Nature is not what comes to mind when an outsider drives into Bel Marin Keys, a tiny community that begins 1½ miles east of Highway 101 in Marin County, reached by a single road that passes a shopping center and small industrial buildings along the way. The wide streets are monotonous, often lined with homes that resemble those of countless 1960s subdivisions. On some blocks, the only hint that creeks and wetlands might be nearby are the red-winged blackbirds that touch down on utility poles. … It’s a bucolic scene — and an engineering landscape that wouldn’t exist if not for the intrusions into former bay wetlands that now are at risk due to sea level rise. That’s why residents of Bel Marin Keys voted to approve a $30 million parcel tax this month aimed at building stronger and taller levees, plus an improved set of locks to keep adjacent waters from spilling into one of the lagoons that give this precarious collection of 700 homes its character.