In 1845, potato plants started dying in Ireland. The next six years became known as the Irish Potato Famine. An airborne fungus called Phytophthora infestans killed the potatoes, and more than a million people starved. A major factor in the famine was a lack of biodiversity.
Although this might sound like something that couldn't happen in today's industrialized world, a lack of biodiversity can still threaten specific plant and animal species. One of today's examples is the Cavendish banana. The most popular banana choice in the U.S., the Cavendish, like all bananas, is a clone of one parent. And all the Cavendish plants in the world are genetically identical. Two blights currently threaten it — although there will still be other bananas should it become extinct.
The drop in biodiversity also ties in to habitat loss. A smaller space in which to live leads to a smaller population size, which leads to less genetic diversity. Human activities like industrialized farming play a role, too, as large fields of one type of plant replace a diverse population of plants and animals.