Was Aldo Leopold Right about the Kaibab Deer Herd?

Abstract

In ecology textbooks prior to the 1970s, Aldo Leopold’s classic story of predator control, overpopulation of deer, and habitat degradation on the Kaibab Plateau during the 1920s epitomized predator regulation of herbivore populations. However, the story disappeared from texts in the late 20th century after several papers noted uncertainties in estimations of the deer population and provided alternative explanations. We re-examined the case study by determining the age structure of aspen (Populus tremuloides Michx.) on the plateau. Aspen comprises the majority of deer browse in the summer, and the absence of a normal cohort of aspen from the 1920s would indicate deer overpopulation. The number of aspen (at 1.4 m) dating to the 1920s was an order of magnitude lower than the null expectation. Other periods of unusual numbers of aspen included high numbers of aspen dating to the 1880s and 1890s (when regular surface fires ceased), few aspen dating from 1953 to 1962 (after a second irruption of the deer population), and very high numbers from 1968 to 1992 (coincident with widespread logging). These convergent lines of evidence support the idea of extreme deer herbivory in the 1920s, consistent with food limitation of deer at high populations (bottom–up control) and predation limitation at low deer populations (top–down control). Some uncertainty remains within the overall story, and this level of ambiguity is common in case studies that involve population ecology, land management, and people at the scale of 1,000 km 2 and 100