Appendix S3. Herbivore survival
METHODS
Grasshopper survival in intra and inter-specific treatments
Using survivorship data, we compared the survival of each grasshopper species in monospecific treatments to its survival in the 3- and 6- species treatments to quantify species
performance in inter-specific treatment relatively to intra-. Grasshopper response to inter- vs.
intra-specific treatments were quantified by calculating the log response ratio for each
grasshopper species following Suding et al. (2003):
𝐺𝑟𝑎𝑠𝑠ℎ𝑜𝑝𝑝𝑒𝑟⁡𝑠𝑝𝑒𝑐𝑖𝑒𝑠⁡𝑠𝑢𝑟𝑣𝑖𝑣𝑎𝑙⁡𝑖𝑛⁡𝑚𝑢𝑙𝑡𝑖𝑠𝑝𝑒𝑐𝑖𝑒𝑠⁡𝑡𝑟𝑒𝑎𝑡𝑚𝑒𝑛𝑡
𝐿𝑁𝑅𝑅𝑠𝑢𝑟𝑣𝑖𝑣𝑎𝑙 = ln⁡(𝐺𝑟𝑎𝑠𝑠ℎ𝑜𝑝𝑝𝑒𝑟⁡𝑠𝑝𝑒𝑐𝑖𝑒𝑠⁡𝑠𝑢𝑟𝑣𝑖𝑣𝑎𝑙⁡𝑖𝑛⁡𝑚𝑜𝑛𝑜𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐⁡𝑡𝑟𝑒𝑎𝑡𝑚𝑒𝑛𝑡)
A LNRRsurvival > 0 indicates that grasshopper species survival was higher when interacting
with other species than when interacting with itself, a pattern that can emerge if feeding niche
complementarity occurs between grasshoppers (Deraison et al. 2015). A LNRRsurvival < 0
indicates the opposite.
Complementarity between grasshoppers is predicted to be higher when grasshopper
exhibited contrasted set of traits, assuming that trait differences between species reflect
species niche differences (Maire et al. 2012) (Fig. 1). We calculated a functional dissimilarity
index to test the effect of grasshopper trait dissimilarity on their survival. The index was
calculated in each multispecies treatments (3- and 6- species treatments) and for each
grasshopper trait (body size, incisor strength and C:N ratio), such as:
𝐷𝑖𝑠 − 𝑇𝑟𝑎𝑖𝑡 = ⁡
∑𝑛
𝑗 (𝑇𝑟𝑎𝑖𝑡⁡𝑣𝑎𝑙𝑢𝑒⁡𝑜𝑓⁡𝑡ℎ𝑒⁡𝑠𝑝𝑒𝑐𝑖𝑒𝑠⁡𝑖− 𝑇𝑟𝑎𝑖𝑡⁡𝑣𝑎𝑙𝑢𝑒⁡𝑜𝑓⁡𝑡ℎ𝑒⁡𝑠𝑝𝑒𝑐𝑖𝑒𝑠⁡𝑗 )²
𝑁𝑢𝑚𝑏𝑒𝑟⁡𝑜𝑓⁡𝑝𝑎𝑖𝑟𝑤𝑖𝑠𝑒⁡𝑠𝑝𝑒𝑐𝑖𝑒𝑠
Dis-Trait is thus the average difference in trait value of a grasshopper species i from its
competing species j divided by the number of pairwise interactions in a given grasshopper
mixture.
We used three models to assess the relative performance of grasshopper species in inter-
vs. intra-specific treatments (LNRRsurvival). We first tested whether the six different
grasshopper species showed contrasting survival in inter-specific treatments compared to
intra- as a function of their species identity and sex. The second model tested the effect of the
mean trait value of the sex/species (body size, incisor strength and C:N ratio) and trait
dissimilarity (Dis-BodySize, Dis-IncisorStrength and Dis-C:N ratio). The third model
included the effect of plant community functional structure (mean plant height, mean plant
C:N ratio, and plant functional diversity) assuming that interactions between grasshoppers are
modulated by the local resources (Ritchie & Tilman 1993).
To model LNRRsurvival we used mixed effect models. Year and block were included as
fixed effects (n = 480). Cage number was included as a random factor. All explanatory
variables were standardized (mean-centred and divided by the standard deviation) and model
residuals were inspected for normality. In all models, interaction terms between year and
explanatory variables were included but they were not significant. The same methodology as
explained in the main text was used for model selection and parameter estimation. Post hoc
analyses were performed as described in the main text.
This analysis focuses on the relative strength of inter-specific interactions between
grasshoppers relatively to intra- as it is expected, following the complementarity hypothesis,
that increasing niche difference between interacting species leads to higher performance in
inter-specific treatment relatively to intra-. Note that this pattern can emerge from two distinct
mechanisms, competition and facilitation (Gross et al. 2007), with similar effect on species
survival and plant community biomass. We acknowledged that our design based on species
replacement and constant herbivore density across treatments cannot formally distinguish
between the two mechanisms.
