Sliding down the side of a ravine, hoping he doesn't hit stickers on the way down, Mark Dion is in his element. Elsewhere in this forest, a team of volunteers has spread out to assist the visiting New York artist in gathering specimens—beetles, spiders, mushrooms, berries, bark, gothic sphagnum moss. A pile of black bear scat, full of berries, is a treasure; it's scooped up and put in a plastic bag to be taken back to the city.

This is the annual trip to the Tacoma watershed taken by the volunteers who staff the Neukom Vivarium at the Olympic Sculpture Park. The Vivarium, an artwork by Dion, is the nurse log that lies in state in a greenhouse next to the railroad tracks at the park. There are 50 volunteers who take shifts wearing orange vests and talking to visitors about the artificial ecosystem inside the greenhouse. They know every inch of this indoor green explosion, which at the moment has reached obscene heights of lushness. They notice when the first elderberry of the season pops. They notice if a millipede looks different from the others. Their seriousness is by turns inspiring, endearing, and hilarious.

A volunteer walks up to Bobby McCullough, the park's gardener, in the parking lot at the watershed. "What are those three little piles that have just appeared beneath the log?" she asks.

"Termites," he says.

"But..." She describes why she does not think it is termites. McCullough smiles politely. "So I've stumped the gardener!"

"It's termites," McCullough repeats. She walks off. This happens often; these volunteers feel very much like this is their work of art, and even when they ask the professional gardener for information, they don't necessarily trust it. "But those volunteers"—the difficult ones—"turn out to be the best ones," McCullough says, laughing. He is a patient man.

At the center of the Vivarium, which opened in 2006, is the 60-foot decomposing/regenerating hemlock on life support. McCullough keeps the temperature inside the greenhouse the same as at the Tacoma watershed where the log was found. But the volunteers hold the keys. If they don't show up, the Vivarium doesn't open for the day. And they give the sculpture a constituency.

"Because of them, some boneheads at the museum in 10 years won't be able to say, 'Let's use this space as a gift shop,'" Dion says. His relationship with the volunteers is one of mutual adoration. They nerd out together over lunch at the watershed, in a room overlooking the Cedar River. Dion, who is also on James Corner's team redesigning the Seattle waterfront, likes to say that Seattle is the nation's most environmentally literate city: Their conversation is only interrupted when someone spies an osprey out the window; everyone's attention turns. (Especially the older fellow who says, hopefully, "Ocelot?")

Back at the park, with specimens in tow in bags and Tupperware, the volunteers gingerly place the bugs and plants into their simulated environment, recharging this unusual long-distance relationship.

"I've read the book on him," one volunteer said of the artist. "He calls this piece an abomination, and people are sometimes surprised when I tell them that." But she sympathizes: She grew up in Federal Way when it was cow pastures, before big-box stores; she's lost her own native habitat.

Another volunteer dumps a bag and observes a tiny, white-green spider curling into a ball, adjusting to its new home.

"Mazel tov," she tells the spider. recommended