Bottom-up and Top-down Cascading Effects of Climate Change on Herbivorous Insects
Global climate change is widely recognized as a threat to human and natural ecosystems, yet the mechanisms underlying such impacts are poorly understood and difficult to predict. My dissertation research aims at understanding the mechanisms driving climate change impacts on herbivorous insects – key drivers of ecosystem processes – focusing on two central mechanisms: a) bottom effects mediated by hostplants resources and b) top-down effects mediated by herbivore’s natural enemies. I hypothesize that accounting for variation in hostplant resources as well as their response to climate change, is essential for predicting these bottom-up and top-down effects of climate change on herbivorous insects. To test this hypothesis, I conducted three separate research projects using two well-studied systems, the milkweed (Asclepias genus) and the heartleaf bittercress (Cardamine cordifolia). I employed species distributions models (SDMs) to test for the role of geographic variation in hostplant availability and quality for the distribution and climate change response of the western monarch (Danaus plexippus), a milkweed specialist and for which hostplant quality varies among species by an order of magnitude. I found that accounting for hostplant’s distributional response to climate change, but not host-quality, is essential for predicting herbivores’ distribution under projected climatic conditions. Again, using the milkweed system, I tested whether variation in plant traits associated with aridity gradients determine the effects of drought on two milkweed specialists: a leaf chewer (monarch larvae; Danaus plexippus) and a sap feeder (oleander aphid; Aphis nerii). I showed that herbivore response to drought can range from positive to negative depending upon the milkweed species they feed on as well as the herbivore feeding mode. Specifically, the leaf-chewer performed better on drought-stressed milkweed species with trait values associated with aridity and such effects were correlated with changes in chemical defenses. In contrast, sap-feeder’s performance under drought was uncorrelated with aridity-associated traits. However, similar to monarchs, oleander aphid’s drought response was correlated with changes in cardenolide concentrations in the same manner such that both herbivores performed worst in milkweed species that increased cardenolide concentrations under drought. Finally, in a common garden experiment, I investigated the chemical basis of bottom-up and top-down effects of drought on plant herbivory using the Heartleaf bittercress (Cardamine cordifolia) system. Results show that drought-stressed in plants decreased herbivory. However, in the presence of predators, herbivory was reduced more in well-watered than in droughted plants. The overall composition of chemical compounds associated with direct (glucosinolates), and indirect (volatile organic compounds [VOCs]) plant defenses were unaffected by plant drought stress. However, specific chemical functional groups in glucosinolates (indoles) and VOCs (alcohols) were affected by reduced water availability, and these may be responsible for the observed changes in herbivory patterns under reduced water conditions. In summary, my dissertation research revealed climate change effects on herbivores insects are mediated by both bottom-up and top-down effects and that accounting for plant variability and their response to climate change is crucial for understanding these mechanisms and predicting herbivore response to projected climate change.