THE NATIONAL PARK system has been promoted as one of America’s best ideas. Wild places throughout the nation were set aside and protected to ensure that all citizens could enjoy the scenery, natural and historic objects, and wildlife through time. This protection was effective at providing safe havens for native species from such threats as timber logging, minerals extraction, livestock grazing, and wildlife overharvesting. Equally important, this ideal increased recognition that healthy park units enhanced the lives of people living in nearby areas by providing sustainable resources such as clean water, wildlife populations, and vegetation communities. As this understanding of...

THE ESTABLISHMENT OF Yellowstone National Park in 1872 began a great quest to make the most of this extraordinary place. Since then, prevailing views of how a national park should be managed have undergone many changes, reflecting both the ongoing public debate about the purpose of a national park and scientific advances in our understanding of how natural ecosystems function. Changes in how the park’s resources have been and currently are managed can be understood through the lens of organizational evolution. Changes in the geophysical processes, political setting, public engagement, and/or scientific knowledge have caused sweeping perturbations in the management...

YELLOWSTONE NATIONAL PARK comprises the core of the greater Yellowstone ecosystem, a relatively large area (56,000 square kilometers) spanning multiple management regimes and constituting the world’s largest intact temperate ecosystem, with a full complement of native species that were historically present (Keiter and Boyce 1991). The largest concentrations of wild ungulates and large carnivores (meat eaters) in the lower 48 states are found here, along with some of the best quality habitat for native char and salmonid fishes. The Yellowstone ecosystem is characterized by numerous high-profile and wide-ranging wildlife species and by large-scale ecological processes. These factors, combined with rapidly...

POPULATIONS, OR AGGREGATIONS of individuals of the same species that interact, are often the basic unit of concern in wildlife management. A population can be managed to make it increase or decrease or provide a sustainable harvest, or it can be left unmanipulated and simply monitored over time (Caughley and Sinclair 1994). Each of these options has been pursued for various wildlife populations in and near Yellowstone National Park, with management objectives for a given population often changing over time. Animals from many populations migrate outside the park for a portion of the year and into areas managed by other...

PREDATION IS AN interaction in which one organism (the predator) attacks and feeds on another (the prey). During the 1930s, biologists were convinced that predators kept ungulate populations at low numbers (Leopold 1933). However, as predators became scarce due to control by humans, there was increasing concern about the role of predators in biological communities (Leopold 1949). Calls to protect predators emerged after Errington (1945, 1946) proposed that predators mostly removed prey animals that would have died anyway due to starvation or other factors, such as severe weather events. Wildlife populations produce a surplus of young that cannot be supported...

THE DIRECT EFFECTS of predation comprise a “fraction of the potential interactions between species” in ecosystems, and numerous factors can indirectly affect aspects of communities other than just prey species (Estes et al. 2004; Hebblewhite and Smith 2010, 75). These effects include competition with other predators that consume the same food resources, changes in competitive interactions among prey species, and interactions with species of decomposers, parasites, and scavengers. Competition occurs when two animals of the same or different species interact and the capability of an individual to survive and reproduce is lowered by the other (Begon et al. 2005). Thus,...

AN OMNIVORE (FROM Latin: omni meaning all, everything; vorare meaning to devour) is a species that eats both plants and animals. Omnivores are typically opportunistic generalist feeders adapted to eat and digest either animal or plant material. Omnivory provides species with diet flexibility and allows them to adapt to varying environments and adjust their diets accordingly. Humans are omnivores, and our diet diversity worldwide exemplifies this flexibility. Most bear species are likewise omnivorous and select plants and animals that are nutritious, abundant, and available.

Trophic ecology is the study of the structure of feeding relationships among organisms in an ecosystem...

WHAT WE SEE today in Yellowstone National Park and surrounding ecosystems is largely a result of natural disturbances that shaped, often in dramatic fashion, Yellowstone’s physical and biological environments. On September 8, 1988, Yellowstone National Park was closed to the public for the first time in its history following a summer of the largest fires ever recorded since the park was established in 1872. By the end of September, more than 319,700 hectares of primarily lodgepole pine (Pinus contorta) forests were burned by over 200 fires fed by a particularly hot, dry, and windy summer. Fires continued to burn into...

