Framed within the fields of Border and Migration Studies and the Environmental Humanities, Border Biomes: Coexistence and Interference on American Migration Trails, examines how 21st -century Mexican, Mexican American, and Chicanx literature and art depict dynamic, unstable geographies that defy common notions of fixed, inanimate borders and become sites of encounters where ontological limits are questioned. Proposing the concept of border biomes, this project thinks through border forests, rivers, and deserts to argue that they are central figures of representation through which the authors and artists I write about challenge rigid categorizations of territorial boundaries, memorialize and expose the intense entanglement of human and nonhuman entities against the backdrop of border demarcations, and defy and reimagine normative ways of coexistence with nonhuman worlds. I make this argument with the analysis of works by Ana Teresa Fernández, Cristina Rivera Garza, Emiliano Monge, Emmy Pérez, Rafael Ramírez Heredia, and Valeria Luiselli, and with the study of the literary and artistic collaborations between Dolores Dorantes and Zoe Leonard, and between Jenea Sanchez and Lauren Strohacker. I study how these authors address the detrimental consequences of border security measures over both people and the environment, the intimate, conflictive, and often lethal encounters between migrants and border ecologies, and the deep-seated relationships linking these biomes with their inhabitants. Through a coalescence of academic inquiry and field work, this dissertation foregrounds the multi-layered connections between biomes, geopolitical articulations, migrants, and border communities, thus becoming a contribution for a nascent field that focuses on the mutual entanglements of human and nonhuman agents surrounded by border settings.