I graduated from high school in Central Iowa, when there were zero deer in my county. Pheasants were getting hard to come by, and there wasn't even much cover left for a rabbit. I attended my 20 year reunion, and all I heard about were the deer. Pretty hard for me to believe too. But the deer have adjusted to man, and live in the weeds, tall grass, and corn.
They also make long migrations from the river bottoms, where they Winter, to spread across the state. Stayed at a friends house last Spring, and he had deer on a tiny food plot all day. Almost never looked at it without seeing a deer. Last Fall he arrowed a 17 pointer off that food plot, which was across an open pasture, 100 yards behind his house. Another friend hunts near Red Rock Resevior. Says his hunting party made one drive, and spent the remainder of the day dragging out about a dozen bucks.
I remember visiting my in-laws over the years. At first there were no deer... then a few... next thing I knew they we infested. I can't explain why, it just happened, and like Iowa beef, those deer are big fat guys. Defies logic though, cause it sure doesn't look like what we think of as deer habitat.
Here is one explaination: If you were to hunt in Iowa, where would you hunt? Almost all the woods in Missouri get hunted, and a good portion of the deer get harvested because of it. Well, in Iowa the deer leave the woods, and hide in standing corn and weeds. So, they live and breed, cuz all the standing corn and weeds don't get hunted.