Line Islands: A Gradient of Human Impact on Reefs

Provided by Enric Sala, National Geographic Society

Reefs on uninhabited atolls exhibited less coral disease, greater coral recruitment, and food webs dominated by top predators such as sharks and groupers. Photo credit: Enric Sala, National Geographic Society

The Line Islands, a chain of a dozen atolls and low coral islands in the central Pacific Ocean, are home to some of the most remote and pristine coral reefs on Earth. The closest continents, North America and Australia, are nearly 6,000 km away. The Northern Line Islands consist of five island formations owned by the United States: Kingman Reef, Palmyra Atolland Teraina, Tabuaeran, and Kiritimati.¹ Kingman Reef and Palymra Atoll are uninhabited, while Tabuaeran and Kiritimati support a growing human population. Overfishing, pollution, and global climate change constitute the greatest threats to the reefs in this area.

In 2005 and 2007 a team of scientists embarked on expeditions to the Line Islands’ reefs with the goal of establishing an ecological baseline for healthy coral reefs and identifying the impacts of human activities on reefs.² The absence of baselines against which to compare today’s coral reefs presents one of the greatest challenges for reef conservation. These baselines would allow scientists to identify characteristics of deteriorated reefs and guide management decisions about their rehabilitation.

The team surveyed the reef ecosystems near the uninhabited atolls of Kingman, Palmyra, and Millennium to construct a baseline, and then examined the reefs near the populated atolls of Tabuaeran and Kiritimati to document changes associated with human activities. These surveys documented everything from the smallest microbes to the largest predators. They included data on fish and predator populations, water quality, macroinvertebrates and macroalgal assemblages, as well as the reef’s diversity and health.

The team’s study revealed a strong association between human population growth and ecosystem decline. Reefs on uninhabited atolls exhibited less coral disease, greater coral recruitment, and food webs dominated by top predators such as sharks and groupers. Top predators accounted for nearly 85 percent of the weight per unit of area for fish, while the same predators accounted for only 19 percent of this weight on populated atolls. Small planktivorous fishes and fleshy algae dominated the reefs near populated atolls. The team’s journey helped to expose how human influence is the most paramount determinant of reef health, and how minimizing human impacts on reefs may increase their resilience to global climate change.

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