The great flightless insular moorhen controversy

tristan moorhen

Off the southeastern coast of Africa, the British Empire still holds onto some islands. The most famous of these is St. Helena, where Napoleon lived out his final days in his second exile thousands of miles from France and Europe and any trouble he might want to cause.

On two of these islands, though, a taxonomic controversy has brewed for decades. On Gough Island and Tristan da Cuhna, insular forms of moorhen (also known as gallinules in much of the US) existed.  They were smaller than the common moorhen, and they possessed shorter wings. In this way, they exhibited both insular dwarfism, a common trait of organism evolving on isolated islands, and the loss of flight that sometimes happens when birds evolve without selection pressures from predation to ensure flight in the population.

The moorhens on Gough Island, nearly 400 miles away, were quite similar to those on Tristan da Cunha, and in the 1950s, some Gough island moorhens were introduced to Tristan da Cunha.

These Gough Island birds did quite well on the new island. where an estimated 2,500 breeding pairs now exist. However, because these birds were introduced from Gough Island, they are not regarded as native and are not protected.

Traditionally, experts have regarded the Gough and Tristan moorhens as distinct species. The Gough species is called Gallinula comeri, while the extinct Tristan species is called Gallinula nesiotis.

However, over the years, it has been suggested that the two were of the same species, and the introduction of the Gough species was actually a reintroduction.

Not many DNA studies have been performed on these birds, but the most notable is Groenenberg et al (2008).   This study examined samples that have been collected over the past two centuries, including a specimen from Tristan da Cunha that was collected in 1864.

The authors found that the two forms were roughly genetically distant from each other as different subspecies of the common moorhen. Indeed, if one were wanting to keep the common moorhen a monophyletic species, one would be forced to include these two insular forms as subspecies of the common moorhen.

The authors found that the Gough moorhen had replaced the Tristan form, and they were different taxa. However, because their genetic difference was equivalent to the genetic difference between some common moorhen subspecies, the authors proposed that these two forms be regarded as subspecies, which they propose as Gallinula nesiotis nesiotis and G.n. comeri.

These birds are most closely related to African and European moorhens than they are to South American ones,

However, the debate gets fairly interesting here because the South American common moorhens are typically considered subspecies of a quite wide-ranging species. When the authors performed their research, the common moorhen was believed to have existed in Eurasia, Africa, and the Americas, but in 2011, the New World population was given full species status, which is usually called the common gallinule. Taxonomists fixed the paraphyly of the common moorhen by creating this new American species (Gallinula galeata).

But this study does not fix the controversy about the moorhens on Tristan da Cunha. Even if the Gough and Tristan populations constitute different subspecies, a real debate can be made as to whether the birds on Tristan da Cunha represent an introduction or a reintroduction.

And this is where the subjectivity part of taxonomy sets in. If the Gough subspecies behaves in the ecosystem in an equivalent way to the extinct Tristan subspecies on Tristan da Cunha, then one could just make the argument that the arrival of these birds in the 1950s was a reintroduction. If they behave in a fundamentally different way from the extinct Tristan birds, then they were simply introduced and certainly don’t require any special protections as a native species under the law.

But the subjective part is where to draw the line between being fundamentally similar or fundamentally different. Yes, the Gough subspecies is genetically different, and it may have some attributes that cause it behave just slightly differently from the extinct Tristan subspecies.

However one answers this question, one should keep in mind that it cannot answered so easily.

This debate is quite strong, not just in gallinules and moorhens, but a big debate exists within some quarters as to whether the feral horses of the American West represent a rewilding from the Pleistocene. Horse evolved in North America and then became extinct at the end of the Pleistocene, but a huge debate exists on how to classify the various late Pleistocene horses. They may have been a single species that was very close to the modern horse. In which case, the feral horses of the American West might be argued to be rewilding population. A more recent study on cheek teeth and ancient mitochondrial DNA of these horses revealed there were three species of horse in North America at the end of the Pleistocene, one of which was very close to the modern horse.

But the Pleistocene ended around 10,000 years ago, and the ecosystems that maintained horses on the range no longer exist in the same way. In this case, one could not honestly say that this was a rewilding. It would be an introduction. (That’s where I stand on the horses as native wildlife controversy.)

However, I’ve often thought about what would happen if we somehow got greater prairie chickens to thrive on the East Coast once again. A subspecies of the greater prairie chicken called the heath hen once ranged all the way down the coast from New England to Northern Virginia. It was a colonial staple, and it was hunted out of existence.

The last population of these birds lived on Martha’s Vineyard, and the last one died in 1932. Attempts have been made to introduce greater prairie chickens to the island in an attempt to restore something like the heath hen to the island, but these attempts have fails.  If such an attempt were successful, it would be very much like the replacement of the Tristan moorhen with the Gough subspecies.  A debate could be had as to whether it was was reintroduction or not, but at least it would be something.

So the story of the moorhens on these South Atlantic islands tells the story of a controversy. It is one that rages in the conservation community all the time. Can you restore an extinct subspecies by introducing another related subspecies?

That answer is never going to be fully black and white. Ecological as well as taxonomic considerations have to be examined. Otherwise, someone could easily make the argument that wolf reintroduction and conservation are silly ideas, because, well, domestic dogs are everywhere. Dogs are a subspecies of wolf, so they just replaced them.

So this controversy will rage hard as we try to deal with this nasty extinction mess. We don’t always have all the answers. We don’t always have the best solutions. But we need to think it through carefully. It’s all gray or grayish.

Natural History

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