Hm, I would guess that it is the neonicotinoids pesticides. If I could point to my general recollection of the decline, I'd very vaguely point to that and when "roundup" became a thing.
It does seem to be even more global than that though. So global warming is probably the main culprit.
Look out the window next time you're in a jet. In america it seems like we have industrialized everything: food production, suburban housing, etc. The scale of our civilization is terrifying, and America is peanuts in terms of population (but NOT in terms of resource consumption per population, the far more important metric).
The fact is economists can't estimate the value of an ecosystem, habitat, or wilderness. It's just untapped money to them if it's torn down. If economics, which directs 95% of our policy, can't get their heads out of the asses and figure out how to properly account for the existential value of nature, we are doomed.
The fact that a pseudo-mathematical/pseudo-scientific area of study, one filled to the gills with venal denialist ultra-right-wing orthodoxy is the driving force of our future is not encouraging.
Economics is useful in the small and medium scales, and in very limited large scales, I'm not going to sit here and tell you price theory, supply/demand, and game theory are all bunk, but the political force that is "economics" is a key contributor to our dire situation.