Details of a Desert Carnivore

Authors


Desert Puma: Evolutionary Ecology and Conservation of an Enduring Carnivore. Logan, K. A., and L. L. Sweanor . 2001 . Island Press , Washington, D.C . 463 pp. $70.00 (hardcover). ISBN 1-55963-866-4 . $45.00 (paperback) ISBN 1–55963–867–2.

From 1985 through 1995, Ken Logan and Linda Sweanor studied puma (Puma concolor) in the San Andres Mountains of New Mexico. Their objectives were to describe and quantify puma population dynamics, social organization, and relationships with their principal prey, desert mule deer (Odocoileus hemionus crooki  ) and desert bighorn sheep (Ovis canadensis mexicana). To this end, they radiotagged most of the puma population and large numbers of their ungulate prey. In early 1991, Logan and Sweanor experimentally removed half of the pumas from the southern half of the mountain range. The resulting 10-year, whole-population study—summarized in Desert Puma—provides a richly detailed picture of the puma's natural history and ecology, with important lessons for conservation and management.

Organized into five parts, 21 chapters, and four appendices, the book is truly comprehensive. Major topics are population structure, reproduction, survival, philopatry, emigration, immigration, population growth, metapopulation structure, behavior and social organization, dispersal, interactions between pumas, prey relationships, and conservation and management.

Logan and Sweanor do an admirable job of summarizing their own work and putting it in the context of previous work on large carnivores. Many of the citations are from the year 2000, and in all cases the authors interpret previous work accurately and respectfully. Because their study was unreplicated and only partly controlled—for instance, there were no radiotagged mule deer or bighorn in the control area—the study qualifies more as a natural history than as an experimental test of scientific hypotheses. Consistent with their descriptive objectives, the authors' basic strategy was to look for patterns and, “once a [pattern] was detected, [to] look…for possible explanations.” Although advocates of strict hypothetico-deductive science may find this disappointing, Desert Puma strikingly illustrates the value of applying excellent natural history to important ecological questions. At worst, natural history is merely an attempt to marshal evidence for a pet hypothesis. In contrast, Logan and Sweanor embody the best of natural history. They carefully scoured the literature for possible explanations for each observed pattern and structured their discussion sections as honest attempts to determine which hypotheses are most consistent with their observations.

This study produces several important insights, perhaps chief among them how puma dispersal structures metapopulations, including the observation that 100% of male cubs that survived to independence dispersed to other mountain ranges. Previously, most puma researchers (myself included), although aware that male pumas are superb dispersers, assumed that a young male would occupy a local vacancy if he found one. But in the San Andres, male immigrants regularly moved in and were accommodated by resident adult males at the same time that locally born juvenile males left the mountain range. Obligatory male dispersal should be an important feature of any future individual-based simulation metapopulation model for this species. The conservation import of dispersal is the only major conservation finding in the book that has been published elsewhere (Sweanor et al. 2000).

In Desert Puma, Logan and Sweanor present strong evidence against two popular notions about pumas. The first shibboleth is that puma social structure limits puma density at a level so low that puma cannot affect prey abundance. Whenever management agencies propose to reduce puma numbers to benefit teetering populations of deer, bighorn, or pronghorn, opponents cite this “fact” to argue that the management proposal is fundamentally misguided. Although Logan and Sweanor give other reasons to be cautious about such management actions, the scientific debate will be more productive now that they have slain the ill-supported social-limitation hypothesis. They similarly found no support, and anecdotal refutation, for the idea that after a puma kills an ungulate the surviving deer and elk move far away from the kill site for days. In most versions of this myth, pumas are portrayed as range managers, forcing ungulates to distribute themselves evenly on the landscape rather than concentrating in sensitive areas. Although the idea is attractive, I have long been uncomfortable with repeating unsupported ideas as scientific fact. I thank Logan and Sweanor for putting this old saw to rest.

They also debunk the reasoning, popular among many managers, that observations of combat among adult male pumas, and of juvenile males showing up in marginal habitat, indicate that puma habitat is saturated and there are “too many puma.” By demonstrating that male dispersal is obligatory and inter-male combat for access to females is unrelated to puma density, this book will help keep bad science out of management decisions.

Desert Puma also paints a new and compelling picture of how drought and predation interact to regulate deer populations in arid systems. When deer thrive in wet years, pumas rely heavily on ungulates of less than a year old and have relatively little effect on adults, which have the highest reproductive value. This allows the deer population to at least hold its own in the face of predation, even when the puma population is also increasing. With the onset of drought, deer fecundity declines, and with fewer fawns available, pumas shift to taking adults, further exacerbating next year's fawn shortage and predation pressure on adults. In a prolonged drought, perennial water sources dry up, ungulates are forced to leave their home range to search for water, and then often concentrate near the few remaining water sources. Both responses increase the vulnerability of deer to puma predation. Thus pumas—at least for 2–3 years—increase in numbers as their main prey decline. It would have been difficult to document this important role for top-down processes in less than 10 years of study of radiotagged predators and prey. This complex story piqued my curiosity about interactions that remain undiscovered, such as those involving species that compete with deer, mesopredators, and lower trophic levels. (Do browse plants need predator-mediated deer declines for establishment?) How will all of this change when puma once again interact with the now-missing grizzly and soon-to-be-recovered Mexican wolf?

