When he goes far enough along the beaten path, Erwin will see one (1) Tropius. It's the obvious cause of the vibrations, as it's currently... Trying again and again to lean up to reach the top of what looks like a plantain tree, to eat its delicious not-banana fruits. It's a bit smaller than an adult Tropius however, so it can't reach quite yet. Even when it gets up on its hind legs to try and snap at the fruit.