Variation in insect herbivory across a salt marsh tidal gradient influences plant survival and distribution

Herbivore damage and impact on plants often varies spatially across environmental gradients. Although such variation has been hypothesized to influence plant distribution, few quantitative evaluations exist. In this study I evaluated patterns of insect herbivory on an annual forb, Atriplex patula var. hastata, across a salt marsh tidal gradient, and performed experiments to examine potential causes and consequences of variation in herbivory. Damage to plants was generally twice as great at mid-tidal elevations, which are more frequently inundated, than at higher, less stressful, elevations at five of six surveyed sites. Field herbivore assays and herbivore preference experiments eliminated the hypothesis that plant damage was mediated by herbivore response to differences in host plants across the gradient. Alternately, greater herbivore densities in the mid-marsh, where densities of an alternate host plant (Salicornia europaea) were high, were associated with greater levels of herbivory on Atriplex, suggesting spillover effects. The effect of insect herbivores on host plant performance varied between the two sites studied more intensively. Where overall herbivore damage to plants was low, herbivory had no detectable effect on plant survival or seed production, and plant performance did not significantly differ between zones. However, where herbivore damage was high, herbivores dramatically reduced both plant survival (>50%) and fruit production (40-70%), and their effects were stronger in the harsher mid-marsh than the high marsh. Thus herbivores likely play a role in maintaining lower Atriplex densities in mid-marsh. Overall, these results suggest that variation in herbivore pressure can be an important determinant of patterns of plant abundance across environmental gradients.