I think that much of the decline can be linked to the decline in subsistence hunting. My father, who is 71, and his brothers hunted to put food on the table. This developed an enjoyment of hunting that they passed down to their sons. In our generation, we have seen a change in the work environment and a shift in land management that has diminished the time available for hunting and the access to quality hunting land. While I absolutely enjoy hunting, I don't hunt nearly as much as my fathers generation. I don't need to put meat on the table in that manner. If I buy property to hunt on, I can't hunt as much because I have to work harder to pay the tremendous expense. Lot's of reasons. In KY, hunting has come under more regulation on government land which has changed access.
As to chemicals, I'm not convinced that the chemicals themselves are causing the demise so much as the practice of land management that is represented by the use of chemicals. In part, farming practices have changed because the farmer has a hard time surviving if he doesn't use every inch of land that will produce. This leads to the use of chemicals to control border/transitional areas that, in years past, were left fallow. The old practices allowed natural areas of transition that are ideal for food growing plants and cover/egress for wildlife in general.
We could argue that government regulation of farm practices has led to much of the changes but I'm speaking of my area of the country just east of the Mississippi.
