Halfway through a five-year effort to preserve wildlife habitat and farmland in Central California, the David and Lucille Packard Foundation issued a report yesterday saying that it could not go it alone and that the land preservation movement needed long-term support from the state government to be successful. The foundation is funding a $175 million program to conserve more than 500,000 acres of land and has already helped to protect 327,000 of them. An earlier report by the California Environmental Dialogue, a coalition of business and environmental groups, concluded that 5.4 million acres of land in the state merited protection, but the price tag for buying them would run to $12 billion.