Many thanks to Amir A. from Israel for sharing today's photograph of Leuchtenbergia principis.
Agave cactus or prism cactus is native to northern and central Mexico. If you are familiar with the world's deserts, you'll know that this distribution means it is found only in the Chihuahuan Desert, an area with exceptional diversity of cacti.
There are a number of ways of measuring biodiversity. In some methods that use numbers of genera and species along with a uniqueness value, Leuchtenbergia would be considered to be contributing to high biological diversity, as it is a monotypic genus (only one species in the genus). Other methods that use genetic diversity as a measure might rate Leuchtenbergia lower; it is closely related to the fairly common Ferocactus (and can hybridize with it). In this way of measuring diversity, an area with Leuchtenbergia and Ferocactus would be considered to have lower diversity than an area with Leuchtenbergia and the so-distantly-related-as-to-be-negligible Fouquieria splendens.