Tropic breezes rustle the palms, a turquoise sea laps white sand, and below the surface a coral reef teems with photogenic fish. What could possibly be wrong with this postcard scene? Enric Sala, a marine ecologist, says such a seemingly flourishing reef might actually be an ecosystem in distress.

This radical notion fits with what Sala, a National Geographic Fellow, has observed during two recent expeditions to Kingman Reef, part of a chain of Pacific atolls and islands (called the Line Islands) that straddles the Equator 1,000 miles south of Hawaii. Places like Kingman, remote and near pristine, preserve a record of the world when the human footprint was light. They provide a reference point against which to measure change and a blueprint for conservation. But they are a scarce resource. "Worldwide, there are maybe 50 reefs in this sort of condition," says Sala. He chose the Line Islands because they provide a gradient of human impact—from uninhabited, unmodified Kingman Reef at one end to ecologically degraded Kiritimati (Christmas Island), with a population of more than 5,000, at the other.

Kingman's 30-mile triangle of coral encloses a lagoon the size of Manhattan Island. Above water, nothing grows. The only dry land consists of a few spits of sun-whitened coral rubble and the dead shells of giant clams. But beneath the surface is a world of rare luxuriance. The reef is a glittering city of staghorn, mushroom, pillar, and plate corals packed so tightly together there is hardly a patch of bare sand. Through their interstices dart fusiliers and damselfish, butterflyfish and parrotfish, and scores more of the plankton sippers, coral nibblers, and algae grazers that populate a reef-fish community.

Patrolling above the coral skyline are the reef 's overlords: gray and whitetip reef sharks and hordes of aggressive red snappers. Indeed, fully 85 percent of Kingman's fish biomass is in the form of these large predators, and three-quarters of the predator biomass is sharks—the opposite of the conventional reef snapshot, with rainbow throngs of aquarium-size fishes frolicking in a coral garden and barely a predator in sight. Kingman's proportion of apex predators, the top tier of the traditional biomass pyramid, is greater than has been found in any other coral reef ecosystem. Here the biomass pyramid is turned on its head.

At first glance, an upside-down pyramid is counterintuitive. On land, we are familiar with the notion that an apex predator, such as a lion, must eat many wildebeests to survive. But imagine a world with one pound of wildebeest for every five pounds of lion. The only way an inverted pyramid can function is if there is rapid turnover of biomass at the lower levels. Prey must be fast growing and quick to replenish; predators must grow slowly and live long. This seems to be the case at Kingman Reef. In the warm tropical waters, many prey species spawn several times a year, replenishing their stocks as rapidly as the predators deplete them. Even so, the prey barely manage to sustain the predators: Researchers found the stomachs of red snappers at nearby Palmyra Atoll, another protected reef, mostly empty. The picture that emerges of life on a healthy reef is one of abundant predators living in perpetual hunger and scarce prey living in perpetual fear.