Probably birds, bats and other species of insects. But there was never agriculture on today’s scale here in the absence of honey bees.
“Probably birds, bats and other species of insects. “
The other insects, particularly, could quite likely be undergoing similar declines, but aren’t noticed because they aren’t used for human uses like honey bees are.
From what I understand, it’s certain pesticides and insecticides that are thought to be responsible for a great deal of the decline, particularly neonicotinoids, which are similar to nicotine.