A dog's nose, of course, is where the trouble all begins.

Homer and Chase are running through Solstice Creek in the Santa Monica Mountains on a warm Sunday afternoon. They are splashing in shin-deep water, darting past salamanders and frogs and jumping over creek-side brush. Inside their long, whiskery snouts, the black Labrador and the Australian shepherd have at least 20 times more scent receptors than a blunt human nose, and the small slits along their nostrils allow them to draw extra air over those sensitive cells.

No wonder, then, the dogs in the creek are experiencing a sensory deluge: the aroma of eucalyptus, the scent of a tree-bound squirrel, a nearby jack rabbit. And then there are the sounds, all the sounds: splashing water, breaking twigs, chirping birds and so much more. A dog has a range of frequency three times that of a human.

Homer and Chase bound through the water, eyes wide, nostrils flared, ears perked. It doesn't get much better than this.

But the fun ends when the soggy pooches dart from the creek and run headlong into Bonnie Clarfield, a National Park Service ranger known as the "dog narc" for her tenacious enforcement of the park's leash laws.

"Where is your owner?" she asks from a nearby trail.

Seconds later, Austin Grady climbs out of the creek, a pair of leashes in his hands, a guilty expression on his face. Clarfield whips out her ticket book and fires off a $50 citation (as of July 1, the fine for violating the leash law in a national park is $75). In the Santa Monica Mountains National Recreation Area, all dogs must be on a leash.

"They were just playing in the water," Grady protests, waving the leashes in the air. "It's unfair to keep them on a leash."

Dogs have accompanied their owners into the wilds for centuries. In 1880, John Muir explored the icy regions of southeastern Alaska in the company of a little black dog named Stickeen. At first, the Scotsman objected to the canine's presence, but after the journey ended, he praised the animal's tenacity and intelligence. That stumpy-legged mutt became Muir's most cherished outdoor companion.

Today, Muir might be surprised at how completely dogs have insinuated themselves on trails and off, across beaches, forests and deserts.

On the shelves of most outdoor stores, dog owners can find a swath of dog hiking products, including portable water bowls, dog tents, leather hiking booties and backpacks designed for four-legged packers. In the past two years, sales of such items at REI alone have jumped 40%. Dog hiking clubs have sprouted across the country, from Maryland to Alaska.

Whether they're wallowing in the creeks of the Santa Monica Mountains or scrambling up the White Mountain range in the Eastern Sierra, dogs have become another trail user competing for access, and owners, who take great pains to train trail-savvy dogs, assume their companions have no greater an impact than they do.

But the growing popularity of this pastime is changing the way we think about dogs and the outdoors.

Stickeen is long gone, and the areas where dogs are allowed to scamper are also disappearing, not entirely because of the behavior of the owners but because of what dogs naturally do.

Blame the nose, but an increasing number of park biologists claim that man's best friend is not compatible with plants and animals in the wilds.

Leash law scofflaws

BACK at Solstice Creek, Clarfield tucks her ticket book into her backpack. She can't begin to guess how many tickets she's written for leash-law violations. Too many by her standards.

"We expect people to do what they are supposed to when they get into a park," she says.

For years, the primary rationale for leash laws was to restrain dogs from sniffing, pawing or mauling other park visitors, particularly young children. The leash issue still creates tension among hikers, and one highly publicized case that reverberated through the dog-hiking community is a reminder of how volatile conflicts over dogs can become.