SANTA ROSA -- It surprised backers of the planned runway expansion at Charles M. Schulz--Sonoma County Airport when federal wildlife regulators earlier this year said the presence of an endangered flower in the project area might be assumed though not found specifically there in surveys, potentially needing more conservation measures estimated to boost the job cost by at least $10 million and delay it a year.

But environmental law experts say such presumed presence of a protected species, though new to a number of North Bay real estate developers, is part of an evolving, if sometimes invasive, regulatory policy over the last two decades.

"The Endangered Species Act requires agencies to resolve all doubts in favor of the species," said Dawn McIntosh, an environmental law attorney in the Los Angeles office of Meyers & Nave. "If the agency does not have staff to prove it's not there, they have to assume the species is there."

That approach has become known as the "conservation principle," she said. The federal Endangered Species Act has developed since its beginnings in 1973 when a "taking" of a listed species was basically direct harm to an animal or plant to increasingly connecting species with their habitats over the past two decades, according to Ms. McIntosh. Local examples of that connection include spawning streams in the Russian River watershed for Coho and steelhead salmon and seasonal ponds on the Santa Rosa Plain for the California tiger salamander.

Yet protected plants are more problematic to prove presence because they can go dormant in the soil or spread from one place to another by animals or the flow of water, according to Ms. McIntosh. So regulators make the case that it's reasonable to assume presence, particularly when a given plant is found nearby but not in a project area.

Presumed presence has been a hot segment of environmental case law in recent years, according to Brenda Davis, a Sacramento-based environmental law attorney who advises the California Farm Bureau Federation.

"This is pushing beyond what the law requires," she said. "This is an ongoing power struggle."

Regulator biological opinion documents that specify little to no impact of a project on species generally can be relied upon if the species or habitat situation doesn't change before construction begins, according to Ms. Davis. But the statuses of listed species are reviewed every five years by law, and any new information could be worked into mitigation plans should the project need federal regulatory review again, such as the involvement of a wetland.

That's why project owners have to stay on top of such new updates, according to Ms. Davis.

Hydrology and animal patterns factored into the big increase in planned mitigation for the Sonoma County Airport expansion project. Project environmental consultants in the Point Richmond office of LSA Associates had conducted three years of biological surveys, the most recent in 2010, of 4.5 acres of seasonal wetlands that would be affected by the project and didn't find the endangered Burke's Goldfields flower there, according to Jon Stout, airport manager. The Federal Aviation Administration, which is paying for about 95 percent of the $53 million expansion by various means, signed off on that determination this past February.

"Fish & Wildlife said the airport does have three acres of Burke's Goldfields, and with observed hydrology and animal movements, the seeds could have migrated to those [project] areas, so to exercise caution, the presence of Burke's Goldfields should be assumed," Mr. Stout said.

The expansion plan called for purchasing credits from a Santa Rosa Plain mitigation bank based on the commonly used ratio of 1.5 acres of bank land for each acre of project land with suitable species habitat. The wildlife service's presumed-presence idea would call for a 3-to-1 ratio, or 13.5 acres worth of credits.

"We've talked to several [mitigation] bankers on the Santa Rosa Plain who say that more and more agencies are assuming presence, and most developers don't have the wherewithal to fight," Mr. Stout said.

Mitigation banks are landowners that have preserved or created wetlands or species habitat and received bank-enabling documentation from regulatory agencies to sell acreage "credits" to project owners.

One such bank on the Santa Rosa Plain is the 38-acre Slippery Rock Conservation Bank, managed by Chris Peterson of Rivendale Homes.

"We have [California tiger salamander] and wetland credits, and we have some flower credits, but we have been trying to find out if we need [the flower credits] for a new housing project that been in the entitlement phase for 11 years," he said. "What an agency will tell you one year may be different another year."

After checking on the availability of Burke's Goldfields credits at Santa Rosa Plain mitigation banks early this year, county officials found that the supply of approved credits for the flower was short, so the county is pursuing the OK to use a bank that faltered when the housing boom crashed in 2006 and the need for credits largely dried up for several years, according to Mr. Stout.

The county could pursue an appeal if the wildlife service biological opinion presumes Burke's Goldfields presence for the airport project, but the FAA advised to just move forward with 3-to-1 mitigation, according to Mr. Stout.

"We were told we could argue this and fight, but we don't have the six to nine months it would take to do that," he said. As it is, the current conservation plan already pushes the project into 2014.

The wildlife service is still mulling over mitigation measures for Burke's Goldfields at Sonoma County Airport, according to a spokesman.

"We are developing a biological opinion for the airport project, but it is not finalized," Robert Moller said.