Establishment of indicators of the evolution of urban biodiversity in Canton Vaud: adaptation of the Syrph the Net method (Diptera, Syrphidae).

To address the current challenges of preserving and halting the decline in biodiversity, the canton of Vaud has drawn up a Biodiversity Action Plan. As part of it, the feasibility of introducing biodiversity monitoring in urban environments was assessed and tested in the field. This monitoring should document the long-term evolution of biodiversity.

It is based on an adaptation of the Syrph the Net (StN) ecological diagnostic tool. The StN tool assesses the proportion of Syrphidae species observed compared with the expected species according to the habitats present. Several indicators were used: (i) global indicators of the ecological quality of all the habitats present at the scale of the canton; (ii) indicators by field of action that provide an indication of the state and changes for habitats designated by urban environment development policies. These indicators were subdivided to assess the quality of biodiversity in relation to its current state (habitats currently observed) as well as in relation to a "target" state, desired in the years to come (required or recommended habitats).

The indicators calculated based on samplings made from 2021 to 2023 using Malaise traps at 38 sites representative of the urban area showed that most of the species expected in the cantonal urban area were currently present, so the ecological quality at the cantonal level (gamma diversity) was considered good. However, only a small number of species were found at individual sites, indicating a poor ecological quality at the local scale (alpha diversity). These mixed results could be explained by a deficit of habitat quality and/or connectivity within the cantonal urban area, which prevents the species actually present from colonizing and occupying all potential sites.

This preliminary study shows that adapting the StN diagnostic tool makes it possible to monitor and assess the state of urban biodiversity in the canton of Vaud. The data collected also improved our knowledge on how Syrphidae use urban habitats.