Aldo Leopold, an ecologist, wrote about his home state of Wisconsin in The Sand County Almanac. In one moving section, he described a triangular cemetery, founded in the nineteenth century, that, because of its unusual shape, contained a tiny patch of prairie, unmowed and undisturbed. Every year, sometime in July, Leopold would watch a single silphium plant bloom there, the only one he found in that part of the state. He used this example to discuss the many plants native to the prairie that have been replaced by a few commercial plants grown by farmers. The disappearance of native plants is an issue of: