This dissertation explores the emergence of three different epidemic diseases in three British colonial cities at the end of the nineteenth century. At first glance these cities – Melbourne, Australia; Bombay, India; and Belfast, Northern Ireland – appear disparate, and their epidemics – Mycobacterium tuberculosis (tuberculosis), Yersinia pestis (plague), and Salmonella enterica serovar typhi (typhoid), respectively – unremarkable in a period otherwise plagued by any number of epidemic diseases. However, I argue that these epidemics arose because of both local and global ecological and cultural pressures, shaped equally by imperial exchange and unique quotidian patterns of human-environmental interaction. To understand how these epidemics are related and why they were consequential, it is necessary to look beyond the human cultural, political, and economic structures that dictated development, organization, and response, and to engage with the underlying ecologies – and consider how they changed. This project examines these layered relationships at a key moment in British imperial urbanism, looking to instances of ecosystem disruption in the form of exponential urban growth and improvement projects as opportunities for disease emergence. Historians have long documented British attempts to impose an ideal of “improvement” upon colonial landscapes; settlers and administrators applied theories of climate, disease, urban planning, and infrastructure to their colonial holdings, undertaking massive (and often expensive) schemes to reroute waterways, construct colonial cities, plant crops, and raise livestock, often destroying entire ecosystems in order to perpetuate their ideas of civilization and productivity. The cases explored in this dissertation reveal one of the many ironies of this system of improvement; namely that, while the British attempted to treat the vastly different ecological systems under their control using uniform ideologies and methodologies, the result was not the development of similar ecological systems, but the emergence of hybrid ecologies that fostered competitive advantage to distinct forms of life. Accessing changing ecologies in British imperial urban spaces and drawing connections between these seemingly disparate events requires engagement with the life-sustaining activities of both human and nonhuman organisms, and thus requires methods that incorporate their influence on shared environments. The main mechanism by which this project proposes to engage these processes is through the adaptation of Niche Construction Theory (NCT), a prominent theory in ecology and evolution, to the study of environmental history. I demonstrate how the theory’s focus on dynamic material change to ecosystems driven by the life-sustaining processes of organisms, both biological and cultural, at local and global scales, provides an ideal framework for the re-integration of humans into their ecosystems while bypassing controversies related to agency dominant in nonhuman studies. I then demonstrate how many of the main mechanisms of niche construction asserted by its major theorists have corollaries in historical and environmental studies methodologies, particularly in the fields’ engagements with the idea of “ecological inheritance” and an interest in tracing the intimacies and lifeways of interacting organisms. Given these common goals, combining the overarching framework of Niche Construction Theory with the nuanced methodologies present in this literature would unite disparate methods in environmental studies under a coherent frame. Niche Construction Theory provides opportunities for more dynamic engagement with nonhumans that shape shared ecologies for historians of the environment, while allowing scholars in this field to introduce more nuanced interpretations of cultural and biological processes to NCT, grounded in historical and anthropological methods.