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Remotely-sensed trends in vegetation productivity and phenology during population decline of the Bathurst caribou herd

November 8, 2021 | 10 minute read

The Bathurst caribou herd of Canada’s Arctic has undergone a dramatic decline in recent decades. Dr. Ryan Danby (RD), Associate Professor in the Department of Geography and planning at Queen’s University, and Dr. Katherine Dearborn (KD), a Postdoctoral Fellow in the Department of Environmental Studies and Sciences at the University of Winnipeg, are authors of a new paper in Arctic Science, which reviews satellite data, and trends in vegetation along the herd’s migration route. They discussed their research in their paper; Remotely-sensed trends in vegetation productivity and phenology during population decline of the Bathurst caribou herd, and its importance with Dick Bourgeois-Doyle (DBD).

Listen to the interview with Dr. Ryan Danby and Dr. Katherine Dearborn or read the interview below:

DBD : Ryan, could you describe the inspiration for the research and the scale of the issues involved with the caribou herd.

RD: Sure, this is the Bathurst caribou herd, which is a large or was a large barren-ground caribou herd, spanning from the Northwest Territories in some years south of great Slave Lake – the large lake that Yellowknife sits on – north to the Arctic Ocean, So a huge range of over 300,000 square kilometres, and in the 1980s, the herd peaked at somewhere around 450,000 animals. And since then, it has declined really quite drastically to the point where the most recent population estimate has them less than 10,000 animals.  I think the estimate is actually at 8,200, something in that order. So just a massive decline of one of the largest herds of caribou in the world to now a remnant of its former self.

These are large barren-ground herds, we call them barren ground because they exist in tundra habitats in the summertime.

From these barren ground areas, then they migrate south to forested areas in the winter. They’re known to undergo large fluctuations like this – a fluctuation in their population sizes is not unnatural. What is unnatural, or outside the range of historic variability with this herd, is the magnitude of this fluctuation, you know, from almost 500,000, almost half a million animals to less than 10,000.

And there are repercussions for that all through the ecosystem, the natural ecosystem, but also the human ecosystem, in terms of hunting, you know, indigenous peoples in the north that rely on this herd for food. There’s a whole sport hunting industry that that revolved around this herd through the 80s. And that has now been shut down. So that the impacts are not just ecological or environmental there, they come to human livelihood as well.

DBD: As you noted, in your paper, there’s myriad possibilities for the cause of the decline. But obviously, something that would spring into a lot of people’s minds is the subject of your study, which was the vegetation.

RD: Yeah, with the vegetation aspect in it, this sort of thinking that maybe changes in their herd’s habitat have had something to do with this decline. There have been some studies that have been done at very fine scales of vegetation change. And when I say fine scale, I mean, setting up long term plots, monitoring those plots, these might be areas that are 10 by 10 metres, you know, the size of a room in your house, or something like that, and monitoring the composition of plants in those areas over time, and seeing how things change over the course of 20 to 30 years.

But remember that this herd ranges over an area of 300,000 square kilometres, it just a massive area. So our ability to use a plot that’s get monitored from year to year, that’s the size of your desk up to the size of a room in your house, it’s tenuous to maintain that same relationship up over such a large area, especially over such a large area that varies quite a lot, you know, ranging from boreal forest in the south to the Arctic Ocean coastline in the north and all kinds of habitats in between.

Extrapolating from the small to this very large area, it’s a little tenuous. So we tapped into the satellite record. There’s literally over I think 3000 satellites orbiting planet Earth right now, a number of them are remote sensing satellites, which are effectively large cameras.  I’m simplifying it a fair bit. But there are cameras that are onboard these satellites, that image and take pictures of the earth using wavelengths that we can’t see, as well as wavelengths that we can see. And they’re constantly monitoring the planet.

And so we were able to use images from the satellites. A satellite in particular that NASA operates to map changes in the herd’s range over its entire range, since the launch of that satellite in 2000.

DBD: So, two things you said there – maybe several things that you said – touched upon the complexity of analyzing the data and interpreting it when as you said that covers a variety of ranges.  One of the things that struck me for the first time in reading your paper was about how different regions serve different purposes to a migrating herd: the calving area in summer area and in the mating area, and then wintering.

In that respect and also with respect to the satellite imagery, Katherine could describe some of the technical issues that you had to overcome? I know that there were gaps in imaging because of aerosol and bare ground.

