Secondary forest development during urbanization sustains apex carnivore populations of Eurasian otters (Lutra lutra)

•We examined the value of secondary forest for conserving apex predators.•We used GLMMs and long term survey data to investigate Eurasian otter occurrence.•Secondary forest can mitigate the negative effect of urbanization on wildlife.•Dispersed urban areas with developed forests could promote more wildlife presence. Better understanding of the ecological and socioeconomic determinants of land-use change is a key to achieve more sustainable development and conservation objectives. Long-term studies on the ability of regenerated secondary forests to sustain biodiversity compared with primary forests have been little explored. South Korea has experienced rapid urbanization and rapid forest regeneration. In this study, we asked two questions: (1) Does secondary forest development alleviate the negative impacts of urbanization on an apex predator; and (2) Can we elucidate a land use planning balance between urbanization and biodiversity conservation using scenario analysis? We studied the Eurasian otter (Lutra lutra) population in South Korea. We applied generalized linear mixed models to five national otter surveys from 1994 to 2019 with 18 environmental variables. Based on the most parsimonious model, we developed 10 scenarios with three patterns of urbanization (dispersed, clustered, and no further urbanization) with or without forest development or with forest degeneration. The most parsimonious model showed that otter occurrences steadily increased with greater forest density, but were negatively correlated with the percentage of urban areas. The scenarios suggested that diffused urban development had a less negative impact on otter distribution than clustered urban areas. Developed forests considerably alleviated the negative impacts of urbanization. In sample sub-basins, scenarios showed that even without further urbanization, otters will take more than six years to occur with the current forest development trends. Hence, sustainable urban planning should include strategic spatial planning for forest development and preservation.