https://phys.org/news/2018-10-uk-bumblebee-population-trends.html

Britain's 25 bumblebee species are some of the nation's favourite creatures and are also vital for the pollination of crops, garden plants and wildflowers. However, they have suffered huge declines over the past century: two species went extinct in the past 80 years, and eight species are endangered. These species were known to have declined in distribution over the long term but little was known about how bumblebee populations have changed more recently.

Hundreds of BeeWalk volunteers together walked nearly 5,000 kilometres each year to gather information about the numbers, species and caste (queens, workers or males) of the bumblebees they saw and identified.