In Africa, the average cost of an AK-47 is a little over $300. A single rhinoceros horn — reputed (falsely) in Asia to have cancer-healing properties — can bring $300,000 on the black market. So a criminal gang or a terrorist group knows what a dead rhino is worth: several hundred assault rifles (less the commission owed middlemen). Al-Shabab, the Somali terrorist group behind the Westgate mall attack in Kenya, uses poached ivory to fund its operations.

So what is a rhino worth to us? At one level, we value rhinos because we want endangered species and wild places to exist, even if we never see them. Respecting and preserving nature reflect an ethical impulse.

But determining the full value of a rhino is more difficult. In some countries, where preserving animals and habitats are keys to tourism, losing these things imposes a steep economic cost. When terrorist groups trade in elephant tusks or rhino horns, the security costs are potentially very high.