Here are a few things you do with a prickly pear cactus: Get poked. Turn its fruit into jam. Use it to clean up dangerous concentrations of selenium in arid California lands.
In California's San Joaquin Valley, a long history of artificial irrigation has impregnated the soil with selenium. In small quantities, selenium is beneficial to humans and animals -- essential, even. In larger quantities, it’s toxic.
Prickly pear cacti, though, can thrive in these soils, even though irrigation has also made the soil and water dangerously salty.