His first book, The Dark Range : A Naturalist's Night Notebook, an exploration of nocturnal natural history and ethology set in the Yolla Bolly-Middle Eel Wilderness of northern California, was written as an M.A.
Wallace has also been categorized as a science writer, and his work contains much scientific information, but it also has folkloric, philosophical, and religious dimensions.
This has led to controversy, as when a March 20, 1983 New York Times Book Review piece on his third book, The Klamath Knot, by then Times reporter Clifford May, accused him of playing "fast and loose" with concepts like evolution and mythology, getting "caught up with what are apparently attempts to alchemize science into poetry."
On the other hand, the eminent botanist and co-founder of the "neo-Darwinian synthesis," G. Ledyard Stebbins, described The Klamath Knot as: "A classic of natural history that will take its place alongside Walden and A Sand County Almanac."
He has been an advocate of parks, wildlife and wilderness protection in several areas, especially the Klamath/Siskiyou Mountains region of northwest California and southwest Oregon, the subject of four of his books (including the Klamath Knot) and Central America from Chiapas to Panama, the subject of three of his books (including The Monkey's Bridge).
Biologist Daniel Janzen called his 1992 book, The Quetzal and the Macaw: The Story of Costa Rica's National Parks, "a major contribution to tropical conservation."
His 2007 book, Neptune's Ark, explores the evolution of western North America's marine megafauna, one of the world's most important although less well known than terrestrial ones.
Botanist Bruce Pavlik called it "a clear and entertaining story about the origin of California's desert that invites the reader into a world of ancient mystery and modern revelation."
It stresses the genre's role as a source and manifestation of a growing conservation movement as Thoreau and Muir developed the idea of the national park in the nineteenth century, when the first parks were created, then as Rachel Carson, Aldo Leopold and others laid down the tenets of modern environmentalism in the twentieth century.
It includes the recognition that nature is not benign or hostile, it is itself, obeying no human laws... Wallace shows that the American landscape and most of its ecosystems have been altered irrevocably by what we have done to them and that we simply do not have anything like enough knowledge to have any idea of the long range effects of our tampering.
Two books on evolutionary history, The Bonehunters' Revenge and Beasts of Eden, try to stress the importance of evolution as a matrix for the human past and future.