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Michael S. Singer






Mutualisms are increasingly appreciated for their contribution to the structure and dynamics of ecological communities. Food-for-protection mutualisms involving ants and sapfeeding insects are a widespread example of a positive interaction with significant impacts on populations and communities. Sap-feeders provide carbohydrate-rich honeydew to mutualistic ants, and this food source restructures other trophic interactions involving ants. Ants engaged in mutualism are more likely to prey on herbivores and reduce overall herbivory on host plants. Despite the widespread importance of ant-sap-feeder mutualisms in many terrestrial ecosystems, a mechanistic understanding of how sap-feeding insects alter ant trophic ecology remains obscure. The carbohydrate-rich food made available by sap-feeding insects has the potential to modify ant behavior, populations and community structure in the presence of so-called sapfeeders. In this thesis, I study the mechanisms of this keystone mutualism in a forest food web to (a) examine patterns of trophic interactions arising from ant-sap-feeder mutualisms (Chapter 1), (b) determine if and how this mutualism changes ant abundance and community structure so as to strengthen predatory effects of ants on chewing herbivores (Chapter 2) (c) determine if and how this mutualism changes ant behavioral responses to prey and competitors (Chapter 3), and (d) evaluate the consequences of these mechanisms for forest tree growth (Chapter 4). In sum, this thesis demonstrates that ant-sap-feeder mutualisms modify tri-trophic food webs in Connecticut forests, that changes in ant community structure and behavior are the primary mechanisms for this variation, and that this interaction has a significant impact on forest productivity over multiple growing seasons.



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