Invasive plants often exert strong negative effects on native species. Researchers investigating the cause of these effects generally look for an unbalanced interaction that explains the invasiveness of a particular exotic species. However, this may lead researchers to focus on negative interactions and to overlook the possibility that the overall effect of the invader constitutes both positive and negative interactions. For example, herbivores often avoid eating invasive plants, and natives growing in close proximity to invaders may likewise be avoided by herbivores in a process is called ‘shared resistance.’ We hypothesized that association with invaders could provide native plants with ‘shared resistance’ or ‘associational defense’ even when the overall effect of an invader is inhibitory. To investigate this hypothesis, we studied the interaction between Euphorbia esula (leafy spurge), an invasive toxic weed that is avoided by grazers, and Balsamorhiza sagittata (arrowleaf balsamroot), a perennial native forb that is favored by grazers. We compared size, level of leaf herbivory, and level of seed-head herbivory between Balsamorhiza growing inside and immediately outside of Euphorbia patches.
Balsamorhiza plants growing inside Euphorbia patches had smaller seeds (p = 0.001), fewer flowers per leaf (p = 0.033), and smaller rosettes (p = 0.002) than plants growing outside of patches. However, they also experienced lower rates of leaf herbivory, apparently from generalist grazers (p = 0.003). Although association with Euphorbia protected Balsamorhiza plants from generalist leaf herbivores, it did not protect them from seed predation by specialists (p=0.744). To our knowledge, this provides the first evidence that native plants experience a potential benefit when growing near an invader; shared resistance can provide release from certain herbivores, potentially mitigating some of the negative effects of association with a competitive invader. Moreover, our results suggest that the effect of shared defense saturates at very low invader densities but that inhibition is strong only at high invader densities. We saw a slight but statistically insignificant increase in Balsamorhiza size at low invader densities (p = 0.147). Therefore, changes in invader density may cause complex changes in the nature of the interaction between natives and invaders. Our results highlight the complexity of the interactions between native and invasive plants and the importance of investigating both positive and negative interactions in an ecological context.