The sequels haven't been coherent about this topic. The plot of the original film talked about a leaf shortage that made the herds leave their homes in search of the Great Valley. Their World was a desolate place, a desert of sorts, with little to no food, and the threat of Sharpteeth always looming around them.
In the sequels 2-5 we see that the mysterious beyond is still a desolate place, roaming with Sharpteeth, with only a few spots of green. Except for the Valley of the Mists, no other place seemed habitable or big enough to hold a herd the size of the population in the Great Valley.
However, from LBT VI, onward, we see that the mysterious beyond has changed. It’s now filled with trees and other leafeaters choose to live there, in spite of the danger from Sharpteeh.
Did the first movie intend for us to think that the dinosaurs were becoming extinct, and that their only salvation was the Great Valley? Or was this a localized event? Why don’t the other herds stay in the Great Valley too, since it’s safer and big enough to hold them? What’s it like in the outside World?