"Should" is a word I'm uncomfortable with. Define your goals, and I can provide an answer. If you want to re-establish the wild ecology of ~4000 years ago? Y

"Should" is a word I'm uncomfortable with. Define your goals, and I can provide an answer. If you want to re-establish the wild ecology of ~4000 years ago? You need to get the people out. Modern human habitation & agriculture claims a right to every last square meter of land, and even the supposedly reserved natural areas are often cut & planted as if they were pseudo- lumber factories. If this is your goal, irradiating wide swathes of land is a good start; There are few simpler or more reliable mechanisms to force sustained evacuation and shunning of an area for the centuries or millennia necessary to re-establish, for example, old-growth forest. Some legal regime might buy you a few decades, but don't kid yourself about the longevity and commitment of a particular political system. The changes in species composition associated with the climate, then with megafauna, and later with the Columbian exchange & globalization, are going to pose some durable problems if you want it to look exactly the same, but you can form a complex ecology with most of the same niches filled with at least similar creatures.