system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do we know exactly that the bee population around the world is decreasing? How do we calculate the number of bees to begin with?
gpt
Bee keepers know how many bees they used to have, and how many bees they have now. Commercial bee keeping is a huge industry (many bee keepers rent their bees out to farmers, for example), and when commercial bee keepers, amateur bee keepers and people who plain notice bees notice colonies are collapsing, it's worth paying attention to.