Vast herds of caribou roaming the barren tundra are among the most iconic images of Arctic wildlife. But today, most of the world's barren-ground caribou and reindeer herds are on the decline.

In some cases, the numbers are dramatic – the Baffin Island herd in Nunavut, for instance, dropped from 235,000 animals in 1991 to just 3,000 in 2014.

But the reasons for this free fall remain unclear. Caribou numbers do fluctuate naturally, but not generally all in the same direction at the same time. Industrial development, hunting pressure and climate change have all been proposed as drivers of the decline, but there's still no consensus about what's really going on.

One possibility that's emerged in recent years is that of a trophic mismatch – the idea that a warming climate could be altering when there's good food available for caribou to eat.

Starting in 2008, researchers working in Greenland found that fewer caribou calves were being born and more were dying because of warmer spring temperatures that were causing plants to emerge earlier. They concluded that by the time caribou arrived on their calving grounds, the vegetation was already older and less nutritious, making it harder for females to sustain their calves.

But now, new research is casting doubt on those findings.

A recent study published in PLOS One has found no evidence of a similar mismatch for the Central Arctic caribou herd in Alaska.

Comparing data from 1977 with new data collected between 2011 and 2013, the authors found no indication that forage quality had declined at the time females calve.

"We really weren't able to pick up on any really ... noticeable shift in the quality of any of the forage species that we were looking at," said author Layne Adams, a wildlife biologist with the U.S. Geological Survey in Alaska.

In fact, Adams said when Central Arctic caribou show up on their calving grounds very few plants have even emerged.

"Right now, they're calving on snow, essentially," he said.

That means the caribou are raising their young largely on stores of fat and protein they've accumulated the previous year – not on new spring growth. So even if plants were to emerge earlier, Adams and his colleagues argue, that shouldn't have a negative effect on caribou.

If anything, they expect the opposite. As growing seasons lengthen, Adams said, "we expect that there actually is going to be more forage of sufficient quality available to them than there is currently."

Adams isn't sure how the researchers studying the Greenland caribou got their results. He doesn't think it makes sense that caribou could be so adversely affected by earlier springs.

"We've actually kind of been questioning that from the get-go," he said.

But not everyone is ready to dismiss the mismatch hypothesis.

"The fact that they didn't see an effect during the course of this study does not mean that it's not there," said Mark Boyce, an ecology professor at the University of Alberta.

He suggested that winter conditions could be an important piece of the story. After an easy winter, the quality of spring forage might not matter so much for healthy caribou.

"But I think that if it's been a tough winter ... and they hit the breeding grounds in poor condition and then the breeding grounds don't have anything for them either, it can be a double whammy," he said.

David Inouye, a researcher at the Rocky Mountain Biological Laboratory in Colorado, pointed out that Adams' study compares data collected over just three years to the data from 1977.

He's "a little reluctant to place a lot of weight on a study that's just three years in a highly variable environment," he said.

A 2013 study of the Greenland herd used a decade's worth of data.

Inouye also pointed to an example of a climate change-driven mismatch in Colorado, where glacier lilies have been flowering earlier, before bees emerge from hibernation to pollinate them.

Those data were collected over nearly two decades.

Inouye said the impacts of mismatches can be "quite significant" on the populations they affect. But they can also be challenging to study, because they involve multiple species and interactions over extended periods of time.

Whatever the reason, the Central Arctic caribou herd is declining, like so many others. In 2010, it numbered 70,000 animals. By 2016, that number had dropped 69 percent – to 22,000.

And if it's not because of a trophic mismatch, then scientists are no closer to understanding why.

"It's a lot of head-scratching," said Adams, though he pointed out that caribou herds do fluctuate naturally, and the herds that roam the Arctic Coastal Plain have been smaller than they are now.

Despite the longer growing seasons, Adams isn't suggesting that climate change will be an overall boon for caribou.

For one thing, warmer temperatures can herald larger swarms of mosquitoes and other biting insects. When the bugs get bad enough, Adams said, caribou tend to head for the windy coast where they can find respite. But that's a trade-off – there's not as much food to be found in the gravelly river deltas.

"Are things going to get worse for them so that they're expending more energy to minimize their insect harassment and spending more time away from places where they can forage efficiently?"

He said it's difficult to predict the overall impact of climate change, given all the variables that come into play.

But to Boyce, it just doesn't make sense to suggest that climate change could be a net benefit for Arctic caribou.

"There's something wrong here," he said. "It would be inconsistent that global warming is enhancing conditions for caribou, given that their numbers are declining here, there and everywhere."

Maura Forrest is a Canadian journalist based in Whitehorse, Yukon. This article originally appeared on Arctic Deeply, and you can find the original here. For important news about Arctic geopolitics, economy, and ecology, you can sign up to the Arctic Deeply email list.

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