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Beating Around the Bush: Arctic Shrubs May Help Explain Caribou Declines

Climate change is creating greener tundra, which may sound like good news for caribou. But in North America, the animal’s preferred diet of lichen is often being displaced by shrubs of birch and alder, which contain toxins that make them unpleasant to munch on.

Written by Gloria Dickie Published on Read time Approx. 6 minutes
Many of North America's caribou herds have seen dramatic declines in recent decades.Bering Land Bridge National Preserve

The caribou herds that graze the Arctic tundra in the summer months have been declining dramatically in recent decades. In 2009, scientists found that 34 of the world’s 43 major herds being monitored were shrinking, falling 57 percent from their historical peaks on average. Some fared worse. Between 1991 and 2014, the Baffin Island herd in Nunavut dropped from 235,000 animals to a mere 3,000. The Bathurst herd in Canada’s central Arctic plummeted from 472,000 in 1986 to 19,000 in 2016. For the indigenous groups who have long depended on these caribou herds as a source of meat and clothing, such changes have substantial implications for their culture.

But while these declines are well documented, the reasons driving such precipitous population crashes are less clear.

Some scientists have put forward theories about a trophic mismatch – warmer temperatures, they posit, are causing plants to emerge earlier, losing their nutritious value by the time caribou calves arrive, lowering their survival rate.

Others argue that a warming Arctic has created a longer growing season, that, if anything, would establish better forage.

“We expect that there actually is going to be more forage of sufficient quality available to them than there is currently,” Layne Adams, a wildlife biologist with the United States Geological Survey in Alaska, told Arctic Deeply last month.

But if that were true, then why the decline?

Now a team of researchers believe they may have found the answer. A new study published in the journal Science Advances has found a strong correlation between sea-ice loss, the widely observed greening of summer Arctic pastures and declining caribou populations in North America. By comparing 35 years worth of climate and population data from 11 North American caribou herds, the study’s authors put forward the hypothesis that as sea ice diminishes, the adjacent tundra pastures warm, causing an increase in plant biomass as permafrost thaws and the growing season lengthens. But instead of seeing a large bloom of the caribou herds’ preferred forage food – grasses, forbs and lichens – the tundra is becoming “shrubified” with birch and alder, which contain toxins that deter mammals from browsing.

So while a longer growing season and warmer temperatures may appear to be a positive change to researchers like Adams, the shift to non-edible shrubs is actually hurting caribou.

Arctic Deeply recently spoke with the study’s lead author, Per Fauchald with the Norwegian Institute for Nature Research, on how this new research contributes to the prevailing knowledge of caribou declines in the North American Arctic.

This animation shows the size of North America’s caribou herds shrink as the tundra they inhabit greens. (Image Courtesy of Per Fauchald)

Arctic Deeply: How does diminishing sea ice affect climate warming in the tundra?

Per Fauchald: The extent of sea ice is really important for the summer climate in the Arctic. When you have extensive sea ice, then it’s cold on the plains or the coastal areas close to the Arctic Ocean, and that has effects on the vegetation and animals. When you have less ice, it’s warmer in the tundra. It’s obvious sea ice has a huge impact on the climate, and that has been shown in several other studies as well.

Arctic Deeply: Can you break down what you found when you compared sea ice concentrations to summer plant biomass and the population growth of caribou?

Fauchald: Sea ice is shrinking in the Arctic Ocean. When the sea ice is shrinking, it’s getting warmer on the tundra. That is good news for the plant growth because you have greener vegetation. We thought that would mean more food for the caribou and herbivores on the tundra, so in theory it should have a positive effect. But it didn’t. Actually, what we found was that the greener vegetation was associated with declining caribou herds. What is the explanation for that? We think that this relationship can be explained by poor pasture quality when the pasture gets greener on the tundra. The composition of the plants available as food for caribou is not as good.

Arctic Deeply: Are you able to quantify the negative effects seen on North American caribou populations from the shrubification?

Fauchald: That is difficult to say. We find that this explains a large part of the variation that we see in population growth in caribou. The growth in caribou is quite variable – we see a lot of fluctuation in these herds. What we find is there’s a close relationship or correlation between the growth in the caribou herds and the vegetation greenness that we can measure from remote sensing. It explains about 50 percent of the variation.

Arctic Deeply: There’s been a lot of back-and-forth over what’s causing population declines in caribou in the Arctic. How does this contribute to the prevailing knowledge of what’s happening in North America?

Fauchald: You have a lot of different thoughts as to what is actually happening to caribou populations. You have discussions going on about density dependent effects, which means that when you have a high population of caribou, there is less pasture available, and some caribou herds will decline. There are also the effects of predators and the effects of climate. In climate for example, winter snow condition is thought to be very important. But what we find is that we can actually explain a pretty large amount of the variation in population growth of caribou just by comparing the greenness of the pasture. This is kind of controversial. Lots of caribou researchers will not agree with us on this, but this is what we found.

Caribou in Alaska’s Denali National Park. (NPS Photo by Jay Elhard)

Arctic Deeply: How exactly will this be controversial? What reactions do you expect to get?

Fauchald: This is a qualitative study, so we are making correlations. The criticism that you get is that you didn’t account for all the different kinds of factors. We really tried to account for the different climate parameters, and that didn’t change our results. But, in a way, that is an endless discussion. And then you can have different local conditions. These results are a common feature for the 11 caribou herds that we have data on in an area from Alaska to Quebec. It’s a huge area. You can have a lot of different local conditions that can be more important for caribou herds locally, of course. For instance, you can have more storms in some places, in other places predators could be more important. But, in general, you find this trend of a warmer climate and more greener pastures and diminishing herds in North America.

Arctic Deeply: Could similar changes be happening with herds in other parts of the world?

Fauchald: We find that these non-edible shrubs are pretty prevalent in North America, but not so in Greenland, Russia or Scandinavia. They are important in North America. It’s possible we’re seeing other reasons for declines in those populations.

Arctic Deeply: What are some of the potential limitations of this study and only working off of 35 years of data?

Fauchald: The climate in the Arctic is changing so fast. The value of these time series is limited because we’re going to have a very different climate in the near future and we don’t have the data to know what is going to happen. We just have to use what data we have and try to tease out some mechanisms that could be important for the future. That’s an important point here. We cannot say for sure that we’re going to have more and more shrubification and diminishing herds – but that is a scenario. That is the problem with climate research these days. Things are happening really fast and we don’t have data for predicting what is going to happen.

Arctic Deeply: What are your next study steps?

Fauchald: First we need to verify the hypothesis that shrubification is problematic for caribou. That could be done with local studies or experimental studies looking at what caribou prefer to eat. We could also think of comparing areas with toxic shrubs to areas that don’t have them. Does that make a difference?

Arctic Deeply: Is there anything else you’d like to add about your research?

Fauchald: It’s important to realize that what we propose as a hypothesis is not something we can be entirely sure about, because things are happening so fast and changing so much. The climate we are going to see in the Arctic in the future will be very different from what we have today. It’s very hard to predict what is going to happen with the caribou. What we can do is try to monitor them, try to come up with hypotheses of what is driving population, and come up with some mitigating effects for the detrimental effects of a changing climate on caribou. Caribou are one of the few species left on earth where you have this huge terrestrial migration. It’s a very special situation in an ecological context. And I think it’s an important point that caribou are also so important for indigenous people in the Arctic. It’s a major species that has been hunted for generations in the Arctic of North America.

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