Thousands roar by Treasure Island every day without a passing glance. That could soon change…radically.

Listen to Alison Hawkes’ companion radio feature on The California Report, Monday morning, and see a slide show of the island’s transformation, below.

San Francisco’s twin islands in the Bay – Treasure Island and Yerba Buena – are not exactly jewels of nature. Although they have stunning views, a half-century of use by the U.S. Navy and years in redevelopment limbo have taken a toll.

Some sites on Treasure Island are severely contaminated, and much of the island is cracked asphalt and derelict buildings. Yerba Buena is solid rock but Treasure Island is entirely artificial, conjured from bay mud as an engineering showcase for the 1939 World’s Fair. As time passes, a corner of Treasure Island is gradually sinking into the sea. Rising sea levels as a result of climate change could subsume the island entirely, returning it back to its natural state, which is to say underwater.

In short, the place needs some serious help and this is where a massive multi-billion dollar redevelopment takes stage. Private developers want to transform the islands into a high-density “eco-city” with as many as 20,000 residents, making use of the best that technology and city planning have to offer in sustainable development.

But some environmentalists are critical of the plans. Mike Lynes, the conservation director of Golden Gate Audubon Society, says bay wildlife has been suffering with the loss of about 40% of open water habitat and 90% of wetlands.

“The nice thing about Treasure Island and Yerba Buena Island is that they’re relatively unpopulated compared to most of the central Bay,” Lynes says. “It was highly developed, especially Treasure Island, so the biological resources there are very limited right on Treasure Island itself, but Yerba Buena has areas rich in bird species and butterflies.”

Lynes says 20,000 people — nearly tenfold the number living there now – will take a toll. High-rise buildings pose a hazard to birds, trash attracts predators to native species, and cats and dogs kill wildlife, he explains.

He’s looked in detail at one aspect of the plans – a high-speed ferry that will take island commuters to San Francisco. Lynes says he’s worried that the ferry will disturb birds that raft together in the bay during winter as they rest up for the spring migration.

But Craig Hartman, a design partner at the San Francisco architectural firm SOM, and the development’s master planner, says the project’s net impact on wildlife will be positive, considering the conditions out there today. Asphalt – which sends contaminated rainwater directly into the bay – will be replaced with parks. Three-quarters of the islands will be open space and new plantings will replace invasive species with natives.

“It’s actually a major transformation of the constructed natural system,” Hartman says. “This is an interesting anomaly because this island is not a natural place and we’re now constructing wetlands and green space that’s never existed there. So it will be a major new sanctuary for wildlife that has not existed in the past, especially for bird life.”

The debate illustrates a longstanding tension within the green community about whether people – by their very presence – are a harm to nature. Or whether they can, with proper planning, play a positive influence on wildlife, even in densely populated areas.

Two brands of environmentalism are at odds – the wildlife preservationists versus advocates of “smart growth” strategies, which include higher-density, transit-oriented communities, to reduce car travel and greenhouse gas emissions. Lynes sees no easy answer.

“It’s more energy efficient if people live in tall buildings than if we live in a more sprawling suburban lifestyle,” he says. “I have particular concerns about wildlife, but I also acknowledge that if were going to have smart growth in the bay area we have to figure out how to balance those values.”