Past present: Extinction debt of forest mammals from urban areas

Increasing amounts of scientific literature focus on ecological processes that shape urban wildlife assemblages. Besides few clear patterns in species dynamics, most literature on the topic focuses on few taxonomic groups and on current landscape structure, leaving huge gaps in our ability to understand, and possibly overcome, extinction processes in cities. Here, we use the city of Florence and its mammalian fauna as a model system to define patterns of local occurrence within large urban areas, testing the hypothesis that past habitat availability may shape the current presence of species i.e., evidencing extinction debt in urban mammals. We conducted a systematic collection of mammal records from Florence, and organized data into two checklists, corresponding to the milestones of urban development history of Florence. We built a land use map for each of these periods, and we modelled total species richness, richness of ecological guilds, and occurrence of individual species, as a function of past and present land use compositions and ecological preferences. We retrieved 1297 records of mammals from Florence, spanning from year 1832 to 2023, and belonging to 62 species. Besides evidencing both local extinction and colonization events, and revealing a net increase of local species richness in time, forest-specialized mammals showed evidence of extinction debt in the city, indicating that current levels of diversity will likely decrease as a delayed response to past habitat loss. Our long-term analysis also revealed the relationship between land use dynamics and the occurrence of some forest species in the urban landscape. We highlight that current species assemblage at urban sites is largely due to the lag between habitat loss and species' responses, particularly for taxa associated with forests, indicating that many species actually represent sorts of “living dead” populations that may be lost if no action is taken to re-establish profitable habitat.