This article doesn't mention the declining insect population at all but I would think it's clear that a quickly declining food source would be a major cause of population decline. I wonder if there's a similar decline in other animals that rely on insects, eg bats. Although, interestingly, the article does mention neonicotinoids as a direct reason for the decline in population. It's hard to imagine humans taking the…
Trantor required continuous supply from 20 agricultural worlds to survive. Pretty much all necessities of life for trantorian were externally sourced, with the possible exception of water.
Once that shut down after the 260 sack and the Imperial Family leaving for Neotrantor, population crashed from 40G to 100M.