What happens to species when climate disruption causes suitable habitat within one country to move or vanish? In Borders of Biodiversity, Will Wright examines the histories of transnational conservation efforts to address the tension between a warming world in which living things are on the move and an increasingly walled world in which their movements are constrained. Focusing on the histories of three border-crossing species—gray wolves, monarch butterflies, and giant sequoias—from the 1850s to the present day, Wright reveals how nonstate actors like citizen scientists think beyond political borders and diplomatic traditions and find collaborations with fellow-minded conservationists by following nature beyond the nation-state. The people at the heart of these intertwined stories in Canada, Mexico, the United States, and Indigenous nations of North America recognize that biota have their own forms of territoriality that should be respected and defended. Wright argues that the realities of climate change are fundamentally at odds with site-specific conservation, which follows the possessive logic of nation-building by bounding space to protect habitats when many ecologies do not naturally fit within traditional protected areas. Taken together, these stories make clear that conservation efforts must forge solidarity across borders for biological well-being—or face the extinction of shared species.