N A HOT DAY in July, several visitors to Yosemite National Park, along the mountainous spine of California, go on a bear walk. No bears are spotted, but the group learns about what they eat (wild raspberries), where they frolic (lush meadows) and what to do if you encounter one on the trail (don’t run!). In the busiest part of the park, tourists trek to waterfalls and queue for snacks and T-shirts at the visitor centre. The bathrooms are dirty, but no worse than when your correspondent visited last summer. All seems well, yet there is trouble lurking.