If we interpret Nature Awareness to detect "animate movement", then I think the counter would actually be the various Awareness skills, and possibly an Orientation roll. Want to "listen" (focus) on worm movements? Go ahead...but roll to pick it out amongst all the noise. Think of it like looking at the motion tracker on Aliens. If lots of big things are stomping all around you, it's hard to pick out the small stuff. Don't count on hearing worms in the middle of a stampede. That said, NA would allow you to stretch your senses far beyond normal mortal means...so if things were quiet, you could hear the worms.
Likewise, you'd have an almost guaranteed chance to detect someone sneaking up on you in the middle of the desert: just not a lot moving out there. However, NA wouldn't help with this on a bustling city street.
I think my interpretation follows the theme of the Spell List well enough, and is workable.
How, then, to sneak up on the Ranger in the desert? Admittedly, it would be tricky. Anything to break Concentration helps, of course: a distracting illusion, a flock of summoned birds, tripping over an imaginary tortoise...Then again, Long Bows have great range.