In a bid to put invasive species on restaurant menus, Silo -- a zero-waste venue in London -- serves up a range of destructive non-native ingredients. "They're all delicious" says Douglas McMaster, Silo's director. At one of the restaurant's special dinners, American Signal crayfish are the focus. Introduced as a food source in the 1970s, the species has spread throughout the UK's waterways, decimating the native white claw variety, of which populations have dropped by between 80 and 90 percent.