An Exploration of Hypotheses that Explain Herbivore and Pathogen Attack in Restored Plant Communities

Many hypotheses address the associations of plant community composition with natural enemies, including: (i) plant species diversity may reduce enemy attack, (ii) attack may increase as host abundance increases, (iii) enemy spillover may lead to increased attack on one host species due to transmission from another host species, or enemy dilution may lead to reduced attack on a host that would otherwise have more attack, (iv) physical characteristics of the plant community may influence attack, and (v) plant vigor may affect attack. Restoration experiments with replicated plant communities provide an exceptional opportunity to explore these hypotheses. To explore the relative predictive strengths of these related hypotheses and to investigate the potential effect of several restoration site preparation techniques, we surveyed arthropod herbivore and fungal pathogen attack on the six most common native plant species in a restoration experiment. Multi-model inference revealed a weak but consistent negative correlation with pathogen attack and host diversity across the plant community, and no correlation between herbivory and host diversity. Our analyses also revealed host species-specific relationships between attack and abundance of the target host species, other native plant species, introduced plant species, and physical community characteristics. We found no relationship between enemy attack and plant vigor. We found minimal differences in plant community composition among several diverse site preparation techniques, and limited effects of site preparation techniques on attack. The strongest associations of community characteristics with attack varied among plant species with no community-wide patterns, suggesting that no single hypothesis successfully predicts the dominant community-wide trends in enemy attack.


Site preparation techniques and natural enemy attack
To compare herbivore and pathogen damage among restoration treatments, we performed two-way ANOVAs for herbivore and pathogen damage, with plant species, restoration treatment or reference site, and species-treatment interactions as independent variables. The interactions among treatment combinations and the response of plant species community composition to these treatments were explored in further detail by . We considered treatment a fixed variable and species a random variable. Because of inadequate replication of E. densiflorum and G. integrifolia in two of the treatments, two ANOVAs were run for herbivore and pathogen damage. One included the four remaining species and all treatments, and the other included all six species but excluded the reference and till + solarization treatments (Tables S1,   S2, S3, and S4). Post hoc Tukey's HSD tests were performed to determine differences among individual treatments and plant species (P < 0.05).
Herbivore attack varied among plant species (F5,40 = 9.03, P < 0.0001) with no main treatment effect; there was an interaction between species and treatment (F48,225 = 2.85, P < 0.001, Figure S1). Herbivore damage was significantly higher on Grindelia integrifolia than on the other five species, which did not differ significantly from each other. Herbivore damage was higher in the herbicide treatment than in the till + thermal treatment only when G. integrifolia and E. densiflorum were included. This difference was driven by high herbivory on G.
integrifolia in the herbicide treatment. When these rare species were excluded and all treatments were included, herbivory did not vary among treatments and there was no interaction between species and treatment.
Pathogen attack varied among plant species (F5,40 = 19.26, P < 0.0001) but not among the ten restoration treatments and reference wetland prairie ( Figure S1). There was more pathogen attack on Agrostis exarata than on the other five species. There was an interaction between treatment and species only when Grindelia integrifolia and Epilobium densiflorum were excluded from the analysis and all treatments were included, because these two species were absent from some treatments (F30,174 = 1.70, P = 0.019); pathogen attack on Prunella vulgaris was higher in the reference than in the restoration treatments. Overall, there was not a strong community-wide effect of treatment on herbivore or pathogen attack.
We did not find a strong community-wide effect of site preparation technique on natural enemy attack in the restoration experiment. Our survey of the ten treatments and reference was motivated by dramatic differences in the plant community composition among treatments during the previous growing season, but these differences were less pronounced during the growing season in which this study was conducted [37,38]. Additionally, the plant community of the restoration experiment became more similar to that of the remnant prairie site with respect to plant species composition [37,38]. Therefore it is not surprising that we found few differences in natural enemy attack among the restoration treatments. While we observed minimal effects of the site preparation treatments on enemy attack on native plants, restoration practices that produce larger differences in plant communities may find larger effects.

Figure S1. Herbivore and Pathogen Damage to Six Native Plant Species among Restoration
Treatments and Reference Prairie. (mean ± standard error) Different lowercase letters represent significant differences in herbivory among restoration treatments with all six plant species included (Tukey's HSD: P < 0.05); when rare plant species were excluded and all treatments were included. There was no significant main effect of treatment. We found no significant main effect of treatment for pathogens, although pathogen damage on Prunella vulgaris was higher in the reference than in the restoration treatments.