WITH THE MELTING of winter snow, plant growth commences in the greater Yellowstone ecosystem, resulting in a greening of the spring landscape. Plants grow in predictable sequences of leaf, bud, flower, and seed formation, which together are referred to as the phenological stages of growth. The timing of phenological events is highly variable and depends on a suite of climatic and biotic factors. At the landscape scale, this phenological development has been characterized as a green wave that travels up the elevation gradient as snow recedes from late winter to summer (Frank and McNaughton 1992). This green wave is crucial...

TWO PROCESSES THAT typify the life histories of many animals worldwide and illustrate the importance of ecological process management are migration and dispersal. Migration has been defined as a seasonal, round-trip movement between separate areas not used at other times of year (J. Berger 2004). Seasonal migrations are the norm for many species to obtain access to greater food supplies, make more efficient use of resources by tracking vegetation growth, and reduce the risk of predation (Fryxell et al. 1988). Dispersal is essentially movement from one area to another without return (at least in the short term; Stenseth and Lidicker...

THE REINTRODUCTION OF wolves to Yellowstone represents one of the most important natural experiments in the history of ecology, offering an unparalleled opportunity to understand how predators change the operation of fundamentally important ecological processes. In particular, wolf reintroduction created the chance to understand how a top predator influences its plant-eating prey and how changes in those prey influence plants. Predators can reduce the numbers of herbivores or change their behavior so that plants benefit from the presence of predators. Ecologists call this series of effects a trophic cascade. Trophic means feeding or nutrition, and the term trophic cascade describes...

AS HERBIVORES, UNGULATES are a highly integrated and interactive component in many ecosystems that impart strong influences on the turnover of soil nutrients, competitive interactions among plant species, and the composition of plant species over time (Hobbs 1996; Pastor et al. 1997; Kie et al. 2003). Herds of grazing ungulates can stimulate the flow of energy and nutrients in grassland ecosystems with high nutrient and moisture availability, with the classic example being the Serengeti in northwestern Tanzania and southwestern Kenya (McNaughton 1976, 1985, 1990; McNaughton et al. 1988, 1997). Grazing intensity in grassland ecosystems is typically much higher (40 to...

A FOOD WEB describes the trophic relationships among the species in a community (Pianka 1978). In most cases, a food web has numerous food chains that represent a single pathway between trophic levels. In essence, a food web depicts energy flow from primary producers (lowest trophic level) to consumers and decomposers (organisms like bacteria that break down dead organisms). Consumers include herbivores that feed directly on producers and carnivores that feed on herbivores or other carnivores. Omnivores are also consumers that occur in many food webs, but they are more difficult to place at any specific trophic level (Pianka 1978)....

DISEASE HAS LONG been recognized as an important process influencing the distribution and abundance of animal populations (Andrewartha and Birch 1954; Anderson and May 1978). Disease organisms, much like predators, have the potential to significantly reduce the size of animal populations when they cause high mortality. However, large mammals have welldeveloped immune systems and are frequently able to recover from infections, thereby reducing the limiting effects of many parasites. Today the greatest disease threat to wildlife may come not directly from infectious organisms but indirectly from humans attempting to reduce health risks to people and their domestic animals. In recent...

THE LANDSCAPE IN Yellowstone National Park is shaped, maintained, and famous for periodic landscape-scale disturbances, both ancient and ongoing. These disturbances occur at a scale larger than a single watershed, with two extreme examples being the fires of 1988, which burned over 404,685 hectares in the greater Yellowstone ecosystem, and ancient Yellowstone volcanism, which spewed ash over half of the continental United States (R. B. Smith and Siegel 2000). Over the geologic timescale (hundreds of thousands of years), changing climate and major geologic processes have dramatically restructured Yellowstone’s landscape and plant and animal communities (National Research Council 2002). Thus, the...

THE NATIONAL PARK Service Organic Act of 1916 states that parks are “to conserve the scenery and the natural and historic objects and the wild life therein . . . by such means as will leave them unimpaired for the enjoyment of future generations.” However, no guidance was provided to define “unimpaired” or indicate how scenery, nature, and history were to be preserved. Management policies later clarified that parks will preserve “components and processes in their natural condition,” which was defined as “the condition of resources that would occur in the absence of human dominance over the landscape” (National Park...