majority of the\nindividuals in the community, the patterns for “all” largely overlap with those of the generalists, resulting\nin similar ranges in the plots. This is therefore not a figure error, but a reflection of the community\ncomposition. We have clarified this in the figure legend (Fig. 2) to avoid misunderstandings.\n-I also have several suggestions for minor changes for improved clarity (see in-line comments below).\nIn-line comments and section-specific recommendations:\nGeneral comments:\nOverall, the intro would be strengthened by drawing more directly to diversity-stability theory (and\nacknowledging the debate/challenges/context dependency within). Because asynchrony is such an\nimportant part of the results, it also seems to need more discussion/description in the introduction.\nIn-line comments\nResponse: We have strengthened the connection to biodiversity–stability theory by explicitly mentioning\nsome of its key mechanisms and context dependencies (L70-L77; L230-L240). We now also provide a more\ndetailed introduction to the concept of species asynchrony and its potential role in stabilizing\ncommunities, thereby better preparing the reader for its prominence in our results (L100-L102):\nIn Introduction:\n“Such stabilizing effects may operate through mechanisms such as averaging effects, negative covariance\namong species, and insurance effects8 and suggest that the current biodiversity crisis9,10 might have long-\nterm negative consequences for ecosystem stability and ultimately human well-being6,11. The full\nconsequences, however, are difficult to assess. This is because, for one, biodiversity–stability relationships\nmay vary with environmental (e.g. climate) conditions and across spatial and temporal scales7,12,13, but we\ncurrently lack insights from many ecosystems.”\n“In general, community stability can be partitioned into species asynchrony, i.e. a temporal misalignment of\nspecies dynamics, and the stability of species’ populations themselves14”\nIn Discussion:\n“Moreover, our study highlights the added value of expanding biodiversity and ecosystem functioning\nrelationships by explicitly considering stability relationships. While high average biomass production may\nreflect favorable conditions in some years, it may mask substantial interannual fluctuations in growth33. The\nrepercussions on herbivore diversity and stability we observed can thus have important ecological\nconsequences—for example with respect to predicting the probability of pest outbreaks following climate\nextreme events and subsequent tree growth variation34—that would not be evident when ignoring such a\ntemporal perspective. Understanding what promotes stability allows us to assess how ecosystems buffer\ndisturbances, maintain trophic interactions, and preserve long-term biodiversity under changing\nenvironmental conditions.”\n110-111 This section leads the reader to believe they will be comparing among plant growth forms. I would\nclarify/modify to discuss tree traits more.\nResponse: Thank you for pointing this out. We have revised the text for clarity (see L117-L122):\n“However, empirical insights on how diversity and stability are interlinked between herbivores and their host\nplants are available primarily for grasslands, which are characterized by a strong yearly biomass turnover of\nherbs and graminoids15. As long-lived organisms, trees are rather constant and predictable resources, and\nassociated herbivores might thus be less affected by changes in the stability of host biomass production.”\nResponse: Further clarified (see L150-L152):\n14\n“3) community stability is primarily driven by asynchrony among dominant generalist herbivores, with\nspecialist species being much rarer and their stability depending more strongly on population stability.”\n184-196 I recognize this is a response to a reviewer, but it needs to be better connected to the hypotheses\nor better explained; as is it comes out of the blue and reads exactly like a response to a reviewer, rather than\nan integrated part of the study.\nResponse: Because we see these analyses more as sensitivity analyses. We realized that moving these\nadditional analyses to the Supplementary Information is the best way to reduce confusion. Hence, we now\npresent these analyses explicitly as sensitivity analyses (see Supplementary sections), pointing to it in the\nfigure caption, while focusing the main text on the core results.\nResults\nGeneral comments\nI recognize that the authors are trying to strike the balance of enough methods details to give context but\nleave detailed methods below in the methods section; however, these results are presented relatively\nconfusingly. Please take another readthrough and clarify and simplify. I suggest defining all abbreviations\nand explaining models more ecologically, rather than statistically (so that the reader does not need to know\nall the details of the models to understand the important differences). Start out more broad (maybe report\noverall direct and indirect effects) before getting into the details of each model. I would also suggest\nconnecting the results directly to the hypotheses to better lead the reader through the figures/results.\nChanges to the topic sentences and/or organization would help.