I should also add that national parks in the US are very different from the UK. In the US they are attempts to preserve a purely natural wilderness, with as little human intervention as possible. Indeed, some claim they go too far in seeking to remove all evidence of human occupation or activity.

In the UK a national park is simply a Town & Country Planning device, and recognises that people live and work within its boundaries.

Studies based on US research need to be interpreted in this light.