Linking wolves and plants: Aldo Leopold on trophic cascades.(Biology in History)

Aldo Leopold, perhaps best known for his revolutionary and poignant essays about nature, was also an eloquent advocate during the 1930s and 1940s of the need to maintain wolves and other large carnivores in forest and range ecosystems. He indicated that their loss set the stage for ungulate irruptions and ecosystem damage throughout many parts of the United States. We have synthesized the historical record on the potential effects of wolf extirpation in the context of recent research. Leopold's work of decades ago provides an important perspective for understanding the influence of large carnivores, via trophic cascades, on the status and functioning of forest and range plant communities. Leopold's personal experiences during an era of extensive biotic changes add richness, credibility, and even intrigue to the view that present-day interactions between ungulates and plants in the United States have been driven to a large degree by the extirpation of wolves and other large carnivores.

Keywords: Aldo Leopold, wolves, ungulates, irruptions, trophic cascades

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Wolves (Canis spp.), once found in all of the conterminous 48 United States, have been largely absent from their original range for many decades (Mech and Boitani 2003, Musiani and Paquet 2004). However, recent wolf reintroductions and range expansions have increased the need to better understand the potential ecological role of wolves and other large carnivores in forest and rangeland ecosystems (Berger et al. 2001, Mech and Boitani 2003, Smith et al. 2003, Soule et al. 2003).

When the presence of top trophic-level predators significantly affects herbivores (the next lower trophic level), and this interaction alters or influences vegetation (e.g., species composition, age structure, or spatial distribution), atrophic cascade occurs (Pace et al. 1999). The conceptual foundation for top-down forcing and trophic cascades is rooted in a landmark paper published by Hairston and colleagues (1960). Robert T. Paine, originator of the term "trophic cascades" conducted an early experiment showing that predators have effects that permeate food webs from the top down (Paine 1966). More recently, researchers have indicated that predation by large carnivores, through the progression of effects across successively lower trophic levels, may be crucial for the maintenance of biodiversity (Estes 1996, Terborgh et al. 1999). In addition to the classic top-down linkages of predators to herbivores to plants, many other interaction pathways resulting from predator effects are known (e.g., increased species interactions, improved nutrient cycling, limited mesopredator populations, food web support for scavengers), and far more are possible and even likely (Rooney and Waller 2003, Smith et al. 2003, Soule et al. 2003, Cote et al. 2004).

Once the most widely distributed carnivores in the continental United States, gray wolves (Canis lupus) were largely eradicated during and following Euro-American settlement. However, in the last decade, gray wolves have been reintroduced in portions of the western United States (Idaho, Montana, and Wyoming), and their range is expanding in the upper Great Lakes states (Minnesota, Wisconsin, and Michigan). In addition, the Mexican gray wolf (C. lupus baileyi) in New Mexico and an experimental population of red wolves (Canis rufus) in North Carolina have been recently reintroduced. Although most research involving wolves and trophic cascades has emphasized ecosystem changes resulting from wolf recolonizations and reintroductions (McLaren and Peterson 1994, White et al. 2003), a few recent studies have described ecosystem impacts that resulted from wolf extirpation in the United States during the early 20th century (Ripple and Larsen 2000, Beschta 2003, Ripple and Beschta 2004a, 2004b). While much of our focus in this article will be on wolves in multipredator systems characteristic of many forest and range settings, we recognize that cougars (Felis concolor), grizzly bears (Ursus arctos), black bears (Ursus americanus), coyotes (Canis latrans), bobcats (Lynx rufus), and wolverines (Gulo gulo) may also have a significant influence on ungulate densities.

Given the current expansions of gray wolf ranges in various areas of the United States (figure 1), we have a unique opportunity to reconsider Aldo Leopold's (figure 2) pioneering work on wolves and other predators. We compiled historical records of wolf kill estimates, by year, from the records of the US Department of Agriculture and obtained information on case studies of ungulate irruptions, by year, for these same western states from Leopold and colleagues (1947). These two data sets were used to compare the timing of wolf kills (and ultimate extirpation) with the timing of deer (Odocoileus spp.) and elk (Cervus elaphus) irruptions to evaluate any temporal patterns in these two variables. Our hope is that a synthesis of the potential cascading effects of wolf extirpation documented by Leopold, within the context of recent research, will provide relevant insights on the reemerging role of wolves in forest ecosystems.

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Early in his professional career, Leopold actively advocated wolf extirpation. At a National Game Conference in 1920, he stated, "It is going to take patience and money to catch the last wolf or lion in New Mexico. But the last one must be caught before the job can be called fully successful. This may sound like a strong statement, but if any of you have lived in the West and see how quickly a piece of country will restock with wolves or lions, you will know what I mean" (Meine 1988, p. 181).

In subsequent years, Leopold studied and observed forest and range ecosystems where wolves had been removed and where they remained. During these latter years, his understanding of wolves and their potential effects on big game production, habitats, and ecosystem conditions changed dramatically. In his widely recognized essay "Thinking Like a Mountain," Leopold (1949) …