For centuries, wolf populations have been under pressure by humans. Wolves are carnivores, feeding not only on a variety of animal species smaller than themselves, but also on very large mammals such as bison, elk, and even moose. Wolves are able to prey on large mammals by hunting cooperatively in packs. However, these large species are also favorite game animals for human hunters. In addition to game animals, their prey may include livestock such as cattle and sheep; some people perceive them as a direct threat to humans as well. These habits led to an effort to reduce or eliminate the species. Wolves were eventually eliminated in most states in an effort supported by the science community at the time. But coinciding roughly with the forester Aldo Leopold's essay, "Killing the Wolf," in A Sand County Almanac in 1948, the view that wolves must be eliminated began to change. Leopold wrote the following: I was young then, and full of trigger-itch; I thought that because fewer wolves meant more deer, that no wolves would mean hunters' paradise. But after seeing the green fire die, I sensed that neither the wolf nor the mountain agreed with such a view.... Since then I have lived to see state after state extirpate its wolves. I have watched the face of many a newly wolfless mountain, and seen the south-facing slopes wrinkle with a maze of new deer trails. I have seen every edible bush and seedling browsed, first to anaemic desuetude, and then to death. Leopold's somewhat poetic understanding of predator-prey interactions marked the beginning of a new scientific view of the role of predators in ecosystems. Scientists came to agree that, in the absence of major predators, prey populations would expand until some other factor (usually food) limited their population growth. As seen in a variety of ecosystems, the elimination of major predators devastated plant populations and led to serious reductions in the other species depending on those plants for food, nesting, migration habitat, or concealment or in any species dependent on the undisturbed ecosystem. The disappearance of wolves, eagles and other hawks, bears, and other top carnivores in much of their former ranges contributed to the passage of the Endangered Species Act (ESA) in 1973, and ultimately, to the re-introduction of wolves into parts of their former range. And predictably, the conflicts over livestock and game that led to the wolf's near extinction a century ago re-emerged. The ensuing conflicts over wolf management are the topic of this report. To protect gray wolves under the ESA, there needs to be some agreement on what a gray wolf is scientifically. Like many large mammals, such as bears ( Ursus arctos ), mountain lions ( Felis concolor ), and white-tailed deer ( Odocoileus virginianus ), gray wolves ( Canis lupus ) have a complicated, even convoluted, taxonomic history. Variations in color, size, and bone structure have led some mammalogists to designate wolves in different areas as different subspecies or populations, whereas other experts would recognize only a single species with variability. A few key terms are central to the taxonomic debate discussed below: A population is a group of "organisms of the same species that inhabit a specific area." A species is a "naturally [occurring] population or a group of potentially interbreeding populations that is reproductively isolated (i.e., cannot exchange genetic material) from other such populations or groups." A subspecies is a "taxonomic category that subdivides species into morphologically distinct groups of individuals representing a step toward the production of a new species, although they are still fully capable of interbreeding. Subspecies are usually geographically isolated." Taxon , or the plural taxa , is defined as "a grouping of organisms given a formal taxonomic name at any rank: species, genus, family, order, class, division, phylum, or kingdom." These terms may appear clear; however, there are no simple measures to draw unequivocal distinctions. Biologists commonly describe their colleagues as lumpers or splitters , based on their inclinations in classifying organisms. As the names suggest, lumpers are those who tend to minimize differences, and see one or a few species, perhaps with some variations, while splitters tend to emphasize those differences, dividing a species into many subspecies, or populations. As one well-known mammalogist once stated, "Splitters make very small units—their opponents say that if they can tell two animals apart, they place them in different genera, and if they cannot tell them apart, they place them in different species. Lumpers make large units—their opponents say that if a carnivore is neither a dog nor a bear they call it a cat." For wolves, which are (or were) found in temperate and polar areas throughout the Northern Hemisphere, some observers (splitters) would argue that there are as many as 24 subspecies in North America and 8 in Europe and Asia. More recently, lumpers have had the upper hand in the scientific community. However, that tide may be changing. In May 2011, the U.S. Fish and Wildlife Service (FWS) proposed recognizing a third species of wolf, the eastern wolf ( Canis lycaon ), in addition to the gray and red wolf. (See "2011 Regulatory and Legislative Activity" below, for more on this announcement.) This muddled state of taxonomic affairs is entirely predictable for several reasons. First, wolves are extremely wide-ranging, both as a species and as individuals, so occasional interbreeding among them could certainly confuse the picture. Second, the consistency of variations over time is hard to determine, since long-range studies of long-lived species are rare. Third, evolutionary change does not stop, and wolves are an adaptable species, as shown by their behavior and by their presence in a tremendous variety of ecosystems. The debate among academic scientists also has an air of informed judgment—and there is no reason to predict that the scientific debate will end any time soon. The Endangered Species Act (ESA or the Act) provides for the protection and conservation of species that the FWS or the National Marine Fisheries Service identifies as threatened or endangered. Under the ESA, if FWS determines that the best scientific information available indicates that a species is in danger of becoming extinct throughout all or a significant portion of its range, FWS lists the species as endangered. If FWS determines that a species is likely to become endangered within the foreseeable future, FWS lists the species as threatened. If the scientific bases underlying a listing change, FWS may take three steps: delist the species, thereby removing it from ESA protection; downlist it, decreasing its protection level from endangered to threatened; or uplist it, increasing its protection level from threatened to endangered. ESA amendments further allow FWS to list distinct population segments (DPS) within vertebrate species for protection. FWS may also reintroduce experimental populations (Ex Pops) to areas that the species no longer occupies within it historic range. The ESA prohibits taking endangered wildlife, which means it is illegal to harass, harm, or kill such species. FWS regulations prohibit taking a threatened species unless special rules are in place. Special rules may be issued for DPSs listed as threatened and Ex Pops. FWS may issue Section 4(d) rules or special rules to provide customized protection that FWS deems necessary and advisable for the species' conservation. Under special rules, taking species, including killing them, may be allowed. The special rules are promulgated in Title 50 (Part 17) of the Code of Federal Regulations. The ESA requires FWS to develop a recovery plan for listed species that will include a description of necessary management actions, measurable criteria for when a species can be delisted, and estimated costs to achieve this goal. In 1982, Congress modified the ESA to allow reintroducing Ex Pops of listed species "outside the current range of such species if the Secretary determines that such release will further the conservation of the species." Congress intended Ex Pops as a way to expand current ranges of listed species without imposing severe restrictions of private and public land. The practice allows introduction of a species outside its current range to restore it to its historic range. Two criteria must be met to establish an experimental population under the law. First, the Department of the Interior (DOI) must authorize the release of the population. Second, the population must be "wholly separate geographically from nonexperimental populations of the species." Congress required the separation so that the introduced population could be clearly distinguished from existing populations of the species. Under the ESA, members of an Ex Pop generally are treated as threatened, and are eligible for special rules. A congressional committee referred to special rules as a way to reduce public opposition to the release of Ex Pops of predators such as wolves: The committee fully expects that there will be instances where the regulations allow for the incidental take of experimental populations.... The committee also expects that, where appropriate, the regulations could allow for the directed taking of experimental populations. For example, the release of experimental populations of predators, such as red wolves, could allow for the taking of these animals if depredations occur or if the release of these populations will continue to be frustrated by public opposition. Section 10(j)(2)(B) of the ESA requires FWS to determine whether the Ex Pop is essential to the continued existence of an endangered or threatened species, otherwise the Ex Pop is deemed nonessential. Currently, there are no essential experimental populations. Critical habitat is not designated for nonessential Ex Pops.