Dominant species are expected to have a large impact on their communities, potentially including trophic interactions among other species. Throughout North America, tallgrass prairies are characterized by the abundance of two dominant C4 grass species, Andropogon gerardii and Sorghastrum nutans. To determine the role of these dominants in structuring the community, we seeded 46 experimental plots (28 m2) with a diverse mix of prairie species, either with or without the two dominant grasses. We investigated whether trophic interactions are impacted by the presence of the dominant grasses, which achieved >50% cover in plots into which they were planted. Such large and common plants could affect the ability of herbivores to detect their host plants. We focused on Solidago altissima, an abundant self-introduced species that is attacked frequently at our study site by mammalian herbivores (mainly white-tailed deer, Odocoileus virginiana) and two gallmaker flies [Eurosta solidaginis (Tephritidae) and Rhopalomyia solidaginis (Cecidomyiidae)]. We compared plots with and without dominant grasses to determine if they differed in rate of mammalian herbivory, rate of gallmaker attack, or rate of parasitism on gallmakers.
Mammalian herbivory was significantly higher in plots without dominant grasses (mean ± SE: 13.2 ± 1.3% of stems) compared to plots with dominant grasses (10.4 ± 1.0% of stems). The rosette galls of R. solidaginis were found on 10.3 ± 0.6% of stems overall, and the ball galls of E. solidaginis on 5.7 ± 0.6% of stems overall; the frequency of these gallmakers did not differ between plots with versus without dominant grasses. Parasitoids were relatively common in R. solidaginis galls (37.1 ± 3.1% of galls) but their frequency also did not differ between treatments.
Overall, we found that S. altissima plants had reduced rates of browsing by mammalian herbivores in the presence of dominant tallgrasses, but insect gallmakers and their parasitoids were not affected. This result could reflect the different means used by mammals and insects to detect and select plants for attack.