The decomposition of dung constitutes an ecosystem service of massive proportions. Previous studies addressing how it depends on individual invertebrate taxa have been focused on small spatial scales, neglecting the impact of large-scale factors like climate. Here, we use the concept of “citizen science” to quantify taxon-specific contributions to dung decomposition at the level of a nation. Young people across Finland manipulated the decomposer communities of cow pats, then measured changes in pat mass over the grazing season. In southern Finland most (90%) of the cattle dung hitting pastures decomposed in just two months, whereas 1100 km to the north the corresponding fraction was smaller (74%). Of the total invertebrate-caused decomposition (13% of dung pat mass, independent of latitude), large tunneling dor beetles in the genus Geotrupes account for 61%, hence removing dung twice as fast as do smaller dung-dwelling beetles and earthworms. Overall, this paper illustrates how ecologists may direct citizen scientists to implement massive ecological experiments. Compared to an approach based purely on professional scientists, we saved three-quarters of the costs. Ultimately, citizen science may offer a key tool for testing current ecological theories at relevant spatial scales—and for disseminating these theories in the process.