SG RUI: Snow melt-induced changes in phenology as direct and indirect drivers of herbivore abundance

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Plants are abundant and widely distributed on earth, despite the fact that many different species of animals feed on them, i.e., herbivores. The world remains green with plants because a variety of factors limit the intensity of herbivory. Understanding these limiting factors is an important goal of ecology. This research focuses on how the seasonal timing of interactions between plants and herbivorous insects could limit herbivory. The project examines interactions between plants and their herbivorous aphids, which form vast colonies on the flowering stalks of the plants, making them an ideal study system for experimental manipulation. The aphid colonies interact with other insects, including ants that provide them with protection from predators. Six years of data show that when snow melts earlier in spring, host plants flower earlier, and this earlier flowering tends to be associated with an increase in aphid numbers. To understand what is causing this association, the researchers will experimentally alter snow cover as a way to vary the timing of plant-insect interactions. This work is important because it adds to our understanding of how changes in the timing of when organisms interact with each other, affect food webs, and the resulting ecological communities. The results will have implications for predicting the abundance of insect herbivores in natural and agricultural systems affected by an earlier arrival of spring. This project will also provide undergraduate students from under-served communities with training in field ecology, and will develop learning opportunities for K-12 teachers and students.Using a combination of observational and experimental approaches, this research will determine the direct and indirect effects of changes in plant phenology on herbivore abundance. In this study system, aphids (Aphis helianthi) colonize the inflorescences of their host plant (Ligusticum porteri), and six years of monitoring indicates that advances in host-plant phenology (earlier flowering) are associated with increases in the abundance of aphids. This correlation between plant phenology and the abundance of herbivores may be due to the direct effects of phenology on plant nutritional quality and/or to the indirect mediation of interactions among ants, predators and competitors. This study will apply a structural equation modeling approach to evaluate hypotheses about how phenological cues influence arthropod abundance through these direct and indirect trophic interactions. The study will directly vary host plant phenology through the experimental manipulation of snow melt using plant populations found along an elevational gradient. These experiments will also employ a factorial design to combine plant phenology treatments with treatments that alter the accessibility of aphid colonies to their mutualists and/or predators. Differences in aphid colony growth among treatments will show the relative magnitudes of direct and indirect effects of plant stage. Laboratory assays of phloem sap and aphid quality (e.g., C/N ratio, secondary metabolite content etc.) will complement these field experiments and provide a mechanism by which host plant phenology could affect aphid population growth or the ant-aphid mutualism.