Enhancing urban forest connectivity: a multispecies modelling approach in Helsinki

Understanding how landscape elements contribute to functional connectivity in a multispecies context is important to inform city planning. We compared the connectivity of urban forest habitats for three vertebrate species with contrasting dispersal modes: the Siberian flying squirrel (Pteromys volans), the common lizard (Zootoca vivipara) and European crested tit (Lophophanes cristatus). We used Graphab and Circuitscape, combined with literature- and expert opinion-based parameters to model their dispersal potential. We compared these models with designated existing connections and rerun the models with modelled future connections by the city of Helsinki, Finland. Our results suggest that despite the relatively high share of forests in Helsinki as compared to many capital cities in Europe and beyond, lack of connectivity may disrupt the movement in different ways for all three species. Isolation was mainly a result of either lack of habitat or too scarce connectivity elements combined with high landscape resistance. Isolation varied depending on species’ specific habitat requirements, interpatch-crossing distance and gap-crossing distance. Based on our models of functional connectivity, the designated existing connections were excessively optimistic. The modelled future connections enhanced connectivity, especially if created with sufficient quality for each species, yet only a small fraction of the modelled future connections was realistically convertible as pathways. Nevertheless, our approach can inform city planning by identifying critical connections and places, where strengthening or building functional connectivity would be beneficial. We suggest that city’s green wedges should be converted into a webbed urban green space network.