Abiotic and multitrophic determinants of geographic distribution in an herbivorous insect

Background/Question/Methods

Characterizing the factors that determine the geographic range limits of species is a major goal of ecology. Historically, abiotic factors (e.g. light environment) have been the focus of research of species geographical distributions. While recent work has shown that pairwise species interactions (e.g. predation) are also important for determining these distributions, the combined action of interactions spanning multiple trophic levels and the environmental mediation of such interactions are poorly understood. In this study, we explored the mechanisms by which light environments (Aspen understory vs. meadows) and tending by ants influence the population growth of the aphid Aphis helianthi feeding on the perrenial herb Ligusticum porteri. We complemented surveys of naturally occurring aphid populations in understory and meadow environments with experimental manipulations of ants and predators in both environments. In so doing, we characterize the effects of (i) direct or plant-mediated effects of light environment, (ii) variation in predation pressure between meadow and understory environments, and (iii) variation in ant tending and predator protection between meadow and understory environments.

Results/Conclusions

Surveys of naturally occurring aphid colonies demonstrated aphid abundance and levels of ant tending to be higher in meadow than understory environments. Our manipulations showed that aphid population performance in the absence of interactions with predators and ants was indistinguishable between understory and meadow environments. In the absence of ants, predators suppressed aphid performance, but this effect was again indistinguishable between understory and meadow environments.  However, ant attendance of aphid colonies was significantly higher in meadow than understory environments, the effect of ants on aphid performance was positive in meadow environments but undetectable in understory environments, and only under ant attendance was aphid performance higher in meadow than understory environments.  In combination, these findings demonstrate that understanding the consequences of abiotic factors on population performance (and, hence, species range limits) requires considering how they mediates ecological interactions in a community context.