\nResponse: We have thoroughly revised the results to improve clarity and flow. Specifically, we now 1)\ndefine all abbreviations upon first use, 2) describe the models primarily from an ecological perspective in\nthe caption of figure 1, 3) begin with a broader summary of overall direct and indirect effects before\npresenting model-specific details (see L160-L170), and 4) explicitly link each set of results back to the\ncorresponding hypotheses (see L161, L171, L187). We also revised topic sentences and adjusted the\norganization of the text to guide the reader more clearly through the figures and results (see L155-L191).\nIn-line comments\nResponse: Defined the abbreviation accordingly.\nResponse: Defined the abbreviation accordingly.\nDiscussion\nGeneral comments:\nGenerally well written and interesting discussion, although I would have liked to see a higher proportion of\nthe overall word count be in this section.\nResponse: We have expanded the discussion section by adding more synthesis of our key findings in the\ncontext of diversity–stability theory (see L216-L220), as well as elaborating on the implications for\nbiodiversity conservation and ecosystem management (see L230-L242):\nthe next sentences are about functional diversity and tree growth.\nResponse: Rephased to promote readability (L203-L207):\n“Our study unravels how the temporal stability of abundance and species richness of herbivore communities\nare linked to changes in the species richness of their host tree communities. Specifically, the results show that\nindirect effects of tree species richness via tree functional diversity and tree growth on herbivore phylogenetic\ndiversity…”\nResponse: We have now revised the text to highlight it more clearly (see L210-L220):\n“ In particular, we found a clear and ecologically intuitive stabilizing effect of tree species richness on the\ntemporal abundance and richness stability of specialist herbivores, whereas such effects were absent for\ngeneralists. For the overall herbivore community, path analysis indicated that this altogether resulted in less\nstable abundance dynamics at higher tree species richness via tree functional diversity, even though the\ndirect relationship between tree species richness and herbivore stability was not significant in our linear\nmodels. These finding underscores the crucial role of host diversity in buffering specialist herbivores against\nenvironmental fluctuations and emphasizes that biodiversity–stability relationships can vary markedly across\ntrophic groups depending on their level of dietary specialization.”\n243-248 Same as comment above; needs to be rooted in ecology (or at least directly connected to your\nhypotheses) to make sense of why you replaced this.\nResponse: We have rephased this section to better connect it to our hypotheses and to highlight the\necological rationale behind the variable choice in models (L263-L272):\n“In the sensitivity analyses replacing herbivore phylogenetic diversity by herbivore abundance or richness\n(Figs S3, S4), the driving role of tree species richness behind these effects became even more evident. This\nwas possibly due to the fact that these simpler metrics of biodiversity were not able to resolve the indirect\npathways via tree asynchrony as accurately as a biodiversity metric accounting for phylogenetic (and\nfunctional) differentiation among herbivore species36,37, underlining the importance of considering multiple\nmetrics of biodiversity. Notwithstanding, the strong influence of tree species richness emphasizes the\nconsequences that biodiversity loss at the host level can have for higher trophic-level diversity and,\nultimately, stability.”\nResponse: We now framed these additional questions as sensitive analyses and moved them to the\nSupplementary Information to reduce confusion (please see our responses to your above comment).\nMethods\nGeneral comments\neven though the methods occur afterwards.\nResponse: We now briefly introduce the study design in the Introduction to provide readers with earlier\ncontext (see L130-L140):\n“We used time series data (17 replicated sampling seasons spread across six years from 2017 to 2022) of a\ndominant group of insect herbivores (lepidopteran larvae) and their host tree communities in a large-scale\ntree biodiversity experiment (BEF-China27). The experiment was conducted at two sites located 4 km apart\nand our study included 52 randomly distributed plots reflecting a tree species richness gradient from\nmonocultures to mixtures with up to 24 species. Specifically, we used linear models and path analysis to test\nthe pathways that directly and indirectly (via tree and herbivore community asynchrony and population\nstability and via herbivore phylogenetic diversity) connect tree species richness and its effects on tree growth\nto the temporal stability of herbivore abundance and species richness (Fig. 1)”\nIn-line comments\nsaplings” early on (maybe in the intro justify why they are a particularly good system), in the results, and add\nat least a sentence in the discussion about how you may expect the results to change as the forest ages.