Sergey A. Zimov, author of this month's essay, is the director of the Northeast Science Station in the Republic of Sakha (Yakutia) in Russia. In "Pleistocene Park" he hopes to reconstitute the long-gone ecosystem of the Pleistocene epoch that supported vast populations of large animals including mammoths, horses, wolves, and even tigers. His bold visions of controlling and restoring ecosystems have earned him coverage in books, documentaries and other media.