Three decades ago, a middle-aged doctor sat outside his northern Michigan home and saw a patch of endangered paradise. A beekeeper and amateur naturalist of prodigious energy, John Tanton had spent two decades planting trees, cleaning creeks and suing developers, but population growth put ever more pressure on the land. Though fertility rates had fallen, he saw a new threat emerging: soaring rates of immigration. Time and again, Dr. Tanton urged liberal colleagues in groups like Planned Parenthood and the Sierra Club to seek immigration restraints, only to meet blank looks and awkward silences. “I finally concluded that if anything...