Twilight of the Mammoths: Ice Age Extinctions and the Rewilding of America . 2005 . University of California Press , Berkeley , CA . 250 pp . $29.95 (hardcover) . ISBN 0-520-2314-4 .
Twilight of the Mammoths is an intriguing but not entirely effective combination of three disparate parts: memoir of scientific discovery, analysis of competing Late Pleistocene extinction hypotheses, and call for radical North American restoration. These various threads are united in falling within the broad interests of the author, who is justifiably well known for his overkill hypothesis (i.e., that human overhunting caused Late Pleistocene megafaunal extinctions) and for exploring the conservation biology implications of Pleistocene and Holocene extinctions. Although the book is inconsistent, the author's informed, passionate perspective remains worthy of consideration.
The first two chapters introduce overkill and provide an overview of prehuman Pleistocene faunas. Subsequent chapters address related topics. These include the importance of ground sloth dung and packrat midden analysis to Pleistocene studies; the paleobiology of ground sloths, which Martin considers the hallmark American megafaunal group; Grand Canyon ecology and paleoecology as an overkill test case; the global correlation between human arrival and extinctions; the question of when humans arrived in the Americas; the interpretation of archeological sites; and the particulars of competing extinction hypotheses. All of this sets the stage for a pair of concluding—and controversial—chapters on species restoration. Several short, free-standing essays are interspersed throughout the book. Most relate tangentially to the study's main subjects, but one offers an exceptionally clear and very useful overview of carbon-dating techniques. Although Martin recounts several key research experiences, Twilight of the Mammoths is only secondarily a memoir. The book's main concerns are Late Pleistocene extinctions and ecological restoration. Accordingly, I also focus on those topics.
The section on extinctions is the stronger of the two. Martin provides a thorough overview of the species lost, although the book deals mainly with North American faunas. Debate about the cause(s) of the extinctions generally centers on the relative explanatory merits of climate shifts and human activities, including hunting, the introduction of destructive exotics (e.g., rats, dogs), and landscape alteration (e.g., as from burning), and all these topics are covered. Although not impartial, Martin's assessment of the debate is informed, fair, and civil. He accurately observes that no existing climate-change model explains the observed extinction pattern but remains willing to have his hypothesis tested. He notes that two lines of evidence would cast serious doubt on overkill: first, unique features of Late Pleistocene climate shifts that could explain the loss of large mammals and, second, evidence for long-term coexistence of humans and megafauna in the Americas or Australia.
The few minor problems with Martin's treatment of the extinctions will pose difficulties primarily for those unfamiliar with the subject. A clear, concise definition of overkill would have been helpful. As presented, the term could reasonably be interpreted as referring either to extinctions resulting solely from hunting or those resulting from a combination of human activities including hunting. Similarly, although Martin is careful to cite works that contradict his overall conclusion, he does not always point out the range of opinion bearing on the specific evidence he presents. For instance, on the subject of prey naiveté, the literature is nowhere near unanimous in accepting that an accurate parallel can be drawn between the North American megafauna that the first Paleoindian hunters encountered and the vulnerable island species that evolved isolated from predators and were easily eradicated by humans as a result.
The final two chapters represent an exchange in a broader dialog on the rewilding movement, which calls for a significant portion of North America to be restored to a wild condition, with core wilderness areas, corridors, and viable complements of native species. Here, Martin proposes a Pleistocene standard for American restoration efforts. The argument has much to recommend it. The ecosystems that European colonists encountered were shaped both by the first Americans and their sudden decimation by Old World disease. Consequently, it is difficult to differentiate between those aspects of early colonial ecosystems that are atypical and those that represent a “normal” condition toward which restoration should aspire. A Pleistocene restoration standard also involves difficulties—even leaving aside the problem of extinctions. Pleistocene floral and faunal assemblages frequently lack modern analogs, and it is unclear how well they can be approximated under current climatic conditions. Martin proposes a radical plan to address these difficulties.
“Resurrection ecology” would restart the evolution of some megafaunal lineages by reintroducing them to North America. The approach would involve a multigenerational commitment, caution, and considerable research. For these reasons it is important to avoid becoming mired in a premature debate about particulars. Martin's suggestions to introduce a host of species—from zebra and elephant to gemsbok and rhino—are tentative and should be read as such. The justifications for such a major enterprise, however, should be both clear and fully articulated, and in this respect the study falters. In addition to the presumed main justification of preserving evolutionary potential, Martin briefly offers a variety of arguments for the plan, but none are fully developed. Those rooted in ethics are the most fragmentary. For instance, it is difficult to determine in what context taxa could be considered to have an inherent right to evolve free of human interference. Martin also briefly offers a series of additional arguments: that resurrection ecology could save endangered Old World species, provide the conservation movement with much-needed optimism, and allow humans to develop deeper ecological understanding by creating a host of real-world experiments. Details are scarce, however. By predicating so ambitious a restoration plan on so fragmentary a series of arguments, Twilight of the Mammoths falls short of persuasiveness.
Martin explicitly states that his support of resurrection ecology is independent of his views on overkill. But he acknowledges that, if the hypothesis were validated, one controversial argument supporting his restoration plan would be that humans bear a moral responsibility to repair the ecological damage they have inflicted. Although Martin expresses reluctance to advance that argument, it nonetheless remains in the rhetorical background, as does a linkage between overkill and resurrection ecology. Martin writes that extinct megafauna are America's evolutionary legacy, commenting: “They are what is natural” (p. 201). But if megafaunal extinction proves not to have been a consequence of overkill or other human activities, the opposite argument—that megafaunal extinction is “natural”—would be more compelling. Certainly, there are many lineages whose evolutionary fortunes we might wish to reverse.
The main unanswered question here is “Absent compelling proof for overkill, why should these lineages receive so much attention?” The lack of a clear answer to that question leads to the book's major rhetorical shortcoming: the appearance it gives that proponents might favor resurrection ecology simply because they think it would be a fine and pleasing thing to do. An esthetic hunch about how to work toward an attractive, newly configured American landscape is far too shaky a foundation for so large and uncertain an enterprise. In addition, some who might support the plan if its underlying logic were clearer might end up opposing it by misreading potential clues the book offers. For instance, Martin writes of the importance of a long-term perspective on Cenozoic mammal evolution, noting that it is vital to conceiving of how we might “design with nature” (p. 186). In context, comments like this one raise the possibility that a call for resurrection ecology might actually be a call for novel ecosystem design and construction—something many supporters of restoration would oppose.
In the absence of details, it is difficult to either accept or reject Martin's perspective. Individuals must decide for themselves whether or not it has a firm basis in ethics or science, a far shakier one in esthetics or whim, or, perhaps, something in between. Nonetheless, a long-term perspective on conservation biology and restoration is long overdue, as is a bolder, more proactive approach. Whatever the particulars of a more ambitious conservation agenda might turn out to be, the majority of conservation biologists would probably agree that while we go about formulating that plan, additional research and larger reserves with greater connectivity would be worthy medium-term goals. Long-term goals can come later. Martin offers his own views on the subject and invites us to consider them and to formulate our own. Although occasionally incompletely argued, the broad messages presented in Twilight of the Mammoths merit consideration—and in that regard Martin's study is a noteworthy success.