Beyond BioBlitzes: expert-led and occasional citizen science both shape urban biodiversity data — a case study from Siena, Italy

Citizen science (CS) is increasingly used for urban biodiversity research. It exploits digital opportunistic-open-data-collec- tion platforms such as iNaturalist that enable broad participation. However, the relative contributions of expert-led initia- tives compared to observations collected outside organised events remain insufficiently explored. This locally grounded exploratory case study examined the contribution of these two approaches in the framework of the Siena BiodiverCity (SBC) project. By analysing biodiversity observations recorded in Siena (Italy) until June 2024, we distinguished obser- vations associated with expert-led activities (e.g. BioBlitzes, workshops) and observations recorded outside the corre- sponding event windows. Participants were further classified as engaged users (active beyond events), event-dependent users (active only during guided initiatives) and occasional users (contributing independently). Taxonomic patterns were compared, as well as SBC data with those from the Global Biodiversity Information Facility (GBIF). Expert-led initia- tives generated the largest share of records and were associated with a higher representation of less frequently recorded groups such as fungi, molluscs and arachnids. However, only a minority of participants remained active after events, and many limited their contributions to the duration of the guided initiatives. Occasional users added a substantial body of biodiversity data, showing the complementary value of occasional CS alongside expert-led events. Many occasional users were classified as probable visitors using an activity-based criterion, although this classification should be interpreted cautiously in a mainland context. SBC also contributed to the GBIF dataset for Siena: iNaturalist-based records from Siena increased thanks to the project and hundreds of species were documented for the first time in GBIF for the Siena municipality through expert-led initiatives. These findings highlight the dual importance of expert-led and occasional CS in urban biodiversity documentation. Future work should combine structured events with strategies aimed at supporting longer-term participation, while also investigating the possible contribution of visitor-generated observations.