In theory, decisions about flora and fauna habitat are purely scientific. In practice, they're political. And that, in a nutshell, is the reality of the Endangered Species Act, passed in 1973 as part of a historic wave of legislation that both protects America's environmental heritage and provides a framework for settling conflicts. Some say it does too little; others, that it intrudes too much. The arguments go 'round and 'round, and underscore a fundamental truth: In the Anthropocene Era, people decide nature's fate. Above: Mexican Spotted Owl The Mexican spotted owl is less iconic than its northern counterpart, which two decades ago touched off a bitter conservation battle over the Pacific Northwest's old-growth forests. But even as it's flown under the radar, the Mexican spotted owl's fate may prove crucial to the future of wildlife in the United States. The owl is at the center of Arizona Cattle Growers’ Association v. Salazar, one of two endangered-species cases now under possible consideration by the Supreme Court. If the court chooses to hear the linked cases, its decision may guide how the U.S. Fish and Wildlife Service — the federal agency tasked with carrying out the Endangered Species Act — evaluates economic impacts when designating crucial habitat for rare animals. Photo: National Park Service

Vernal Pool Fairy Shrimp The other case under consideration by the Supreme Court, Home Builders Association of Northern California v. United States Fish and Wildlife Service, involves no immediately charismatic species, like spotted owls. Its animal protagonists are three endangered species of shrimp found only in California and southern Oregon, in vernal pools — ephemeral basins of fresh water that fill in the spring, hosting unique communities of creatures. That this case involves inch-long crustaceans, and that the vernal pools are tied to their fate, crystallizes an essential endangered-species tension. "The crucial thing is not … the single threatened species, but the fact that its threatened extinction is a kind of thermometer registering the health of the whole ecosystem which supports it," wrote Peter De Vos and Loren Wilkinson in Earthkeeping in the Nineties: Stewardship of Creation. "Such situations present us with a real dilemma: How much right does a species have to live?" Photo: U.S. Fish and Wildlife Service

Furbish Lousewort Even if fairy shrimp are not charismatic, they're at least animals. The furbish lousewort, found only on the banks of the St. John River (.pdf) in northern Maine and New Brunswick, is a plant. But plants can be endangered, too. When the Furbish lousewort — once considered extinct — was rediscovered in 1976, it was enough to halt a proposed $227 million hydroelectric dam that would have flooded much of its habitat. After a decade of legal wrangling, plans for the dam were officially abandoned. Photos: Maine Department of Conservation, Wikimedia Commons

Desert Tortoise Another battle between endangered animals and energy development is taking place in the U.S. Southwest. But unlike the hydroelectric dam blocked by the Furbish lousewort, green energy is on offer: solar farms planned for regions inhabited by desert tortoises. One proposed compromise is tortoise relocation, but that's easier said than done. The animals can live for 60 to 80 years, following paths they've trodden for decades. New habits come hard. Photo: Arizona Game and Fish Department

Snail Darter In 1973, the same year the Endangered Species Act was enacted, biologists discovered the snail darter, a tiny fish living only in a short stretch of the Little Tennessee River. That stretch was about to be dammed by the Tennessee Valley Authority. The TVA argued that, because dam construction started before the act's passage, the law didn't apply. The ensuing fight reached the Supreme Court, which ruled in favor of the Endangered Species Act. Doing otherwise would have required the court “to ignore the ordinary meaning of plain language," wrote Chief Justice Warren Burger. Congress was happy to do so, however, granting the project a specific exemption from the Endangered Species Act. The dam was finished late in 1979, and captured darters were transplanted to Tennessee's Hiwassee River, where they became successfully established. Five years later, the snail darter was removed from the endangered-species list. Photo: U.S. Fish and Wildlife Service

Fluvial Arctic Grayling Grayling are found in the Arctic, but the descendants of populations stranded when glaciers last retreated across North America still live in parts of Montana's Big Hole River. Called fluvial grayling, these fish are, according to the U.S. Fish and Wildlife Service, legitimately endangered, but the agency can't do anything about it now. Thus they're "warranted but precluded," in a state of bureaucratic limbo. Fourteen species, including wolverines, have received this designation during the Obama administration. Montana's grayling, along with the snail darter, are subjects of an essay by conservation journalist David Quammen on the value of species. "The grayling, face it, is useless. Like the auk, like the zebra swallowtail, like Angkor Wat," he wrote, criticizing the notion that a species should have use to be valued. "A snail darter arrived at the hard way, the Darwinian way, through millions of years of random variation and natural selection, reaching its culmination in a small homely animal resembling a sculpin, is something far more than a net asset in potential utility." Photo: Matt Ruuhela

Mexican Gray Wolf At once symbols of the wild; big, beautiful dogs; and livestock-raiding villains, few animals are as loved and despised as wolves, whose western U.S. populations are alternately protected and killed. One proposed compromise over wolves has been to pay ranchers for animals lost to predation. However, the Defenders of Wildlife suspended its compensation program in August 2010. For a variety of reasons, from the logistical difficulties of proving causes of death to cultural distrust, it just wasn't working. But some researchers think another, more-sophisticated approach may work: Rather than paying ranchers for lost livestock, pay them for the number of wolves seen on their land. Photo: U.S. Fish and Wildlife Service

Ivory-Billed Woodpecker How do you protect an animal that may not even exist? That question has been posed since 2004, when an ivory-billed woodpecker — the largest of all American woodpeckers, once found throughout the southeastern United States, last seen in 1938 and considered extinct — was glimpsed in Arkansas by ornithologists from Cornell University. Subsequent attempts to find the birds have proved inconclusive. But if any do survive, then it's imperative to protect the last few remaining patches of old-growth bottomland forest, the only habitat rich enough to support a creature so large it was nicknamed the "Good God Bird." Bird hunters — who do, after all, have a rich track record as conservationists — sued in July 2010 to stop the construction of a power plant on Arkansas' Little River, near the location of unconfirmed sightings in the 1980s and 1990s. Painting: John James Audubon/Wikimedia Commons

Polar Bear Polar bears are the poster animals for wildlife threatened by climate change. Whether they should be considered endangered is as politically and socially charged a question a any ever posed by our environmental responsibilities. After all, what threatens them is nothing less than society's dependence on fossil fuels. However, even if the bears' adaptability enables them to survive, and allows us to evade the conflict between law and lifestyle, the reprieve will be temporary. Many other Arctic animals certainly will be endangered. Photo: U.S. Fish and Wildlife Service