\nResponse: Both experimental sites were established in 2009 and 2010, respectively, by planting young\ntree saplings (approximately one year old at the time of plantation). Thus, during the period of our study,\nthe stands represent young experimental forests that were past the sapling phase and forming a closed\ncanopy.\nfrom rare species?\nResponse: We agree that the wording “unrealistically” was misleading. We have now rephrased this\nsection to be more specific, clarifying that low stability is indeed expected for rare species. At the same\ntime, we now treat this analysis as a sensitivity test and have moved the corresponding text to the\nSupplementary Information for clarity (see Supplementary section L16).\n507-526 I assume this is a response to a previous reviewer, but to me the four models, rather than\nstructuring a single model based directly on the interactions of interest in the hypotheses, feels\ndisconnected from the main aims of the paper. Maybe having one sentence in the introduction that clarifies\nthat you are specifically interested in the direct, indirect, microclimate, or both (or something that captures\nall four models) effects would help. This is also not clearly connected to the main results presented in the\nfigures (rather than the supplementals).\nResponse: Thank you for pointing this out. We now provide a brief rational in the Introduction for why we\ncompared multiple model structures, and directly reference the overview Figure, which previously was in\nthe supplementary material but is now integrated in Fig. 1. We also rephrased the model description in\nthe methods to better connect these alternative models with our hypotheses (L552-564):\n“First, we considered the initial model with all potential links included (Model 1). Second, we assumed that\neffects are entirely mediated by stability components, except for effects such as microclimate buffering that\nmay remain in tree species richness. Therefore, we removed the direct connections between trees and\n17\nherbivore abundance and richness stability, except for effects of tree species richness (Model 2). Third, we\nassumed that there are no such effects of tree species richness, so we additionally excluded the direct\npathways between tree species richness and herbivore stability metrics (Model 3). Fourth, we assumed that\ntree species richness only has indirect effects via tree stability components and functional composition. We\nhence removed the links between tree species richness and herbivore MPD/abundance/richness and\npopulation stability (Model 4). The same model structure selected for overall herbivores was applied to\ngeneralists and specialists to ensure comparability across groups.”\nFigures\nGeneral comments\nFigure 1 clearly sets up the “monoculture versus everything else”, which isn’t the way the analyses were run\n(more continuous). Consider which is the correct choice for the hypotheses and main messages of the paper.\nResponse: Now we slightly modified the figure 1 to represent low tree species plots rather than only\nmonocultures.\nherbivore community stability. Herbivore abundance asynchrony among species and community stability\nare coupled with the diversity and temporal growth rate stability of their host tree communities (Hypothesis\n1), with herbivore community stability ultimately a destabilized by low tree species richness and b stabilized\nby high tree species richness (Hypothesis 2). Specifically, in a monocultures and species-poor mixtures, the\npronounced fluctuations in tree growth may lead to reduced asynchrony among herbivore species, thereby\ndestabilizing the herbivore community. Conversely, in b more diverse mixtures, complementary dynamics\nresulting from the asynchrony among tree species could contribute to the stabilization of plant communities.\nThis stabilization effect extends to herbivore communities, promoting greater asynchrony among herbivore\nspecies and ultimately enhancing the stability of the herbivore community (Hypothesis 3). These\nrelationships were investigated with c path models. Structure based on theoretical expectations and\ncorrelations among herbivore- and tree-based variables: mean phylogenetic distance (MPD), species\nasynchrony, population stability of herbivores; tree functional diversity (FD), species asynchrony, population\nstability of trees. Arrows indicate expected causal relationships. Blue lines are covariances retained in the\npath models. We assessed with four alternative models how direct vs. indirect effects of host tree-based\nmetrics influence herbivore community dynamics. Model 1: both direct and indirect pathways from trees to\nherbivore stability; Model 2: restricting tree growth effects to indirect pathways via herbivore population\nstability and asynchrony, except for tree species richness; Model 3: assuming even species richness acts only\nindirectly; Model 4: assuming that tree diversity influences herbivores solely through effects on tree\nfunctional diversity, asynchrony, and population stability. Model 4 was selected for our final analyses.\nFigure 2d,e,f. If dark red is “all”, why do the lines and data only span the range of the generalist species?\nIs this just a figure error or does this represent the analyses? Needs a better explanation of “all” either way.\nResponse: The category “all” refers to the entire herbivore community (i.e. generalists + specialists).