Habitat specialisation and dispersal capacity drive rapid carabid beetle responses to urban forest fragmentation

The effects of habitat fragmentation on insects are well documented, yet most studies overlook extinction debt. We investigated carabid beetles in 25 remnant urban forests in Helsinki, Finland, spanning unfragmented, recently fragmented, and historically fragmented sites. Across 3162 individuals and 34 species, we analysed species richness, community composition, and traits including dispersal capacity, habitat specialisation, and body size both at the community and population levels. We found no extinction debt: species richness remained stable, but community composition shifted rapidly. Forest specialists declined non-linearly within three decades post-fragmentation before partially recovering, whereas open-habitat species showed the opposite pattern. Dispersal-limited species lost richness without compositional change, while highly-dispersive species maintained richness but altered community composition. Individual size, mass, and dispersal traits showed no consistent patterns. Our study demonstrates rapid, trait-mediated responses to fragmentation in short-lived beetles and highlights the importance of considering specialisation and dispersal in urban conservation planning.