Snowmelt timing determines aphid abundance through multitrophic interactions

Abstract

Ecologically important relationships are being altered by climate change. It is important to understand these relationships in order to predict future changes in species distribution and abundance. Previous research has shown that aphid (Aphis asclepiadis) abundances correlate significantly with snowmelt date in subalpine Rocky Mountain ecosystems, with late snowmelt years correlating with larger aphid populations in summer. Although consistent across years, the mechanisms underlying this phenomenon have not been previously identified. We suspected early snowmelt decreases aphid abundance by inducing early season host plant drought stress. In this study, we used an elevation gradient to mimic year-to-year variation in snowmelt date. We experimentally added soil moisture to separate the effects of early season drought stress on aphid populations from other effects of elevation and snowmelt date. Our manipulations resulted in significant responses in aphid abundance, ant abundance, aphid predators, and plant quality yet revealed host plant drought stress was a minor mechanism by which snowmelt determined aphid abundance. Instead, host plant phenology and mutualist behavior appeared to greatly influence aphid abundance. This research highlights the importance of considering multi-trophic interactions in determining the effects of climate change and how changes of snowmelt dates may be affecting insect communities.