Shhh, and Not Because the Fauna Are Sleeping

February 22, 2011, 3:18 pm

At times, deep within this vaulted chamber of redwoods, it is almost quiet enough to hear a banana slug slither by. For the National Park Service, that stillness is as vital a component of the site as the trees’ green needles, or the sudden darting rays of sunlight.

A decade after the agency resolved to restore natural sounds to this park in a metropolitan area of seven million people, managers at Muir Woods, in Marin Country just north of San Francisco, have made big strides in vanquishing intrusive noise. Now the background sounds are dominated by the burbling rush of Redwood Creek, the soft sibilant breeze that stirs the redwood branches, the croak of a crow.

Humans do contribute, too, but, with the exception of toddlers’ squeals, their voices tend to be pitched lower than usual.

The impact of noise on wildlife ranging from birds to whales to elk has been a growing focus of scientific study. Increasing evidence suggests that animals in natural settings modify their behavior, though sometimes only briefly, in response to human commotion.

In a 2009 article in Park Science, researchers explained that animals react to human intrusions as if they were suddenly being threatened by predators.

“These disturbances evoke antipredator behaviors and interfere with other activities that enhance fitness,” the article said, like foraging for food, mating and tending to the young. When such disturbances grow frequent, the researchers warned, “population consequences may result.”

By 2001 or so, Muir Woods had in fact long been abandoned by otters and piliated woodpeckers, and park managers had grown concerned that sightings of a pair of northern spotted owls, an endangered species, were becoming more and more infrequent.

There were other worries besides noise levels. An asphalt walkway was cramping the growth of the redwoods’ surprisingly shallow roots in some places, causing at least one tree to topple. And park visitors were straying from the path into the groves, compacting earth that was meant to be loose and harming the redwoods further.

But the noise question was the most vexing. The pathway could be altered, and was: in many places a slightly elevated boardwalk has replaced it. Visitors are firmly advised to stay on the paths. But the clatter and rumble of garbage can lids and maintenance vans remained.

Today, no Dumpsters or garbage cans are to be found along the trails. Maintenance vehicles powered by electricity glide by almost silently. Workers in emergency vehicles do not idle their engines while resolving whatever problem brought them to the park.

Once the diesel engines had been stilled, visitors began falling into line, heeding a subtle signal that human noises are superfluous here.