Ok i have had an idea about replacing the trees for a more appropriate blockages.
Instead there could be giant fissures/cracks in the earth between regions (as in evidence of the split between countries coming)
this could only make sense between regions and so ill have to move some blockages around
This is a great idea!
As for the bridge situation I can think of 2 solutions. Go with the ancient race solution and change "Terror Island" to "Atlantis" or "Hiva". The other would be to use pterodactyls for bridges (since they can fly over impasses) and Sea dinosaurs for ports.