City Nature Challenge
Started 2016 as LA-vs-SF friendly competition; now an annual end-of-April 4-day bioblitz where cities compete on most observations, most species, most participants. Using iNaturalist. Run by California Academy of Sciences + Natural History Museum LA.
Cities are ranked on three public global leaderboards (observations / species / participants). Strong city-pride angle; cities literally publish their rank each year. Includes city-to-city challenges (UK has a UK-only league via NBN).
Each participating city has a local organizer/team that recruits residents, runs guided BioBlitz walks, partners with libraries/parks/schools so residents collaborate face-to-face for the four days. Cities like El Paso run zoo-based events; Bachman Lake hosts community BioBlitzes.
Largest active inter-city sustainability competition by participant count. Under-mentioned variants beyond basic 'iNaturalist'. The CITY-level structure with local organizers is the differentiator the user said to find.
Others in the same family
STADTRADELN (CITY CYCLING)
Run by Climate Alliance, a 21-day campaign where municipalities sign up to compete on per-capita and absolute cycling kilometers. Mandatory team affiliation; tracked via the CITY CYCLING app GPS or paper log.
Eco-Olympics / Green Cup (campus residential)
Month-long competition between residence halls (dorms) on percent reduction in electricity and water vs baseline. Tracked via real meter data, often visualized on color-changing orbs (Oberlin BuildingOS).
Energy Sparks
Charity-run platform that pulls schools' smart-meter data into an energy-analysis dashboard for pupils and staff, with explicit competition layer.
Trash Hero World (chapter league)
Swiss-registered nonprofit where each town runs a chapter (resident-led volunteer team) doing weekly cleanups, supported by a global mentor team. Chapters share branding, handbook, methodology.
Klima-Taler
City-branded climate app powered by motiontag that auto-detects walking/cycling/public-transport trips and converts CO2 saved into 'Klima-Taler' coins redeemable at local pools, museums, shops. Each participating city gets its own white-label instance; users join open teams.
BetterPoints
Council-funded mobile app that auto-detects walking, cycling, bus, train, car-share trips and converts to BetterPoints redeemable at local retailers. Councils customize challenges per borough.