The COVID-19 pandemic has changed the way large citizen science events can be carried out—reducing gatherings of large groups and shifting toward individual, small-group, and online participation. This paper aims to describe changes in participant engagement in the City Nature Challenge (CNC) in Tokyo. The CNC is a four-day international event held in April to document biodiversity in cities using an online citizen science platform, iNaturalist. To assess the impact of COVID-19, we compared the number of participants, observations, species, and identification rates in 2019 (pre-pandemic) and 2020 (during the pandemic). We also used cluster analysis to elucidate participation patterns, and we assessed changes in the geographical distribution of observation sites. The results showed: (1) the number of participants and observations decreased by 63% and 68%, respectively; however, the number of species was almost the same in the two years, and the identification rate increased 154% in 2020 relative to 2019. (2) The most enthusiastic participants contributed in similar amounts in 2019 and 2020, but participation by less enthusiastic volunteers drastically declined. (3) The spatial distribution of observation sites changed from cluster-like to scattered. Understanding participant engagement during the pandemic could help to improve data quality, reduce geographical bias in observations, maintain records, and recruit more users in future years. Online citizen science could provide opportunities for many citizens to get outside and participate in conservation science during and after the pandemic.
