As the last wolves die off on Isle Royale and the Lake Superior island's moose herd continues to grow rapidly, another odd phenomena is occuring.
Isle Royale moose are getting smaller.
Research to be published in an upcoming edition of the journal Global Change Biology shows that the size of moose skulls has shrunk 16 percent over the past 40 years.
The Michigan Technological University research, which analyzed the skulls of 662 full-grown moose on the island, also found that moose that experience a warmer-than-average first winter tend to live shorter lives.
Michigan Tech scientist Sarah Hoy concluded that the ever-evolving moose relationship with wolves, coupled with climate change, is spurring smaller adult moose. Only skulls from moose that made it to at least age 5 were studied.
"We found that the decline in body size appeared to be the combined effect of changes in winter temperatures, as well as" periods of high moose populations due to fewer wolves on the island, Hoy told the News Tribune. "Moose are thought to be more nutritionally stressed when their population is really large because there is more competition for food."
Hoy said the conditions that moose are born into "have a massive impact on not only how big you are but also how long you're going to live," Hoy said. "This idea isn't new. What we're trying to do is establish how climate warming is affecting this iconic, cold-adapted species. We found evidence suggesting that moose experiencing a warm first winter tended to be smaller as adults and live shorter lives."
The study was possible because fellow Michigan Tech scientist Rolf Peterson has been studying the island's moose and wolves for 50 years, more recently joined by Jon Vucetich. They have collected, measured, analyzed and stored hundreds of moose skulls over that period as they also counted wolves and moose. The same researchers discovered that wolf numbers crashed in recent years due to genetic defects caused by inbreeding, which rendered pups unable to survive. Inbreeding occurred because new wolves have not ventured to Isle Royale with fewer periods of ice connecting the island to the North Shore due to more warm winters.
Scientists for decades have known that the same species tend to be bigger in northern climates than southern. Minnesota whitetail deer are noticeably larger than the same species in Georgia, for example. But on the island, where wolves are the only predator and moose the only major prey, it's not that simple.
While it might seem odd that the island's moose are rapidly increasing in number but shrinking is size, scientists say it actually makes sense. As wolves have disappeared from the island (likely only one wolf remains) moose numbers have tripled in the last 10 years to 1,600 this past winter. Competition for food contributes to stress and malnutrition, resulting in smaller moose.
Moose numbers have been through several up and down cycles over those 40 years, based on wolf numbers.
Peterson says it's probably the combination of several periods of fewer wolves, leading to too many moose, leading to moose malnutrition that has been the primary problem for Isle Royale moose. Throw in warmer winters more often and moose are showing noticeable change in body size.
"Warmer temps in winter probably reduce foraging opportunities for moose (resulting in) poorer nutrition. Also, shorter winters probably translate into more winter ticks, which also reduce energy levels," Peterson told the News Tribune. But he said smaller moose are mostly "a story of poor nutrition with high population size as there is less forage available per moose."
Vucetich said the problem of too many moose and malnutrition may get worse now that wolves are all but gone from the island due to inbreeding and genetic defects. He and Peterson have lead the effort to introduce new wolves to Isle Royale. That decision is expected early in 2018 from officials at Isle Royale National Park.
"We're likely looking at a population in transition," Vucetich said of Isle Royale moose. "And the healthiest transition would almost certainly involve restoring wolf predation to Isle Royale."