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Why did the passenger pigeon die out? Humans really did cause the extinction of this species

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January 11, 2018 Norwegian University of Science and Technology

The passenger pigeon was once among the most numerous species on Earth. The last passenger pigeon died in the Cinncinati Zoo just over 100 years ago. How did it all go so wrong?

….In 2014, a study in published in the scientific journal PNAS strongly suggested that humans were simply the final straw in destroying a species that was already vulnerable and headed to oblivion…..The researchers asserted that…the population of the species varied greatly, similar to lemmings, but over a longer period of time.

[They used] the PSMC method [that] can use the information in the genes of a single individual of a species to map the history of the species….The problem is that the PSMC method can’t be used on passenger pigeons. The new research in Science provides completely different results.

…In passenger pigeons, most of the genetic diversity was found at the ends of the chromosome. The middle of the chromosome showed little variation from one generation to the next as a result of the selection on these genes….The researchers behind the [new] article in Science didn’t use the PSMC method. Instead, they used mitochondrial DNA from 41 passenger pigeons as their starting point. …

…Scientists previously believed that the larger the population of a species is, the more genetically diverse it will be. But this theory has turned out to be wrong, as the recent passenger pigeon research has shown…..[in this new Science study] the large population size appears to have enabled passenger pigeons to adapt and evolve more quickly and thus remove harmful mutations. In species with fewer individuals, chance can cause a less beneficial mutation to persist, but chance plays less of a role in species with greater numbers of individuals….

….”The passenger pigeon died out because of people,” is Gilbert’s short version. The passenger pigeon wasn’t in trouble prior to Europeans arrival in North America. Nothing suggests that the species was struggling in any way….

In the 19th century passenger pigeons were so numerous that there were contests to shoot as many of them as possible during a certain period of time. In one competition, the winner had shot 30 000 birds.

…the story of the passenger pigeon has contributed to a greater understanding that even prolific species can become extinct…

…People ate passenger pigeons in huge amounts, but they were also killed because they were perceived as a threat to agriculture. As Europeans migrated across North America, they thinned out and eliminated the large forests that the pigeons depended on. The pigeons lived primarily on acorns.

As the species was already dying out, 250,000 birds – the last big flock – were shot on a single day in 1896. That same year, the last passenger pigeon was observed in Louisiana. It was also shot.

The pigeons were probably dependent on a large flock size to reproduce. Their instincts didn’t work when only a few individuals remained here and there. The last passenger pigeon died in the Cincinnati Zoo in 1914.

Gemma G. R. Murray, André E. R. Soares, Ben J. Novak, Nathan K. Schaefer, James A. Cahill, Allan J. Baker, John R. Demboski, Andrew Doll, Rute R. Da Fonseca, Tara L. Fulton, M. Thomas P. Gilbert, Peter D. Heintzman, Brandon Letts, George McIntosh, Brendan L. O’Connell, Mark Peck, Marie-Lorraine Pipes, Edward S. Rice, Kathryn M. Santos, A. Gregory Sohrweide, Samuel H. Vohr, Russell B. Corbett-Detig, Richard E. Green, Beth Shapiro. Natural selection shaped the rise and fall of passenger pigeon genomic diversity. Science, 2017; 358 (6365): 951 DOI: 10.1126/science.aao0960

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