Aphid fecundity and grassland invasion: invader life history is the key

Loss or gain of pathogens can determine the trajectory of biological invasions, and invasion by novel hosts also can alter pathogen dynamics to facilitate invasion. Recent empirical and theoretical work has implicated infection by barley and cereal yellow dwarf viruses (B/CYDV), a group of generalist pathogens of the Poaceae family (grasses), as a necessary precursor to the invasion of over 9 million hectares of California's perennial grasslands by exotic annual grasses. The mechanism underlying this pathogen-mediated invasion hypothesis is elevated vector fecundity on exotic annual grasses. While empirical evidence supports this hypothesis, the links between aphid fecundity, host identity, and host resource supply have not been thoroughly assessed. We performed field and laboratory experiments to examine the fecundity and preference responses of three of the most common aphid vectors of B/CYDV, Rhopalosiphum padi (L.), R. maidis (Fitch), and Sitobion avenae (Fab.), to a combination of host life history (annual and perennial), host provenance (native and exotic), and nutrient supply (mineral N and P fertilization), controlling for host phylogenetic lineage. Aphids consistently had higher fecundity on annual grasses than perennials, regardless of host provenance, age, or nutrient fertilization. In addition, aphids preferentially colonized annual hosts when offered a choice among host species. Multi-generation studies have found that nutrient addition affects both host quality and composition in natural communities; our experimental results indicate that the indirect effects of nutrient fertilization in determining host community composition are of more importance than are the direct effects on host quality to aphid population dynamics. To summarize the applications of our results, we demonstrate that, in contrast to the current focus on the qualitative differences between invaders and natives, the impact of invasive exotic grasses is not due to host provenance, per se, but arises because the annual invaders differ qualitatively from the native species in interactions with shared pathogen vectors. More generally, our work demonstrates the importance of isolating whether the fate and impacts of an invader are, at their root, due to the provenance of the invader, or due to other characteristics that determine its functional uniqueness in the context of the native community.