Hundreds of cities are competing this weekend to collect the most plant and wildlife observations in an urban “bioblitz” as part of a global citizen science challenge. From Dundee windowsills to San Francisco parks, people are being urged to document whatever flora and fauna they can spot around them in urban areas and upload the photos to the iNaturalist app to help identify rare species. The City Nature Challenge, which started as a competition between Los Angeles and San Francisco in 2016, has become a global event that will see nature lovers in towns and cities on all continents race to spot plants, fungi and animals this weekend. Last year, nearly 1.7m observations were made around the world by 67,000 people, who identified more than 50,000 species. Last year, La Paz, Bolivia, came first with 137,345 observations, more than twice as many as Cape Town in South Africa, which came second with 66,144, and Boston in the US, which made 46,896 observations. In the UK, London, Manchester, Nottingham and Swansea are among the 19 participating urban areas. They will compete with cities including Mumbai, which has a population of leopards, and Foz do Iguaçu in Brazil, which boasts the Iguaçu Falls that separate Argentina and Brazil. Dundee, which will be the second most northerly city taking part this year after Whitehorse in Canada, has issued a rallying cry to citizens to take part over the bank holiday weekend and said it would inform the city’s biodiversity action plan. Heather Anderson, Dundee neighbourhood services committee convener, said: “We hope that residents get involved in this citizen science initiative and scour their own gardens as well as the city’s parks and green spaces. “The challenge will help to increase the overall knowledge of Dundee’s wildlife and what species can be found in the city area. Information gathering is vital to better develop our wider environmental aspirations,” she said. Improving access to green and blue spaces and enhancing biodiversity in urban areas was agreed as a target for Cop15 in Montreal. The pictures collected on iNaturalist, a joint initiative by the California Academy of Sciences and the National Geographic Society, will help towns and cities better understand the wildlife they live with. Several species that were thought to be extinct, such as the Dumbéa River pipefish and the small whorled pogonia, a plant last seen in 1902, have been rediscovered thanks to people using the iNaturalist app. The City Nature Challenge takes place from Friday 28 April to Monday 1 May. From 2-7 May, species will be identified, and the winner will be announced on 8 May.