In 1958, an American ornithologist found pigs devastating a remote Pacific atoll and the removal of 58 pigs was followed by a seabird increase from 650 to 137,000

Sometimes, the greatest victories for conservation efforts aren’t born out of painstakingly laid plans or a multi-million dollar government initiative. They’re often the result of a researcher on a scientific expedition arriving to study one species and noticing another species needs urgent help to survive the threat. That appears to be the case for at least one bird on a remote Pacific atoll in 1958.

The island almost nobody visited

Clipperton Island is a tiny, isolated atoll in the eastern Pacific Ocean, hundreds of miles from any continent. The uninhabited island is a French territory consisting primarily of a sandy ring surrounding an interior lagoon. Despite its isolation, the atoll supports large colonies of seabirds.


Throughout its history, the only people who visited for any length of time were passing ships, a few guano miners, and settlers who ended up pretty much abandoned after that whole venture went bust. They were all forgotten and more or less left alone for years at a time. At some point, the guano miners introduced pigs to the island, presumably as a source of food for the workers. The animals later posed a serious threat to ground-nesting seabirds.

Who was Kenneth Stager

Kenneth Stager was an American ornithologist who served as the curator of bird collections for the Natural History Museum of Los Angeles County. He studied birds and their distribution, and in 1958 he went on a scientific expedition to study seabirds and their nesting colonies at Clipperton Island, specifically brown boobies and masked boobies. The findings of this survey unexpectedly turned the expedition into a conservation intervention.

The feral pigs that had roamed the island for an unknown period had a substantial negative effect on the seabird colonies. They consumed eggs and chicks and, in some cases, killed adult ground-nesting birds. When Stager arrived, the brown booby colony size had been reduced to approximately 500 individuals and the size of the masked booby colony to approximately 150. These numbers combined to total a mere 650 birds at an island which reputedly would have sustained a large colony in the past. Rather than simply documenting the problem, Stager and the expedition team took direct action.

Stager and the expedition team eliminated all 58 feral pigs. He hunted them all out of the land and shot a total of 58 island swine.

What happened after

When a group of scientists returned to the Clipperton Atoll approximately one decade later, in 1968, the island’s booby colonies seemed to have grown substantially, estimated at 25,000 brown boobies and a little over 4,000 masked boobies. From that point forward, the colonies continued their rapid expansion. The 2003 estimate was approximately 25,000 brown boobies and 112,000 masked boobies, or roughly 137,000 individual birds in total.

The intervention removed a major source of mortality, and subsequent surveys documented the recovery of the seabird colonies over the following decades.

There’s more to the story

The ecological consequences of the intervention were more complex. The exact scale of the original decline is hard to pin down with certainty. Early descriptions of Clipperton relied on rough estimates and traveler accounts, while later surveys used far more systematic methods. That means the 500 and 150 figures should really be treated as historical estimates rather than numbers.

The story is more complex than a simple before-and-after account. The initial population estimates are uncertain, while subsequent surveys provide stronger evidence of a dramatic increase in the seabird colonies following the removal of the pigs.

The intervention also produced an unexpected ecological consequence. Once the pigs disappeared, the land-crab population surged, reportedly reaching somewhere around 11 million crabs and they consumed much of the island's vegetation. The pigs may have been keeping the crab numbers in check the whole time. The episode illustrates the complex cause-and-effect relationships within island ecosystems. Removing one ecological pressure can produce changes elsewhere in the ecosystem. Understanding those consequences requires long-term observation rather than assumptions about how an ecosystem will respond.

Why this story still gets told

Stager’s trek to Clipperton ultimately wound up mattering to more than one isolated landmass. It marked one of the first strong, compelling case examples where ridding an isolated island of its invasive species permitted native life forms that have rightful claim to that location to recover, tangibly and measuredly. It wasn’t some offhand tourist taking an impromptu detour; Stager visited as part of a formal scientific expedition researching bird fauna, and the removal of the pigs was a consequence of his activities.

That confluence of an intervening researcher’s actions and decades of carefully gathered data in its wake inspired two scientists, Bernie Tershy and Don Croll, to later establish an entity that has now dedicated itself for nearly two decades to doing the very thing on the world’s islands, now a much more science- and monitoring-driven venture than Stager’s moment in time.

Stager's experience and the subsequent documentation of the seabird colonies' recovery helped inspire scientists Bernie Tershy and Don Croll. In 1999, the idea for a dedicated organization focused on removing invasive species from islands took shape, and Island Conservation was established in 2000.

It is crucial to differentiate between original survey estimates and those from later surveys. The approximately 137,000 birds recorded in 2003 represent the estimate from that particular survey, rather than a current or definitive population figure. What is clear is that removing the pigs eliminated a major source of mortality, while subsequent surveys recorded substantial increases in seabird colony sizes compared with the historically estimated populations.

This serves as both an excellent illustration of conservation and an equally strong illustration of why modern approaches depend extensively on data collection beyond a simple moment and heroic attempt, however warranted. The impulse the researcher expressed in those years was valid. The confirmation just took decades.