Hmmm...Food for thought : how species comes to “invade” new territories.
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How a giant eagle came to dominate ancient New Zealand
Evidence suggests eagle was part of a wave of feathered invaders.
The now-extinct Haast’s eagle hunting moa in New Zealand, which lacked other large predators. Today scientists are looking at the ancient history of the islands’ birds to better understand how “natural” biological invasions happen.
New Zealand has long been known as a place for the birds—quite literally. Before people arrived 700 years ago, the archipelago hosted an idiosyncratic ecosystem, nearly free of mammals. More than 200 bird species filled a food web all their own. Rather than cows or antelopes, there was a family of flightless birds known as moa. And in place of apex predators like tigers, New Zealand had Haast’s eagle.
Ever since a group of farm workers drained a swamp in the late 1860s and uncovered its buried bones, this eagle has captivated researchers. Julius Haast, the explorer and geologist who published the first notes on the species, described it as “a raptorial bird of enormous dimensions.” Today, biologists estimate that the eagles weighed up to 33 pounds—roughly 50 percent more than any raptor known today. But with a wingspan of only two to three meters—just beyond the range of a bald eagle—this was an oddly proportioned bird.
The shape of Haast’s eagle was one of many puzzles that scientists faced as they studied this long-extinct species, preserved in just a few skeletons, plus scattered bits and pieces. For nearly a century, there was a debate over whether such a large bird could fly; even after that feud was settled, questions remained about whether the bird was capable of killing moa, which in some cases would have been more than 15 times larger than the eagle itself. Now, new scientific techniques, combined with a clearer understanding of New Zealand’s geological history, has placed the Haast’s eagle amid a much larger ecological discussion: how species comes to “invade” new territories.
Scientists now believe that this superlative bird was one in a wave of feathered invaders that conquered New Zealand over a relatively short period. And this was not the only wave of invasions. Haast’s eagle—despite being gone for centuries—has revealed that we live in a much more connected world than we once thought, says biologist Michael Knapp of the University of Otago, who has studied the eagle. If such seemingly isolated islands have repeatedly attracted so many incoming species, he says, then “natural invasions” must be a major force in ecosystems across the world.
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