New Zealand has long been known as a place for the birds, literally. Before humans arrived 700 years ago, the archipelago was home to a peculiar ecosystem, almost free of mammals. More than 200 bird species filled their own food web. Instead of cows or antelopes, there was a family of ratites known as moa. And instead of apex predators like tigers, New Zealand had Haast’s eagle.
Ever since a group of farm workers drained a swamp and uncovered the buried bones in the late 1860s, this eagle has fascinated researchers. Julius Haast, the explorer and geologist who published the first notes on the species, described it as “a bird of prey of enormous proportions.” Today, biologists estimate that the eagles weighed up to 33 pounds — about 50 percent more than any bird of prey we know today. But with a wingspan of just two to three meters — just beyond the reach of a bald eagle — this was an oddly proportioned bird.
Haast’s eagle shape was one of several puzzles 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 has been a debate as to 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 is said to be more than 15 times larger than the eagle itself. Now, new scientific techniques, combined with a better understanding of New Zealand’s geological history, have placed Haast’s eagle in a much larger ecological discussion: how species ‘invade’ new areas.
Scientists now believe this superlative was a bird in a wave of feathered invaders that conquered New Zealand in a relatively short period of time. And this was not the only wave of invasions. Haast’s eagle 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 inbound species, he says, then “natural invasions” must be a major factor in ecosystems around the world.
Digging for answers
New Zealand has always held an important place in scientists’ understanding of extinction. When Western scientists first encountered moa, the idea that species could become extinct was only a few decades old. Their skeletons quickly became a hot item. “You could pretty much name your price,” says paleobiologist Paul Scofield, chief curator at the Canterbury Museum in Christchurch. “It was really what made our museum possible.” Haast himself launched the museum and amassed the first collection by exchanging moa fossils for several other archaeological and zoological curiosities.
New Zealand retained unusual species, including the famous kiwi that cannot fly. Combined with these existing eccentrics, the moa fossils helped establish the idea that New Zealand was a lost world, a place where ancient creatures, sheltered by distance from the rest of the world, managed to survive mass extinction events. Subsequent geologists confirmed that these rocky islands were once part of a supercontinent they called Gondwana, but broke up about 80 million years ago. In 1990, a television series described New Zealand’s islands as “Moa’s Ark,” popularizing the catchy name of the long-held model of how the bird-filled ecosystem came to be.
By the late 1990s, however, scientists realized that there was a period during the Oligocene, about 25 million years ago, when geological and climatic changes could have flooded all of New Zealand. Such a flood would have wiped out most – if not all – species on the islands. The theory, which came to be known as the “Oligocene drowning,” met with resistance from some scientists, sparking a heated debate over exactly how much land was covered.
Fortunately, new technologies emerged to answer that question. Scientists began extracting and sequencing DNA from fossils; this meant that researchers could compare ancient DNA with modern genomes and create family trees of the evolutionary relationships between living and extinct species. Such “phylogenies” could roughly determine when two species split from their common ancestor — data useful in settling the battle over New Zealand’s geologic history.
In 2005, a team of scientists published a paper comparing DNA sequences extracted from two Haast’s eagle fossils with the genomes of 16 modern eagles. The scientists determined that the closest relatives of the large lost bird were Australian species, as expected. The genomic data suggested that the family tree had split over the past few million years. Subsequent analysis established the divergence time about 2.2 million years ago.
Score one for the Oligocene drowning hypothesis: The eagle appeared to have arrived after the proposed immersion. But later analyzes of several other New Zealand species showed divergence times on the order of tens of millions of years. Some species then persisted throughout the Oligocene.
In 2014, geological evidence had convinced most scientists: Yes, much of New Zealand had drowned, but small patches of land — perhaps 20 percent — had survived. While a few species from the islands date back as far as Gondwana, many others, including the Haast eagle, were newer arrivals.
