"Across the world, invasive species pose a danger to ecosystems. The UN Convention on Biological Diversity ranks them as a major threat to biodiversity on par with habitat loss, climate change and pollution. Tackling this isn't easy, and no one knows this better than Hugh Warwick, a conservationist who loathes the idea of killing, harming or even eating animals. Yet as an ecologist, he is acutely aware of the need, at times, to kill invasive species...