Humanity today is witnessing the greatest mass extinction of species since the dinosaurs demise. Most of the vanishing breeds are wild, and many are found in remote locales like tropical rainforests. But some threatened species live much closer to home, in farmyards. About 30 percent of the world's domestic animal breeds risk extinction, the United Nations estimates. And their figures suggest that each week, one domestic animal species is lost forever. Some efforts are being made to stem the tide. Living on Earth's Daniel Grossman visited a farm in Maine which is saving threatened livestock for future generations. The genetic diversity of farm animals is also rapidly shrinking, perhaps putting our food supply at risk. To fight this trend, some farmers are raising what they call heirloom livestock.