The humble sparrow.
In 1958 Chinese leaders introduced the "eliminate sparrows campaign" (as part of the Four Pests campaign), because sparrows were thought to consume too much grain and fruit. The campaign severely depleted the sparrow population, pushing it to near extinction.
However, the Chinese soon found out that sparrows eat a lot of locusts, being practically the locusts' only predator. With all the sparrows gone, the locust population exploded. Massive locust swarms moved across China and devoured any grain that they came upon. The result was the Great Chinese Famine, one of the greatest man-made disasters in human history, which killed millions of Chinese people.
The Chinese government eventually resorted to importing 250,000 sparrows from the Soviet Union to replenish their population.