One other potential factor not yet discussed is overpopulation of other species using the same habitat. For 20 years now we have heard the stories of the snow geese exploding population destroying their northern nesting grounds. Don't they eat the same lichen as caribou? So two overpopulated species impacting the same resource and they should both be headed to a cyclical population crash. I think the geese have a better chance of making an adjustment to survive. In 2000 I watch a flock land in a rice field in AR. They formed a tornado shape as they circled to land and the flock stretched to the horizon. I watched for a good 45 minutes and as darkness fell they were still coming from the horizon. I have no idea how many but it may have approached 100k. Next morning the rice field was bare.
As I said earlier, glad I got to experience the greatest big game migration in North America when it was at the peak for my lifetime.
