US used to maintain bat houses so they kill insects and drop guano. Wars were fought for guano caves. Then synthetic nitrates (Haber process) happened and the bottleneck keeping population below 1 billion went away.

Reportedly, one of them had a bat infestation that produced a thick layer of guano in his attic, which he planned to use as fertilizer. He probably should have inspected his attic wearing a N95 respirator.