The City Nature Challenge as an urban BioBlitz: evaluating Citizen Science contributions to biodiversity monitoring in Berlin.

backgroundThe human-induced loss of biodiversity demands innovative, resource efficient monitoring approaches, such as Citizen Science, to complement traditional biodiversity monitoring methods. BioBlitz events are an established Citizen Science format for biodiversity monitoring that generates as many species observations as possible in a short period of time and within a limited area through collaboration between scientists, the public and nature enthusiasts. The most prominent example of a large-scale worldwide urban BioBlitz event is the City Nature Challenge (CNC). This study investigates whether and how BioBlitz events, specifically the CNC Berlin, can complement existing biodiversity monitoring by addressing three key questions: (1) how species composition compares to existing datasets, (2) how data quality varies across taxonomic groups and participants, and (3) how participant engagement shapes data generation. We further assess taxonomic biases in the CNC Berlin 2023 and 2024 datasets to evaluate their contribution to urban biodiversity monitoring. For a better understanding, the CNC Berlin data is compared with a reference dataset from GBIF (Global Biodiversity Information Facility), an international open-access database compiling biodiversity records from multiple observation platforms. The reference dataset is limited to Berlin, covers a comparable time period and includes biodiversity data from a number of Citizen Science observation platforms.

resultsParticipation in CNC Berlin has increased substantially, with the number of contributors rising from 184 in 2023 to 361 in 2024, reflecting growing involvement. Analysis of the uploaded observations during CNC 2023 and 2024 in Berlin revealed clear differences in participant activity levels: while highly active individuals were few, they contributed a substantial portion of the total observations. The comparison of the observed species in taxonomic groups from CNC Berlin 2023 and 2024 with the GBIF reference dataset revealed the impact of highly active species experts participating in BioBlitz events on particular species groups like fungi and lichens. This highlights the influence of species experts on BioBlitz datasets. In total birds, reptiles, fish, and mammals were more likely to reach "Research Grade" status on iNaturalist, whereas arachnids, protozoans, insects, and chromista had substantially lower Research-Grade rates. Of the 2,440 species listed in the GBIF reference dataset, 1,027 species (42%) were also recorded during CNC Berlin 2023 and 2024. Observations included threatened amphibians, such as the Spadefoot Toad (Pelobates fuscus) and the Great Crested Newt (Triturus cristatus), rare birds like the Whinchat (Saxicola rubetra), and several invasive species, including crayfish (Procambarus spp.) and Giant Hogweed (Heracleum mantegazzianum). Notably, the potentially invasive mysid Hemimysis anomala was recorded in Berlin for the first time ever in 2024. As observations from CNC 2023 had more time to reach "Research Grade" than those from 2024, differences between years should be interpreted with caution.

conclusionBioBlitz events and Citizen Science in general, can provide valuable data that can complement traditional biodiversity monitoring and help to close research gaps, particularly by documenting endangered, invasive and non-native species, and other conservation-relevant taxa. We encourage other researchers to investigate large BioBlitz datasets like CNCs in other parts of the world or even comprehensively on a larger geographic scale.