Urbanized and post-industrial sites often host considerable biodiversity but are too frequently dismissed by conservation professionals, in part because current species assemblages differ from the site's natural history. Given the dramatic and often irreversible changes to these sites, we conclude that historic ecosystems do not provide a useful reference for restoration. However, seen through a novel ecosystem lens, these landscapes already have conservation value and thus require nuanced restoration planning that recognizes their current and potential community composition. We highlight slag-dominated sites in the brownfields of the Calumet region as an example of a post-industrial landscape that may serve both as a recreational area for humans and a refuge for native biodiversity.