Indirect positive effects ameliorate strong negative effects of Euphorbia esula on a native plant

Abstract

Invasive plant species can have strong direct negative effects on native plants. Depending on the nature of interactions among competitors and consumers within a community, strong indirect interactions may either augment or offset direct effects. We used path analysis to estimate the relative importance of direct and indirect effects of Euphorbia esula, an unpalatable invasive plant, on Balsamorhiza sagittata, a native forb, through “shared defense” and by suppression of native competitors. Our results indicate that E. esula had strong direct negative effects on B. sagittata, but also that its net effect was reduced by 75% because of indirect positive effects. This reduction was because in equal parts of lessened competition from other native plants eliminated from E. esula stands and to lower levels of herbivory inside E. esula stands, apparently caused by indirect defense of B. sagittata by E. esula. To our knowledge, this is the first evidence that invaders may indirectly reduce herbivory on native plants, a phenomenon that may commonly occur with unpalatable invaders. Furthermore, our results highlight the potential complexity of interactions between native and invasive plants.