Is climate change the primary driver of moose decline in Minnesota?

The Department of Natural Resources is increasingly making the case that a warming climate is exacerbating a long list of moose ailments—from ticks, brainworm, and other parasites, heat stress, and unidentified diseases.

According to the DNR, they are all contributing to a dramatic collapse in moose numbers, one that threatens to all but extirpate the species in Minnesota within 20 years.

While climate change is very real, particularly in northern Minnesota, where average temperatures have jumped as much as 1.5 to 2.0 degrees F. in recent years, it strikes me as a bit too convenient an explanation— and one that in all practical terms, lets DNR managers off the hook for the collapse of one of the state’s most iconic species. After all, far better to suggest that forces outside the agency’s control are causing moose decline then point to forest management or other policy decisions that may be impacting the quality of moose habitat in Minnesota.

At this point, there are still too many inconsistencies and unanswered questions to draw any firm conclusions about what’s actually happening here.

Some of the more obvious questions are these:

‰ Why isn’t climate change having a similar impact on moose elsewhere?

Climate change isn’t just taking place in Minnesota—yet the decline in the moose population seems to be centered right here. While calf-cow ratios are declining in northwestern Ontario (which may reflect the increased calf harvest by hunters there), researchers there have yet to document any significant decline in moose numbers north of the border.

To the east, moose populations are growing significantly in eastern Canada and in New England, areas which are not exempt from climate change. Even if they were, temperatures in northern Minnesota are still significantly colder than in any part of New England. If moose are subject to heat stress here, why aren’t they subject to similar stress in New England? Instead, while moose numbers and range are declining in our region, they are growing even in parts of Connecticut, a state at the same latitude as southern Iowa, and with a climate that is just as mild.

‰ How do genetic differences play into the decline?

Moose in New England and eastern Canada do represent a slightly smaller subspecies (known as the eastern moose) than the western subspecies found here in Minnesota. That certainly raises questions for further inquiry into the genetic differences between the subspecies and how those differences may be allowing the eastern variety to thrive and expand at the southern edge of its range, while the western variety is facing serious decline here in Minnesota. Is it possible the eastern subspecies would better survive in a warming Minnesota?

‰ Could the decline be part of a cycle?

The populations of many boreal forest species rise and fall dramatically over time. On Isle Royale, where the moose population has been tracked since 1959, moose numbers have ranged from a high of almost 2,500 animals in 1997 to barely 400 ten years later. That’s a collapse that pales in comparison to what we’re experiencing in Minnesota, and the population has since shown modest signs of recovery. About 750 moose were counted on Isle Royale in 2011.

In Minnesota, we grew used to a rising moose population in the latter half of the 20th Century, but that was primarily a recovery from the sharp declines experienced in the first half of the century, mostly due to habitat loss and overhunting.

The dramatic nature of the moose decline in Minnesota lends some credence to a natural cycle we don’t yet understand. It’s worth noting that our region’s moose population has fallen by roughly half since 2006. How is it that moose were apparently doing fine six years ago, but suddenly they can’t take the heat? While there is certainly the possibility of a tipping point effect, the weather in our region was actually milder, on average, in the early 2000s than it was over the past several winters (last winter being an obvious exception).

The tipping point explanation would make sense if we’d seen a big spike in temperatures since 2006, but we haven’t experienced that. Certainly 2012 has been off-the-charts, but any effects from this year wouldn’t have shown up in the most recent moose survey, which was conducted last January.

‰ What role do wolves play in the decline?

The public frequently points to wolves as the cause of the decline, and it’s certainly true that moose are doing well in New England, and north central North Dakota, where wolves are absent. It’s also true that wolves and moose have survived together just fine for centuries, and there’s no reason to believe that interrelationship is now suddenly out of whack. Indeed, the available evidence suggests wolves are not a major factor in adult moose mortality here, since only 11 of the 150 adult moose radio-collared in Minnesota were apparent victims of wolves. That’s one less than succumbed to vehicle accidents.

While that doesn’t rule out the impact of wolves on calf survival, it remains unclear why it would be a problem now when it wasn’t much of an issue ten years earlier, when calf survival was much higher. The wolf population may have increased statewide over the past ten years, but that’s not the case in moose country, where wolf numbers have been largely at capacity for a long time.

‰ What role does forest management play?

In the 1980s, moose were doing well in Minnesota, despite the fact that there was much more old forest than there is today. While moose certainly like to browse on young forest species, they also spend a lot of time in heavy cover. In the 1980s, there was more cover than browse and clearcuts and fires attracted a lot of moose, leading to the belief that more young forest would mean more moose.

Of course, nothing is ever that simple in nature. What species need is the right balance of everything, and there’s reason to believe that the state’s forest management over the past 25 years tipped the scales too much in favor of young forests. As areas were converted from mostly old forest to almost all young forest, moose numbers declined. The area along the 200 Road, south of the Echo Trail, is a good example. Twenty-five years ago, this area was mostly big woods and just about any opening was a good place to see moose. But after 20 years of extensive logging, it’s all young woods and moose are tough to find anywhere there. In fact, the DNR just eliminated that management unit from its moose hunt since hunters haven’t had any luck finding moose there in recent years.

And consider that growing moose population in New England. I’ve been in New England a number of times in recent years (my wife hails from Connecticut and has lots of family in the region). I’m always amazed at the forests of New England, which make northern Minnesota’s current woods look like brushland by comparison. The woods are deep and dark and clearcuts are hard to find. And still the moose population is growing out there— so much for the theory that moose need lots of young forest browse to thrive.

Ongoing moose research in Minnesota is starting to track how moose use the various habitats out there in our forests, which might shed some light on all this within the next year or two. Maybe then, we’ll have a better idea if habitat changes are part of the equation.

Until more data is available on this and other possible factors behind the moose decline, it’s too early to point fingers at climate change. There are just too many outstanding questions and inconsistencies to start drawing convenient conclusions.