Invasive species might not be entirely awful, after all

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Over a century ago, New Zealand was faced with a very tiny, very adorable problem. As is the case for just about any island onto which our species has settled, we brought a menagerie with us. Rats and mice are now everywhere humans are, along with dogs and cats, but the main problem for New Zealand was bunnies. Rabbits were everywhere. At that time, the favorite method for dealing with an invasive pest like a rabbit was to introduce a predator known to have a predilection for gobbling up that invasive pest. That’s why, in 1884 and 1885, the government of New Zealand released at least 224 stoats from Great Britain.

Stoats (Mustela erminea) are mustelids, which means they’re related to animals like weasels, otters, badgers, ferrets, minks, and wolverines. On their native Great Britain, more than 80% of the stoats’ diet was made up of the European rabbits, and that’s despite the fact that the rabbits themselves are invasive in Britain. They were first brought to the British Isles some 2,000 years ago and became fully naturalized by the mid-12th century. So New Zealand’s plan was to round up a bunch of British stoats and hope that they could clean up the island’s rabbit problem…like a bunch of very cute municipal workers.

In the third year of New Zealand’s rabbit solution, another 3,000 stoats and weasels were sent from Lincolnshire alone (because historical records are vague, it isn’t clear how many of that 3,000 were stoats and how many were weasels; either way, it was a lot of each). Over the next 20 to 30 years, thousands more would be released; nobody quite knows how many. By the end of the 19th century, so many stoats had been released and in so many places that they’d completely colonized both of New Zealand’s main islands.

While New Zealand’s new stoats went about their business – chowing down on rabbits and making little stoats – their cousins back home in Britain would eventually face a problem of their own.

In the early 1950s, the myxoma virus began to appear in Britain. When the virus infects a rabbit, it causes a disease called myxomatosis, which is often just called “myxo.” The illness spreads among rabbits either through direct contact or indirectly, when bitten by a flea or mosquito that had previously fed on an infected individual. Infected rabbits usually die within two weeks, but not before developing skin tumors, blindness, fatigue, and fever.

Between 1953 and 1955, the illness wiped out nearly 99% of Britain’s rabbit population, with the support of a weary UK government, who knew the kind of havoc that the invasive rabbits could wreak on agriculture. In some places, the virus was intentionally introduced or spread as a means of pest control by placing sick rabbits into burrows with healthy ones.

But fewer rabbits meant less food for their predators, including stoats. Young stoats died of starvation, and fewer pregnant or nursing females were observed than in the years before the disease made its way through the rabbits. Though stoats did began to rely more heavily on invertebrates, birds, and rodents, that behavior change wasn’t enough to keep their numbers from crashing. The average number of stoats caught on game estates decreased by 84% each year for a decade following the outbreak.

Typically, invasive species suffer from a lack of genetic diversity compared to their source populations because populations start from a small set of founding individuals. But in a unique twist, it turns out that New Zealand’s stoat population has greater genetic diversity today than the one in Britain. That’s the conclusion of University of Auckland biologist Andrew J. Veale (who is now a postdoc at the University of British Columbia) and his colleagues, who have reported their findings in the journal Molecular Ecology.

Thanks both to historical records and to genetic samples taken from stoats in both places, the researchers concluded that the increased diversity in New Zealand wasn’t due to multiple source populations for their stoats (they all came from Britain, none from Europe), not because of an increased rate of mutation in the New Zealand population after introduction. Instead, they think it is in large part due to the severe genetic bottleneck occurred after the British population crashed in the 1950s.

It’s not a crazy idea. Another study found that voles introduced 5,000 years ago to Scotland’s Orkney Islands also retained diversity that had since been lost from their ancestral mainland populations.

What this means to the researchers is that introduced populations can in some circumstances act as an “ark for genetic diversity.” It’s an idea that could help save struggling species, and not just stoats. The grey partridge and cirl bunting were also once common throughout Europe and the UK. Changed agricultural practices have led to their decline in their native range, while introduced populations elsewhere continue to thrive. “As we move towards conserving genetic biodiversity, the genetic diversity conserved in introduced populations may also be considered valuable and therefore worthy of re-introduction,” writes Veale.

The researchers caution, however, that the suitability of an introduced population to act as a genetic ark is only possible because the demographic history of both populations is known. Deprived of the relevant information, the genetic data alone might have led researchers to the erroneous conclusion that the more diverse population in New Zealand was the source population, while the British stoats were the invaders. For older populations whose expansions were perhaps not facilitated by humans, genetic diversity itself is not sufficient to infer the historical processes that operated upon it. – Jason G. Goldman | 08 May 2015

Source: Veale, A. J., Holland, O. J., McDonald, R. A., Clout, M. N., & Gleeson, D. M. (2015). An invasive non-native mammal population conserves genetic diversity lost from its native range. Molecular ecology. DOI: 10.1111/mec.13102.

Header image: British stoat via