Effects of herbivore identity on plant fecundity

Herbivores are pervasive, yet their effects on plant fecundity are often variable. One potential source of variation in herbivore impacts results from differing feeding modes of herbivores. We examined the relative importance of inflorescence-feeding insects versus large ungulates in affecting the fecundity of Balsamorhiza sagittata (Asteraceae), a dominant native perennial forb in western Montana, USA. We quantified these effects across four sites that varied in elevation to determine how consistent herbivore impacts were across these divergent sites. Insect herbivores were present in flower heads at all sites but they significantly depressed plant fecundity at two of the four sites. At the two sites where herbivore suppression had significant effects, this treatment let to 1.6 and 3.4-fold increases in seed production, respectively. In contrast, across all sites ungulates had minimal impacts on balsamroot flower and seed production. Seed addition experiments revealed that at some sites and in some years B. sagittata is seed-limited, as there was a positive relationship between seed input and seedling recruitment. Thus, reductions in seed production from heavy insect herbivory may limit balsamroot recruitment in some locations. Overall, results show that inconspicuous insects have stronger effects on balsamroot fecundity than do mammalian herbivores, but the magnitude of negative impacts at both the individual and population-level vary by site.