After decades of trapping, the last known American marten was spotted on Isle Royale in 1917. Fifty years later, in 1966, the National Park Service planned to re-introduce martens in the Lake Superior National Park, but no one knows if it agency has ever followed by. Then, in 1993, the martyrs were confirmed on the island for the first time in 76 years.
Whether these small carnivores, living in the forest, historically valued for their fur, had been there all the time, found their way back or were introduced in the sixties without anything ever recorded, the last quarter of a century mystery.
But in new research published today (August 23, 2018) in the journal Scientific reports, Researchers from the University of Wisconsin-Madison of the Department of Forest and Nature Ecology, who collaborated with the National Park Service, followed the recolonization to martyrs that probably arrived in the 1990s, shortly before they were noticed.
Genetic studies of martens of Isle Royale and nearby populations in America and Canada showed that the contemporary population came from nearby Ontario, Canada. The animals probably ran on an ice bridge in the winter, the researchers speculate.
The results provide the much-needed context of the natural history of an island that has long been considered a pristine wilderness, but with a long history of ecological interruptions and recoveries. The island park is best known for the ebb and flow of its wolf and elk populations, which have been followed for 60 years.
With the arrival of additional wolves to Isle Royale in the coming months, the new research provides ecologists and land managers with a more complete picture of how dynamic even seemingly isolated island ecosystems can be.
After all, the researchers say, if the killer in domestic cat sizes can find his way, islands like Isle Royale may be less isolated and static than we think.
Jonathan Pauli, professor of forest and nature ecology at UW-Madison, has studied martyrs for years as part of attempts to understand how wild animal communities respond to human disturbance. In 2015, his group provided evidence that martyrs had long since escaped detection on islands in southeast Alaska prior to deliberate reintroduction efforts in the 20th century. And in work published in 2016 with graduate student Phil Manlick, Pauli questioned the effectiveness of periodic increases in re-introduced marten populations in Wisconsin, where the once-out-of-breed carnivore remains an endangered species.
Part of the problem when studying martens: they are so difficult to recognize in the wild.
"I have never seen a Wisconsin marten without catching one," says Manlick.
Manlick, Pauli and co-author Mark Romanski from the National Park Service collected mardi scat samples on the island from 2012 to 2008 and again from 2012 to 2013. They collected additional samples from nearby marten populations in Upper Peninsula, Canada, Minnesota, Wisconsin and Michigan. Samples from another species, the Pacific marten, located in Colorado, served for comparison. In total, the researchers collected samples of 230 people.
They analyzed DNA that had been extracted from the samples to obtain a genetic fingerprint of the different populations. The team then evaluated three possible colonization scenarios: that an old population of martyrs had escaped detection after probable extermination, that the planned re-enactment of 1966 actually took place and was successful, or that a more recent reintroduction was taking place.
An initial analysis of the martens' DNA showed that the Isle Royale population was related to martens from Ontario. But the DNA fingerprint of the island population was different, unique enough to suggest that they might have been isolated from the mainland to have sufficient time to diverge into their own subpopulation. This striking pattern supported the idea that a hidden population of martyrs had escaped detection in the 20th century.
"If something has been isolated there for thousands of years, there will be some (genetic) mutations that build up in that population, where they will look different than everyone else around them," says Manlick, explaining how, over generations, mutations slowly take mutations. "And that's what we expected to see, but we did not do that."
A closer look at DNA from the mitochondria – the energy-generating part of the cell has its own, small genome – revealed that the martyrs of Isle Royale were part of one larger, ordinary population around Lake Superior.
"They look exactly like all the other martens in the region that come from Ontario and Minnesota and Michigan, and all these groups of marters all look the same" genetically, says Manlick.
The low genetic diversity of the martyrs of Isle Royale indicated that their population had experienced a serious genetic bottleneck. An analysis of the bottleneck estimated that the population of the island began with only two individuals – or even one pregnant woman – and that they migrated less than a generation to the island before samples were collected for the first time in the early 2000s.
In other words, the martyrs of Isle Royale repopulate the island just before they were rediscovered, in 1993, after 76 years of absence. Although it is impossible to prove, the researchers venture that the martens run 16 miles over an ice bridge from Ontario, a familiar path for other animals. Such connections with the country are less common in a warming climate.
"Much of what we understand about the island and its ecosystem evolves as we develop new analysis techniques." At the beginning of the project, I pulled in for a remnant of the population, but to find out that martens had just arrived, I blew in. thoughts, "says Romanski. "I am used to being wrong, but in this case I certainly do not mind, because the story is richer."
"If they naturally colonize the island, this has consequences for the management of Isle Royale in general," says Pauli.
This is because ongoing management decisions, such as the relocation of more wolves to the island planned for this fall, are partly based on assumptions that the ecosystem of the island will not recover from disruptions without human assistance. But the little marten found its way back. This also applies to other species. The take-home lesson, the researchers say, is that even protected, isolated ecosystems can be resilient and rarely static.
"This suggests that these islands may not be as isolated as we think," says Pauli, "and that they are indeed much more dynamic than we reward them."