The Agricultural Preservation and Open Space District has a unique opportunity to help restore San Pablo wetlands to a semblance of their former glory.

If the Board of Supervisors approves the purchase of a 650-acre ranch near the onetime proposed casino site on Lakeville Highway, the public would gain an important link in the 400-mile-long San Francisco Bay Trail.

More importantly, the Dickson ranch would be another piece in the puzzle of extensive wetlands conservation efforts.

It's difficult to imagine, but from the mid-1800s through the 1950s, about 85 percent of San Francisco Bay's wetlands were filled in or dried out. In the northern reach, known as San Pablo Bay, dikes were formed to hold back the waters to create salt ponds and arable farmland.

The tide turned when the public recognized the importance of wetlands to wildlife, to migratory birds and to the health of the ocean. At the same time, restoration opportunities became available when the Navy decommissioned Skaggs Island and Mare Island.

The Open Space District, along with other state and regional agencies, rightly made the purchase of properties that front the bay a high priority.

Because water is difficult to control, it's important that long stretches of neighboring properties be owned by the public. The Dickson ranch is a critical link in restoration efforts.

For this reason, it's difficult to know whether one critic is right in arguing that the $5.1 million purchase price - of which the district's share would be $2.6 million - is too high. While it's a lot of money to pay for a hay and cereal grain ranch, is it too much to pay to preserve a unique piece of natural history?

As with all purchases, the district relied on an independent appraiser to assess the property's value. The reality is, given the nature of the land, comparisons to other properties are difficult to make.

The bay will never entirely regain the wetlands that have been covered with roads, homes and businesses. But every effort should be made to restore as many acres as possible to places where birds, fish and wildlife can once again thrive.