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Ask anyone who loves a good action movie and they will tell you that it just takes one lone wolf to save the day, but in this particular case it wasn’t Bruce Willis, but a real wolf called ‘The Old Gray Guy’.

The comparison to a movie is perhaps not so far-fetched, as the story plays out like most action flicks – a devastated community is on the brink of extinction until a lone saviour strolls in to save the day. In this case the lone saviour was The Old Gray Guy who managed to cross an ice bridge onto the remote island of Royale, which lies off the coast of Michigan in Lake Superior.

His arrival revived the flagging fortunes of the wider wolf population, which had been hit by disease and inbreeding, and triggered cascading effects that improved the health of the overall forest ecosystem.

At the time of his arrival, most of the indigenous wolves had been weakened by the effects of having too small a breeding group, and this led to an explosion of the moose population, which in turn had devastating effects on the local fauna.

According to a study published in Science Advances, “Issues like inbreeding and low genetic diversity are an important concern for scientists, but this is the first study that shows when you have these genetic issues, they don’t just impact the particular population and increase the risk that they will go extinct: they also have these really big knock-on effects on all the other species.”

By the 1980’s, the wolves were in trouble due to the arrival of canine parvovirus which drove their numbers down from a high of 50 to around 12. Though the disease eventually disappeared, the population didn’t recover right away. The reason was severe inbreeding, which caused lower reproductive success, as well as poorer health outcomes such as spinal deformities of the kind often seen in purebred dogs.

As Sarah Hoy, an ecologist at Michigan Technological University notes in the study, “If you’re a wild wolf and you’re having to take down prey like a moose that’s eight times your size, that can make life in the wild really tough for you.”

But then in 1997, a lone wolf crossed an ice bridge that briefly connected Canada with the remote Isle Royale. Identified as “M93” by scientists, he was affectionately nicknamed “The Old Gray Guy.” He quickly became the breeding male in one of the island’s three wolf packs and went on to sire 34 pups, greatly improving the genetic health of the population and the kill rate of its prey.

By reducing the moose numbers, the wolves helped bring the forest back into balance, which was most notable in the effects on balsam firs – the species commonly used as Christmas trees. With fewer moose, the trees began growing at rates not seen in decades, which is vital for the renewal of the forest and the myriad plant and animal species that depend on it.