Was Aldo Leopold Right about theKaibab Deer Herd?Dan Binkley,1,2,3* Margaret M. Moore,4William H. Romme,1,2andPeter M. Brown1,51Department of Forest, Rangeland, and Watershed Stewardship, Colorado State University, Fort Collins, Colorado 80523, USA;2Graduate Degree Program in Ecology, Colorado State University, Fort Collins, Colorado 80523, USA;3Natural Resource EcologyLaboratory, Colorado State University, Fort Collins, Colorado 80523, USA;4School of Forestry, Northern Arizona University, Flagstaff,Arizona 86011, USA;5Rocky Mountain Tree-Ring Research Inc., 2901 Moore Lane, Fort Collins, Colorado 80526, USAABSTRACTIn ecology textbooks prior to the 1970s, AldoLeopold’s classic story of predator control, over-population of deer, and habitat degradation on theKaibab Plateau during the 1920s epitomized pred-ator regulation of herbivore populations. However,the story disappeared from texts in the late 20thcentury after several papers noted uncertainties inestimations of the deer population and providedalternative explanations. We re-examined the casestudy by determining the age structure of aspen(Populus tremuloides Michx.) on the plateau. Aspencomprises the majority of deer browse in thesummer, and the absence of a normal cohort ofaspen from the 1920s would indicate deer over-population. The number of aspen (at 1.4 m) datingto the 1920s was an order of magnitude lower thanthe null expectation. Other periods of unusualnumbers of aspen included high numbers of aspendating to the 1880s and 1890s (when regular sur-face fires ceased), few aspen dating from 1953 to1962 (after a second irruption of the deer popula-tion), and very high numbers from 1968 to 1992(coincident with widespread logging). These con-vergent lines of evidence support the idea of ex-treme deer herbivory in the 1920s, consistent withfood limitation of deer at high populations (bot-tom–up control) and predation limitation at lowdeer populations (top–down control). Someuncertainty remains within the overall story, andthis level of ambiguity is common in case studiesthat involve population ecology, land manage-ment, and people at the scale of 1,000 km2and 100years. A complete version of the Kaibab deer storyand its history would be a valuable, realistic casestudy for ecology texts.Key words: deer population; irruption; KaibabPlateau; Grand Canyon; fire history.INTRODUCTIONThe story of the Kaibab deer herd formed a cor-nerstone of population ecology from the 1920s(compare Elton 1927) through the early 1970s(compare Dasman 1964; Kormandy 1969; Owen1971; Ricklefs 1973). The classic form of the storywas often attributed to Aldo Leopold (1943), whoreported that reducing the population of predatorson the Kaibab Plateau led to an irruption of thedeer population, degradation of habitat, and lowercarrying capacity for deer. The deer herd on theKaibab Plateau was the focus of major controver-sies between the National Park Service and theUSDA Forest Service, a landmark dispute betweenArizona and the federal government over wildlifeand hunting rights, and even a colorful attempt todrive deer across the Grand Canyon that was fic-tionalized in a novel by Zane Grey (Grey 1924;Young 2002). Rachel Carson (1962) used theKaibab deer story in Silent Spring to convincereaders of the importance of predators, and thestory appeared in most ecology textbooks to showReceived 5 August 2004; accepted 29 December 2004; published online15 March 2006.*Corresponding author; e-mail: [email protected] (2006) 9: 227–241DOI: 10.1007/s10021-005-0100-z227that top predators are critical for controlling eco-system food webs.Confidence in the story began to erode whenCaughley (1970) highlighted the paucity andinconsistency of the original data. He suggestedthat if a deer irruption actually occurred, the drivermight have been release of the deer from foodcompetition with livestock after massive livestockreductions at the end of the 19th century. The roleof predation in regulating food webs was alsochallenged by ecologists and land managers in the1960s and 1970s. Increasing confidence in thecontrol of food webs by primary production ratherthan predation contributed to the adoption of the‘‘natural regulation’’ policy by the US. NationalPark Service. Populations of ungulates were ex-pected to stabilize at ‘‘natural’’ levels controlled byfood supply (National Research Council 2002).Some texts began to use Leopold’s Kaibab deerstory not as an example of ecological principles butas a warning about the dangers of unwarrantedcredulity (Dunlap 1988; Botkin 1990). Colinvaux(1973, 1986) went so far as to claim that the Kaibabstory had been exposed as fiction, that the anecdotewas known to be without value, and that the his-tory was not true. Most recent textbooks simplyomit mention of the once-classic tale (Molles 1999;Krebs 2001; Ricklefs 2001; Smith and Smith 2001)or use it as an example of poor ecology (Stiling2002). Some scientists remain convinced of the roleof mammalian herbivores in shaping the structureand function of ecosystems (for example, Paine2000), so the underlying idea of top–down controlof populations retains some support even if theKaibab example was inconclusive.Does the classic Kaibab deer story warrant con-fidence as a landscape-level demonstration of theimportance of predation in regulating food webs, oris the value of the story limited to a historicalaccount of misguided ecology? Mitchell and Free-man (1993) reviewed a wealth of primary sourcesapparently not examined by Caughley (1970) andconcluded that a deer irruption and die-off hadoccurred, but that the drivers included bothreductions in predation (including human hunt-ing) and livestock competition, followed by over-browsing, severe drought, and a change in thelong-term fire regime. Young (2002) provided themost thorough synthesis of the story; his primaryconclusion was that whatever the real dynamics ofthe deer population might have been, the Kaibabdeer story is an excellent example of the interactionof science, conservation, politics, and management.We tested a hypothesis that might shed light onwhether the deer population irrupted in the 1920s asAldo Leopold (and almost all of this contemporaries)believed. Young shoots of aspen (Populus tremuloidesMichx.) comprise the majority of deer browse duringsummer on the Kaibab Plateau (Rasmussen 1941;Bostick 1949; Hungerford 1970) and we hypothe-sized that aspen regeneration would necessarily beimpaired