Above- and belowground herbivory alters the outcome of intra- and interspecific competition between invasive and native Alternanthera species.
Effects of herbivory on competition between invasive and native plants have seldom been examined from an above-belowground integrated perspective. We examined the interactions between a monophagous beetle, Agasicles hygrophila, or an oligophagous beetle, Cassida piperata, and a root-knot nematode on the intensity of intra- and interspecific interactions between the invasive Alternanthera philoxeroides and its native congener, Alternanthera sessilis. Plant-plant competition was assessed using the relative neighbour effect (RNE) index. Competitive effects (positive RNE indexes) from conspecifics for A. philoxeroides were detected under herbivory by A. hygrophila alone. The ramet number, stolon length, and/or the biomass of A. philoxeroides were reduced compared to plants without herbivory. The interactions between the two plants without herbivory were facilitative (negative RNE indexes), and the facilitative effect became stronger such that A. philoxeroides produced more biomass under combinative herbivory by C. piperata and the nematode. However, significant competitive effects from conspecifics were detected for A. sessilis under all the AG-BG herbivory treatments, while no apparent competitive or facilitative effects from A. philoxeroides were detected for A. sessilis under all the AG-BG herbivory treatments. These results suggest that intra- or interspecific competition of invasive and native plants can be greatly affected by AG-BG herbivory, and thus interactive effects of AG-BG herbivory and plant competition may influence invasive process of alien plants in the field.