A friend tells you that vintners use yeast that produce alcohol to turn grape juice into wine.

A friend tells you that vintners use yeast that produce alcohol to turn grape juice into wine. He also observes that wine must still have living yeast in it because vintners don't pasteurize wine to kill the yeast. To make wine himself, he explains, he added wine to grape juice with the expectation that the yeast would turn the juice into wine. It didn't work, and your friend wonders why not. In terms of population ecology, what can you explain to your friend about the death of his yeast?