RESULTS
Survival in inter-specific treatments relatively to intra- (LNRRsurvival) ranged from values
close to 0 (i.e. survival in inter-specific treatment = survival in intra-, e.g. in case of C.
dorsatus males and females) to significantly positive values (i.e. inter-specific survival >
intra-) (Fig S3a). It suggested that survival was generally higher when grasshoppers where in
inter-specific mixtures. Grasshopper survival response (LNRRsurvival) was best explained by
the functional identity and dissimilarity between herbivores in mixtures as well as the local
characteristics of plant communities (Fig. S3b & Table S3). We found a negative effect of
herbivore incisor strength and C:N ratio on LNRRsurvival. Herbivore community with weak
incisor and low C:N ratio showed a lower survival in intra-specific treatments compared to
inter- while survival in intra-specific treatment equal survival in inter- for herbivores with
strong incisor and high C:N ratio. We also observed a significant and positive effect of
herbivore functional dissimilarity (Dis-Trait). The more the herbivore species differed in their
incisor strength (Dis-IncisorStrength), the higher LNRRsurviva was, indicating that herbivore
survival was improved in inter-specific mixtures as compared to intra-. Opposite effect was
found for herbivore C:N ratio dissimilarity (Dis-C:N ratio). Herbivore body size was not
selected in any of the models (Table S3). Finally, we also found that herbivores survival was
affected by the functional structure of plant communities. LNRRsurvival decreased with
increasing in tall and functional diverse plant community (high mean plant height and plant
functional diversity). Thus, survival was higher in inter-specific treatment compare to intra- in
small and functionally poor vegetation, while survival became similar in inter- and intraspecific treatment as plant communities get taller and/or functionally richer. Other plant traits
(e.g. LDMC, C:N ratio) were not selected in the final model (Table S3).
Table S3. Results of linear mixed effect models testing the effect of herbivore functional
dissimilarity index (Dis-Trait), herbivore species mean trait values, plant community
functional identity (community-weighted means) and diversity on grasshopper survival in
inter-specific treatment relatively to intra- (LNRRsurvival). Incisor strength and C:N ratio
correspond to the mean trait value per species/sex of herbivore incisor strength and carbon
nitrogen ratio respectively. The model also included the cage number (from 1 to 70 cages) as
a random effect.
LNRRsurvival
Model parameters
d.f.
F value
P value
Year
1
3.03
0.828
Block
4
0.94
0.361
I(Year-Block)
4
6.67
<0.001
Grasshopper species incisor strength
1
16.40
<0.001
Grasshopper species C:N ratio
1
8.96
<0.001
Dis-Incisor strength
1
4.75
0.007
Dis-C:N ratio
1
6.54
0.008
Mean plant height
1
2.64
0.003
Plant functional diversity
1
2.31
0.07
Residual
464
AICc
-221.1
Fig. S3a. Average LNRRsurvival over the two years of the experiment for each grasshopper
species and sex (M : males and F: females) (Herbivore species identity: F1,5 = 4.18, P < 0.001;
Herbivore species identity: Sex: F1,6 = 6.67, P < 0.001; Year: F1,1 = 3.12, P = 0.07; Block: F1,4
= 0.84, P = 0.50; Year:Block: F1,4 = 5.88, P < 0.001). Significant differences (P < 0.05)
between herbivore treatments are represented by different letters (a, b, c), calculated by
multiple comparisons for parametric linear mixed effect models. Cb refers to Chorthippus
biguttulus, Cd: Chorthippus dorsatus, Ci: Calliptamus italicus, Ee: Euchorthippus
elegantulus, Pg: Pezotettix giornae, Pp: Pseudochorthippus parallelus.
Fig. S3b. Scaled estimates (±SE) for the grasshopper functional dissimilarity index (DisTrait), grasshopper species mean trait values, plant community functional identity
(community-weighted means) and diversity on the LNRRsurvival. Grasshopper species incisor
strength and C:N ratio corresponded to the mean trait value per species/sex. Red color
indicates variables related to herbivore communities and green color indicates plant
community attributes.
References
1. Deraison, H., Badenhausser, I., Börger, L. & Gross, N. (2015). Herbivore effect traits and
their impact on plant community biomass : an experimental test using grasshoppers. Funct.
Ecol., 29, 650–661.
2. Gross, N., Suding, K.N., Lavorel, S. & Roumet, C. (2007). Complementarity as a
mechanism of coexistence between functional groups of grasses. J. Ecol., 95, 1296–1305.
3. Maire, V., Gross, N., Börger, L., Proulx, R., Wirth, C., Pontes, L. da S., et al. (2012).
Habitat filtering and niche differentiation jointly explain species relative abundance within
grassland communities along fertility and disturbance gradients. New Phytol., 196, 497–509.
4. Ritchie, M.E. & Tilman, D. (1993). Predictions of species interactions from consumerresource theory: experimental tests with grasshoppers and plants. Oecologia, 94, 516–527.
5. Suding, K.N., Goldberg, D.E. & Hartman, K.M. (2003). Relationships among species traits:
Separating levels of response and identifying linkages to abundance. Ecology, 84, 1–16.