\nPlease also see our response to you comment above.\n“‘All’ herbivores includes the entire community (generalists and specialists) analyzed together....”\nFigure 4 write out functional diversity on the figure legend\nResponse: Done.\nspecies richness on the abundance and richness stability of all, generalist and specialist herbivores,\nrespectively. Effect sizes were calculated by summing indirect effects of tree species richness via tree\nfunctional diversity, tree asynchrony, tree population stability, herbivore phylogenetic diversity, herbivore\nasynchrony, herbivore population stability, and herbivore abundance stability. The different colors show\neffects of tree species on herbivore stability via tree functional diversity, tree asynchrony, and tree\npopulation stability, respectively. Effect sizes were calculated as the product of standardized path\ncoefficients connecting each predictor with herbivore components, summed over the individual predictors\nof each component for positive and negative effects on herbivore stability metrics, respectively. Black T-\nshaped lines indicate the total effects of tree species richness on herbivore stability metrics. Note that tree\nspecies richness, population stability, abundance stability and richness stability of herbivores were log-\ntransformed. Stability measures are based on the inverse of the coefficient of variation (eqn. 3). See\nSupplementary Results and Fig. S7 for effects including non-significant pathways.\nSupplemental Figures\nGeneral comments\nFig S1. Say which was the final model structure used.\nResponse: Clarified accordingly, now in Fig.1\n“Model 4 was selected for our final analyses.”\nfigures or captions. Also same question as in figure 2: why does “all” not encompass the specialist species?\nResponse: We now removed tree species richness from the caption. Please see our responses above\nregarding the “all” herbivores\nFig S3. I would change the title of this to encompass the replacement directly rather than in parentheses.\nResponse: Changed as suggested.\nFig S4. Same comment as S3\nResponse: Changed as suggested.\nFig S5 Same comment as Fig 2, and aren’t some of these bivariate relationships in Fig 2? (e.g., e?, which also\nhas no label)?\nResponse: The Figure depicts results for the alternative stability metric. Added the label for e.\nFig S6 Be clear about ho these are different from Fig 3.\nResponse: We clarified it further in the figure legend to easily distinguish it from Fig. 3.\nAsynchrony and functional diversity couple herbivore community\ndynamics to host plant diversity\nMing-Qiang Wang, Georg Albert, Carlo L. Seifert, Douglas Chesters, Helge Bruelheide, Jing-Ting Chen,\nAndréa Davrinche, Sylvia Haider, Shan Li, Yi Li, Goddert von Oheimb, Tobias Proß, Keping Ma,\nXiaojuan Liu, Arong Luo, Andreas Schuldt*, Chao-Dong Zhu*\nRESPONSE TO REVIEWER 1\nReviewer #1 (Remarks to the Author):\nThank you for this new, revised version. My comments have all been addressed satisfactorily. I have on final\nremark: In response to a reviewer comment, the conclusions were extended. In my opinion, the conclusions\nincluded in lines 353-358 do no reflect the study results, as the destabilization of herbivore communities due\nto biodiversity loss only holds for specialized herbivores. I suggest to include a little more differentiation\nhere.\nResponse: Thank you for this insightful remark. We agree that the destabilization of herbivore\ncommunities due to biodiversity loss primarily applies to specialist herbivores. We have now revised the\nconclusion to specify that monocultures are especially vulnerable to instability driven by specialist\nherbivores, while mixed-species plantings can mitigate such risks by diluting host availability.\n“Our findings suggest that monoculture plantations, despite their widespread use in forest management22,\nmay be more prone to herbivore instability driven by specialist herbivores that can rapidly build up on their\npreferred hosts under low tree diversity. In contrast, mixed-species plantings dilute host availability and\nthereby stabilize herbivore communities, highlighting the value of biodiversity-oriented management for\nmore resilient forest ecosystems.”\nRESPONSE TO REVIEWER 4\nReviewer #4 (Remarks to the Author):\nOverall statement:\nThe authors have sufficiently addressed my previous concerns and the other reviewers’; the manuscript will\nmake a very nice contribution to the field, and I once again commend the authors on their work. Below are a\nfew minor suggestions for small clarifications.\nResponse: We appreciate the careful reading and have addressed all minor suggestions accordingly to\nfurther improve the clarity and precision of the manuscript.\nIn-line comments:\n76 and we lack, rather than but we lack\n141-152 May want to consider specifying a, b, c if possible for Figure 1 rather than refer generally to Figure 1\nmultiple times in these sentences\nResponse: We have now specified the corresponding panels when referring to Figure 1 to improve clarity\nand readability.\n236 the repercussions of what? Clarify this sentence/few sentences\nResponse: We have specified this point more clearly by indicating that it refers to “the repercussions of tree\ndiversity”.\n2"}]

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