Until recently, the number of citizen scientists in Australia was estimated at between 100,000 and 130,000 people. But this is a major underestimate. My survey of about 20 key organisations suggests there are likely more than a million in Australia. There are only a limited number of professional (paid) scientists. But anyone with a smartphone can log observations, and professional scientists increasingly work alongside citizen scientists to collect and analyse valuable conservation data. Science lives far beyond the lab, and it’s not just done by scientists.In this series, we spotlight the world of citizen science – its benefits, discoveries and how you can participate.Citizen science isn’t new. Perhaps the best-known citizen scientist was Charles Darwin, who neglected to finish his medical degree in favour of studying corals. Later, the prolific letter writer built a network of passionate naturalists and collected their observations to gather evidence for his theory of evolution. But what is new is how easy it is to get involved – and how many people are now lending their time and skills to the cause. It opens up the possibility of science by the people, for the people. Citizen scientists can sample local waterways and lakes to find invertebrates – and gauge ecosystem health. Australian Citizen Science Association, CC BY-NC-SA How are citizen scientists contributing?Melburnians may know the name Ferdinand von Mueller as the first director of the city’s Botanic Gardens. But Mueller started as a passionate botanist who migrated from Germany in 1847, determined to catalogue every plant species in Australia. After years collecting samples, Mueller realised the task was too big. So he, like Darwin, set about building a network of passionate collectors. Over the next 40 years, more than 1,500 amateur botanists sent him samples. This helped Mueller catalogue hundreds of species new to Western science and produce the first comprehensive surveys of the continent’s vascular plants. Former Chief Scientist Alan Finkel believes Mueller effectively planted citizen science in this country. Since then, citizen scientists have contributed significantly to science. In 1870, Victorian farmers identified the giant Gippsland earthworm and sent it to professional scientists for confirmation. In 1973, a central Queensland fencing contractor rediscovered the bridled nailtail wallaby (presumed extinct), while a Daintree grazier rediscovered the unique idiot fruit tree – which had not been formally recognised – after its fruit poisoned his cows. In 1994, bushwalker and amateur botanist David Noble was exploring a remote canyon in the Wollemi National Park west of Sydney when he found the last remaining stand of a tree long thought extinct. The Wollemi pine (Wollemia nobilis) dates back to when dinosaurs roamed Earth.Even now, passionate citizen scientists are still making new discoveries. In his free time, Jürgen Otto hunts for peacock spiders around Australia. He and his collaborators have named 64 species of the tiny, colourful spiders famed for their courtship displays. This year, Jan Pope and her daughter Sophie Kalkowski-Pope found a huge and unusual coral meadow in the Great Barrier Reef. So what, exactly, is citizen science?A good definition is “public participation and collaboration in scientific research with the aim to increase scientific knowledge”.Key to this are core principles, such as active involvement of citizens in scientific endeavours that generate new knowledge and genuine scientific outcomes. Earlier generations might have called citizen scientists “amateur naturalists”. The term citizen science became popular for its less binary framing. Some people prefer “community science”. Another phrase is also gaining momentum. “Indigenous science” has clear overlap with Traditional Ecological Knowledge. As Jingili man and zoologist Joe Sambono has observed, Indigenous science shouldn’t be set in opposition to Western science, given science is Latin for “to know”: all groups of humans […] have recognised patterns, verified through repetition, made inferences and predictions and developed branches of knowledge that helped them to make sense of the world around them and their place within it.Indigenous science is grounded in continuous, trans-generational, place-based observation built over tens of thousands of years. Cultural burning uses weather, fuel and ecological indicators to gently burn Country while supporting biodiversity, while seasonal calendars tied to flowering plants and wildlife movements shaped practical decisions about when to collect bush tucker, travel and care for Country.In our experience, enduring insights emerge from the overlapping space between Indigenous, citizen and mainstream science. Getting involvedCitizen science can be as simple as a keen birder posting sightings of a rare orange bellied parrot to eBird or a fisher posting a sighting of a bull shark to iNaturalist where citizen and professional scientists can see it. But it can also be volunteering to help in large-scale practical projects organised by professional scientists. These include tracking bushfire recovery, reporting weed infestations, monitoring koalas or fishes, assessing microplastic hotspots and tracking water quality.It took six years (2008–14) for Australian citizen scientists to collectively contribute 10,000 species observations on iNaturalist. Now, more than 10,000 are posted every day. Citizen science can be done solo with a smartphone – or as part of a group working with professional scientists. Here, citizen scientists undertake a bioblitz in Cooloola, Queensland. Michelle Neil, CC BY-NC-ND From a movement of volunteers to a trusted resourceWhen this flood of data began, some scientists were sceptical. Could it be trusted? As time has passed, standardised data collection and guidance from professional scientists have built trust in the data. Citizen science is now recognised for its contributions to health, astronomy, agriculture and – especially – nature conservation. Australia’s Strategy for Nature, Threatened Species Strategy and State of the Environment reports recognise the worth of citizen scientist data. In an era of misinformation and declining trust in institutions, citizen science offers people a chance to engage directly with evidence and the natural world. To date, little funding goes towards making the most of what citizen scientists can offer. Nationally, the government will spend A$15 billion on science this fiscal year. Of this, we estimate less than $10 million will go to citizen science. What could citizen scientists do next?Much more is possible. The million Australians involved in these projects are active in the world and interested in helping protect nature. These attributes mean they could take a large role in tackling climate change – especially at local and regional scales – by observing changes, working on projects to reduce emissions, and helping communities adapt to climate change.To paraphrase the famous anthropologist and citizen scientist Margaret Mead: “Never doubt that a small group of citizen (scientists) can change the world – in fact it is the only thing that ever has.” Stephanie von Gavel and Dr Annie Lane contributed to this article. They are the chair and immediate past chair of the Australian Citizen Science Association.Adam Smith is a member of the Australian Citizen Science AssociationGregory Andrews is affiliated with the Canberra Writers Festival and Lyrebird Dreaming Pty Ltd.. Steve Turton is an elected member of the national committee of the Australian Citizen Science Association. He has received funding from the Australian government.