In a world fraught with human impact, predicting the response of organisms to environmental changes is a fundamental challenge. A dominant approach is to investigate how demographic and genetic factors determine the adaptability of a single taxon. However, organisms do not live in isolation. Hence, community ecologists investigate how the structure of species interaction networks affects community stability. While the interplay between species-level processes and community-level structures governs the response to environmental change, these two approaches have been disconnected. Moreover, how specific kinds of environmental heterogeneity and perturbations modulate this interplay is understudied. We propose to address these gaps by integrating theory and experiments using bacteria as a model organism. Bacteria are a dominant life form, colonizing almost every environment. In addition, genetic change in bacteria is partly driven by horizontal gene transfer (HGT)--a community-level process whose interplay with other interactions (e.g., competition) has not been studied at a community level. We will develop a theoretical framework to explore how species-level factors interact with the environment, affecting competitive ability and emerging community structures. We will use bacteria to experimentally test communities' responses to multiple kinds of environmental change, testing our theory. Our work will establish a framework for a mechanistic understanding of the interplay between species-level processes, community-level structures, and the environment. While targeted at bacteria, insights from this work will have broad implications for other organisms. This work is only achievable through a unique international collaboration that bridges ecology and evolution, microbiology, and statistical physics.