The book contains many other new and interesting findings, although with less conservation import than those already mentioned. For instance, the large home ranges of adult male pumas reported in all previous studies suggest that most females have no opportunity for mate choice. But in this study, only 5 of 19 adult females during a typical year had home ranges entirely encompassed within the home range of a single adult male. The other 14 females were accessible to, and perhaps could choose among, two or more adult males. This study is also the first to document geographic clusters of matrilineal female groups whereby young females establish home ranges in areas already occupied by their mother or siblings. The documentation of high overlap among home ranges of neighboring adult males makes me suspect that previous observations (including my own) of low overlap among home ranges of neighboring adult males may have been artifacts of undersampling.

The book provides great gobs of raw data that future models, meta-analyses, and cross-species comparisons in ecology and biology will re-mine for decades to come. For instance, I was able to use the nearly raw data to compute the maximum instantaneous rate of increase (rmax) for the treatment area in the year immediately following the experimental removal (a statistic the authors did not provide) on the basis of January counts (rmax = 0.73), calendar year (0.33), and biological year (0.46). If you want a more traditional rmax measure (i.e., ignoring immigration), all the data you need are in the book. This same level of detail, and the book's sheer length, may put off nonscientists who are not carnivore enthusiasts. This would be unfortunate, because Logan and Sweanor took pains to make the book accessible to a nonscientific audience—for example, by defining each new concept in clear, nontechnical terms at first introduction.

I was not persuaded, however, by all of Logan and Sweanor's interpretations. For instance, they argue that “the best explanation” for male dispersal is “avoidance of inbreeding.” Their own observation that resident males bred with their daughters suggests that natural selection can overlook inbreeding. More important, natural selection on inbreeding defects (which may not become manifest for several generations) seems a tenuous mechanism to compel obligatory dispersal. It seems more likely, as implied in several parts of the book, that a young male cannot challenge resident males for breeding space until he spends a year away from home gaining size, strength, fighting experience, and perhaps increased exposure to sex hormones.

Rigorous peer review would have sharpened some of the interpretations (such as that just mentioned) as well as several descriptions of the methods. For instance, the procedure for estimating error in radio locations was described simply as follows: “all radiotelemetry locations had estimated error radii of 500 m or less.” It is not clear if or how this error relates to the “overlapping error polygons” used to infer that two pumas were associating with each other. Although most (perhaps all) of these problems are minor, they make it hard for readers to interpret results on their own.

This leads to my only serious complaint about the book: I wish Logan and Sweanor had first published a series of tightly focused peer-reviewed papers and then written a shorter book for a broad audience. I believe this would have increased the impact of their main new findings. For instance, the paper by Sweanor et al. (2000) provides conservation biologists with a concise, defensible argument for incorporating puma dispersal in conservation efforts. Similarly, we need companion papers with titles such as “Puma Social Structure Does Not Prevent Overexploitation of Prey” and “Puma Predation Does Not Redistribute Ungulates on the Landscape.” And wildlife managers need papers titled “Bogus Indicators of Puma Habitat Saturation” and “Interaction of Drought and Predation Regulates Deer Populations.” I can't help but fear that some important ideas will be buried in this massive volume and that lack of peer review may impede broad acceptance of ideas that are almost certainly correct or allow incorrect ideas to prosper too long. With several peer-reviewed publications in the literature, a shorter book could then bring natural history and a conservation message to readers in a more lively format, lavishly illustrated with anecdotes about individual pumas.

The penultimate chapter of Desert Puma is a well-reasoned call for specific conservation actions. As a supporter of The Wildlands Project vision, which uses carnivores as umbrellas for planning of large interconnected reserves, I applaud Logan and Sweanor for stating that the top conservation need is to “immediately identify, map, and conserve puma habitat and landscape linkages.” They cogently argue against complacency by pointing out the dramatic impact of a single highway project in their study area. I have often presented my studies of pumas in southern California as indicating the sort of future that the rest of the West must work to avoid. For pumas, the remote landscape of central New Mexico is as good as it gets. Learning that even this landscape is under assault has heightened my sense of urgency and commitment to conservation action.

With only about 32,000 pumas in the United States, these animals clearly have an influence larger than their numbers or biomass in the ecosystems they inhabit. Logan and Sweanor have considerably advanced our appreciation of those influences. They debunk some old myths and provide many new insights and hypotheses. They present evidence for both top-down and bottom-up regulation (predation will not be pigeonholed). They are conservationists at heart, but avoid letting their values cloud their interpretation. I highly recommend Desert Puma as a must-read for any student of large carnivores or their prey and for citizen activists in regions that are—or ought to be—inhabited by puma. Any conservation biologist would find it useful and thought-provoking.

Ancillary