An invasive herbivore structures plant competitive dynamics

Abstract

Species interactions are central to our understanding of ecological communities, but may change rapidly with the introduction of invasive species. Invasive species can alter species interactions and community dynamics directly by having larger detrimental effects on some species than others, or indirectly by changing the ways in which native species compete among themselves. We tested the direct and indirect effects of an invasive aphid herbivore on a native aphid species and two host milkweed species. The invasive aphid caused a 10-fold decrease in native aphid populations, and a 30% increase in plant mortality (direct effects). The invasive aphid also increased the strength of interspecific competition between the two native plant hosts (indirect effects). By investigating the role that indirect effects play in shaping species interactions in native communities, our study highlights an understudied component of species invasions.

Keywords: aphid; competition; herbivory; invasive species; milkweed; species interactions.

Figure 1.

Effect of invasive A. nerii on native A. asclepiadis. Mean aphid abundances (total number of aphids of that species per plant) over the six-week experiment when species are grown with or without interspecific competition (averaged across all three plant treatments). Data points are mean values ± one standard error. (Online version in colour.)

Figure 2.

Performance of plants in the absence (a,b) and presence (c,d) of the invasive aphid A. nerii for common milkweed (dark, green) and butterfly milkweed (light, yellow). Upper panels show live biomass (a) and plant survival until the end of the experiment (b) in treatments where A. nerii was absent: control plants grown with no aphids (dotted lines) and treatments with only A. asclepiadis (dashed lines). Lower panels show (c) live biomass and (d) plant survival in treatments with A. nerii present: A. nerii only (dashed lines) and mixed aphid treatments (solid lines). Data points are mean values ± 1 s.e. (Online version in colour.)