Climatic change threatens biodiversity worldwide. In the forests of the northeastern United States, climate change is expected to increase mean annual temperatures by up to 4.5˚C and change precipitation seasonality. These changes in climate are likely to have impacts on the biodiversity of the region. In order to better understand the impacts of climate change on biodiversity, I used ants, an indicator taxonomic group, to predict how ant communities and ant-mediated ecosystem processes change as the climate warms. In the first chapter of this dissertation, I review the major ecosystem processes and services mediated by ants using the Millennium Ecosystem Assessment framework. In chapter two of this dissertation I present the results of a major ant sampling effort along environmental gradients of the Appalachian region of the northeastern United States. In 2010 I sampled ant communities in forested and open habitats at 67 localities from Virginia north to Maine and developed macroecological models which show that ant community composition in forested habitats can be explained by the region’s climatic properties. In chapter three, I intensively sampled open and forested plots at Harvard Forest LTER and Myles Standish State Forest in eastern Massachusetts. In chapter four, I present the results of a warming mesocosm experiment using the ant species Formica subsericea. I found that as warming increases, soil movement and soil respiration increases but decomposition and nitrogen availability decreases. In the final chapter of this dissertation, I use different functional diversity and species distribution models to classify the ant communities of the region into different functional groups and explore how their distributions will change in future climates. In this dissertation, I show that ant diversity and ant-mediated ecosystem processes are likely to change under future environmental and climatic conditions. I used observational, experimental and modeling approaches to evaluate and predict the consequences of climatic change on the biodiversity of ants in the northeastern U.S. Ants are considered to be amongst the little things that run the world, and the impacts of climatic change on their communities, abundances, distributions are likely to have major impacts on the forests of the region.