Before humans started killing wolves in the early 1900s as part of a government-sponsored program, the exact numbers of gray wolves living in Yellowstone were unknown, but estimates range from 300 to 400. Elk populations at that time hovered around 10,000 animals. The size of a population in a given geographic area is determined by the interplay of factors that simultaneously increase the number of individuals in a population (births and immigration) and those that decrease numbers (deaths and emigration). An understanding of these factors helps us predict population size at any given time. Ecologists who study changes in population size and makeup (population dynamics) find that the population size of some species increases and decreases rather predictably (barring a catastrophic event), while others tend to fluctuate more randomly, affected by a variety of factors.

In the early 20th century, Congress allocated $125,000 for the Predator Control Program, which employed poison—and later hunting—to eradicate predators and rodents that might harm crops or livestock. Wolves were one of the targeted species because wolves preyed upon livestock. Wolf eradication was also supported because it boosted deer and elk populations for human hunting. “It was park policy to kill all predators, and wolves were their biggest objective,” Smith explains. Between 1914 and 1926, at least 136 wolves were killed in Yellowstone. In 1944, the last known wolf in the Yellowstone area was killed.

Every population has a minimum viable population, or the smallest number of individuals that would still allow a population to be able to persist or grow, ensuring long-term survival. This is an important concept when considering how to conserve endangered or threatened species. A population that is too small may fail to recover for a variety of reasons. For example, some species’ courtship rituals require a minimum number of individuals for success. Other activities that depend on numbers—like flocking, schooling, and foraging—fail below certain population sizes. Genetic diversity (inherited variety between individuals in a population) is also important: A population with little genetic diversity is less able to adapt to changes and is therefore more vulnerable to environmental change. A small population is also subject to inbreeding, which allows harmful genetic traits to spread and weaken the population. Ecologically, however, the minimum viable population may not be as significant as what is known as an ecologically effective population—one large enough to perform the important ecosystem services it normally contributes to its community. One small pack of wolves in Yellowstone National Park probably has little impact on elk populations.

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Critics began raising concerns about the Predator Control Program in the 1920s, and wolves in Montana and Wyoming became protected under the Endangered Species Act (ESA) in 1973, but it wasn’t until 1987 that the U.S. Fish and Wildlife Service proposed reintroducing an “experimental population” of wolves into Yellowstone, in its Northern Rocky Mountain Wolf Recovery Plan.

Not everyone was in favor of reintroducing wolves to the landscape of the American West, and opposition remains today. Ranchers in particular were and still are wary of the damage wolves might do to livestock. In some years, wolves were responsible for thousands of herd deaths—although not nearly as many as are killed by coyotes, says the U.S. Fish and Wildlife Service. Current programs compensate ranchers for lost livestock, but they must prove that their animal was killed by a wolf, which is not always easy to do.

After the Fish and Wildlife Service unveiled its reintroduction plan, Congress funded the organization to prepare an environmental impact statement (an evaluation of the positive and negative impacts of a proposed action) on what was likely to happen if wolves were reintroduced to Yellowstone. By considering all aspects of wolf ecology, scientists were able to organize the reintroduction to maximize survival of the species. For instance, scientists believed that it would be best to “softly” release wolves in the park by holding them temporarily in packs in areas that would be suitable for them to live instead of doing a “hard” release in which the wolves could immediately disperse anywhere in the park. The soft release curtailed the wolves’ movement and helped them survive and acclimate to the move.

After many years of tracking wolves in Yellowstone, Smith has a good idea of where to look for the wolf packs. And the helicopter hovers a mere 15 meters (50 feet) off the ground, making it easy to spot roaming groups of wolves.

The number of wolves distributed throughout Yellowstone National Park is their population density—the number of individuals per unit area. Population density is an important feature that varies enormously among species, or even among populations of the same species in different ecosystems. If a population’s density is too low, individuals may have difficulty finding mates, or the only potential mates may be closely related individuals, which can lead to inbreeding, loss of genetic variability, and, ultimately, extinction. Density that is too high can also cause problems, such as increased competition, fighting, and spread of disease. Deer, elk, and moose populations in the United States, whose density has increased in recent years because of exploding numbers combined with shrinking habitats, now frequently suffer from an infectious disease known as chronic wasting disease.

In addition to size and density, another important feature is population distribution, or the location and spacing of individuals within their range. A number of factors affect distribution, including species characteristics, topography, and habitat makeup. Ecologists typically speak of three types of distribution. In a clumped distribution, individuals are found in groups or patches within the habitat. Yellowstone examples include social species like prairie dogs or beaver that are clustered around a necessary resource, like water. Wolves travel in packs and therefore have a clumped distribution. Elk, one of their prey species, congregate as well; living in herds offers some protection against the wolves.

In a random distribution, individuals are spread out in the environment irregularly, with no discernible pattern. Random distributions are sometimes seen in homogeneous environments, in part because no particular spot is considered better than another. Species that rely on wind and water to disperse their offspring—like wind-blown seeds or the free-floating larvae of coral—also often have a random distribution. Uniform distributions, rare in nature, include individuals that are spaced evenly, perhaps due to territorial behavior or mechanisms for suppressing growth of nearby individuals (seen in some plant species). INFOGRAPHIC 9.1

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