Interactive effects of herbivory and competition on blue vervain (Verbena hastata L.: Verbenaceae)

Abstract

We conducted an herbivore-exclusion plus neighbor-removal experiment to determine whether herbivory and competition had independent or interactive effects on a wetland plant, blue vervain (Verbena hastata). Eight-week-old plants were added to 1.5 m2 experimental plots with neighboring reed canary grass (Phalaris arundinacea) either left intact or removed and herbivores either not excluded or excluded. Neighbors were removed with herbicide and hand-weeding while herbivores were excluded by surrounding a plant with 6-mm wire mesh and spraying the plant weekly with a systemic insecticide. Treatment effects were determined by monitoring plant survival over two growing seasons and by measuring inflorescence mass in the second season. The two treatments had a synergistic, interactive effect on plant survival (G=25.7, P<0.001). Four of the five plants per treatment survived when exposed to herbivory alone or to competition alone, but no plants survived when exposed to both herbivory and competition. The two treatments had a substitutive, interactive effect on inflorescence mass (F=11.26, P<0.0005). Mean inflorescence mass was only 0–1 g plant−1 when plants were exposed to herbivory alone or to competition alone or to both herbivory and competition. Plants not exposed to either herbivory or competition had a mean inflorescence mass of 60 g plant−1. Since effects of herbivory and competition were interactive rather than independent, future studies measuring either herbivory or competition effects on wetland plants should include both herbivore-exclusion and neighbor-removal treatments.