Along Interstate 5 in southern San Diego County, there is what seems to be a saltwater marsh. Native grasses are flourishing, and a symphony of sound comes from insects and frogs. But the marsh is fake. It was built by the state as compensation for wetlands beloved by birds for nesting and mowed down by man for freeways. The dirt was trucked in, the water channeled through man-made diversions. Sadly, none of this seems to be fooling the birds. Neither the endangered light-footed clapper rail nor the endangered California least tern--supposedly the prime beneficiaries of all the fuss--has been spotted at the marsh since its creation four years ago.

This artificial wetland is a harbinger of the fate that awaits Southern California's wildlife. Shoved off its territory by people, wildlife will increasingly have to depend on man-manipulated environments for survival. In many ways, these well-intentioned, high-tech fixes smack of desperation, built to offset the taking of natural habitat. In the San Fernando Valley, a 4-foot-deep "lake" with a drain on its bottom is filled with treated effluent every autumn for migrating birds and emptied every summer to keep down the mosquitoes. A proposal to restore steelhead trout to Malibu Creek would use a $500,000 elevator to lift the fish over a man-made dam that interrupts their run.

Conservationists applaud such efforts, reasoning that man-made wildlife habitat is better than no habitat at all. But the success of these costly, Disney-like creations is not always assured. What took nature eons to build is not easily duplicated by man in months or even years. Worse, these and other so-called "mitigation" projects may lull government planners into believing they can allow the scattered remains of Southern California's wild lands to be paved over at no cost to their original inhabitants.

Agency's 'Vietnam'

Already, the toll is high. Tree frogs, pond turtles, horned lizards and rosy boas that a youngster 25 years ago might have played with in his Los Angeles County back yard are seen rarely, if ever, by his children in that same back yard today. There is much in nature now that may not be there for the next generation. More than half the state's rare or endangered plants and animals reside in Southern California, and wildlife officials are not optimistic about saving them. "Southern California is this agency's Vietnam," said David Klinger, spokesman for the U.S. Fish and Wildlife Service. "We have intense problems there, but we don't have the ability to control (the causes) . . . development, urbanization, people pressures."