The population of the world’s seabirds has plummeted 70 percent in the past 60 years, a Canadian-Australian research team has found.

The drop is equivalent to the disappearance of around 230 million seabirds since 1950, according to a statement from the University of British Columbia, where some of the researchers are based.

The sharp decline has been driven by human disturbances to the environment: Most seabird species simply cannot reproduce fast enough to replace all the animals getting tangled up in fishing gear, injured by marine plastic pollution, outcompeted for prey by rapacious fishery fleets, or hunted by introduced species such as cats and dogs.

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Global warming and development pressures on avian habitat are also harming seabirds. The human population more than doubled during the six decades encompassed in the study, from around 2.5 billion in 1950 to 6.8 billion in 2010, and the United Nations estimates that about 44 percent of the population lives within 90 miles of a coastline.

During the same period, heat-trapping carbon dioxide in the atmosphere — the leading driver of climate change — grew from under 320 ppm in the 1950s to 385–390 ppm in 2010, according to the records kept by federal scientists at the Mauna Loa Observatory in Hawaii. (The current level is just over 400 ppm.) The resulting sea-level rise and warming temperatures have affected seabird nesting sites, prey fish, and migration patterns.

In their study, published in July in the journal PLOS One, University of British Columbia graduate student Michelle Paleczny and her colleagues aggregated six decades of English-language seabird population records. They ended up with significant data on more than 500 breeding colonies of 162 seabird species, half the total number of 324 species and representing 19 percent of the world’s seabird populations.

Once the researchers analyzed the best-monitored populations, they found massive changes to five of the world’s most abundant avian populations. Sooty terns of French Polynesia and South Orkney; soft-plumaged and Kerguelen petrels of South Sandwich Island; and Guanay cormorants in Peru “accounted for over 30 percent of the total numbers of seabirds in the sampled population in 1950,” wrote the scientists, but “all of these populations were reduced to less than 5 percent of their initial size by 2010.”

While some smaller seabird populations grew during the same period, increases in these less numerous birds were nowhere near enough to offset the losses in the major populations.

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Certain conservation efforts have been successful, Paleczny and her coauthors noted, such as seabird hunting bans, eradicating introduced species that prey on seabird nesting colonies, and reducing entanglements in fishing gear. But these would need to be expanded by several orders of magnitude to reverse the overall trend.

“When we see this magnitude of seabird decline, we can see there is something wrong with marine ecosystems. It gives us an idea of the overall impact we’re having,” Paleczny said in the statement. “Our work demonstrates the strong need for increased seabird conservation effort internationally.”

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