KD: Yeah, first of all, just the choice of which satellite sensor data set to use is in a really important one. Because as Ryan mentioned, there’s a bunch of them up there that measure wavelengths that are relevant for getting a sense of vegetation productivity in a given area. And so they all have their kind of drawbacks and benefits. I guess there’s sort of three things to consider in my mind. One is the length of the satellite record because different satellites have been up for different amounts of time. And we needed one that covered almost like a good couple of decades of time when the herd was really, really declining rapidly.

The other thing is basically the revisit time of that satellite. So how often does the satellite take an image of a given area because some of them really only have a revisit time of a couple of weeks. They only sort of take an image of the same spot on the planet every couple of weeks. And in a really short growing season like in the tundra, having just a couple of data points, or, you know, a handful of data points throughout a growing season is not really enough to get a sense of the things we wanted to measure, which are how productive was the vegetation in a given growing season and how long was that growing season.

So that’s the other thing. And then the third thing to consider is the spatial resolution of the imagery, because different sensors have different pixel sizes, essentially. And because as Ryan mentioned, we know that these areas are quite heterogenous.

They’re not just like, all the same everywhere, we wanted a fairly fine resolution data set. So with all those things in mind, we settled on the MODIS sensor, because it has a pretty fast revisit time it, it actually takes an image of the entire earth surface approximately once per day. Now a lot of that is cloudy; there’s always clouds getting in the way.

And it also has a fairly fine spatial resolution of 250 by 250 metre pixel size.

So we were pretty confident. And it was launched in the year 2000, as Ryan mentioned, so we had a good long record. So that one kind of fit the bill.

DBD: That’s a NASA satellite?

KD: Yeah, it’s a sensor onboard, actually on two of NASA satellites, that measure those wavelengths that are really important for getting a sense of vegetation productivity,

DBD: You weren’t just measuring the productivity, of course, you were looking for different plant species. And the resolution is such that you can actually identify them?

KD: Unfortunately, no, that’s what really the drawback of using satellite imagery is that you don’t know, if you see a change or if you see an increase in productivity in a given area, you don’t know which species are responsible for that.

And you also don’t know whether it’s a species that’s always been there that’s just growing more productively over time, or whether new species are moving into an area, which is why it’s really important to actually go out on the ground and look at what those changes actually mean, in terms of sort of which plant species are there and how plant species composition is driving those changes you see in the satellite record.

DBD: So given the high level – in two different ways – of the data that you collected, what would you describe as the significant findings of the research?

KD: Well, we – and Ryan can jump in here – but we certainly found an increase in vegetation productivity, I think it was about 13 to 16% of the annual range of the herd showed sort of significant increases in vegetation productivity. And then we also found an increase in the length of the growing season, I think it was about the same amount, somewhere in the nature of about 16% of the herd’s range.

We really didn’t see a decrease in either of those things. Anywhere that the amount of decrease in productivity was 1% or less. And same thing for like the growing season, it didn’t decrease anyway, we only saw kinds of increases

RD: Except in areas that had burned, as I mentioned earlier that the herd ranges between boreal forest in the winter and tundra in the spring, summer and fall, and those winter areas are subject to forest fire, and there’s been a whole lot of forest fires. And we can certainly see that this year across Canada, increasing amounts of forest fire and the areas that did experience a decrease in productivity coincided perfectly with those areas that have experienced fire over the last 20 years or so.

DBD: The fire-ravaged areas that had an increase in productivity would have been fires much longer in the past?

KD: Yeah.

DBD: So those are two areas that would, to my mind, help narrow down of further research looking at the areas that had the 16% increase in productivity and fire areas, you didn’t posit cause and effect or even a hypothesis as to what was causing the decline in the herd. But do you have any thoughtful commentary to make that?

Or is it a little premature?

RD: It’s probably a little premature. We’re cautious about inferring cause and effect here –  in the sense that this is certainly a strong correlation between the period of herd decline and the period that we’ve monitored with respect to vegetation change, and the types of changes that we’ve experienced that they’re, they match quite well.

But one of the challenges, even though there’s benefits, one of the challenges of working with such a broad scale is that we can’t pinpoint, we can’t test those hypotheses that scientists really like to sort of dive into – the nitty gritty –  what this study allows us to do is to target areas now where we can do that.

So it provides the basis now for identifying both geographic areas but also topical areas where now we can we can identify future studies to try and pinpoint more cause and effect here.

One of the things we’re doing is work on the range. So we’re not just desktop ecologists, we’re actually field ecologists, we’ve mounted two – three actually now – expeditions by canoe, through the Bathurst caribou range to sample areas that are that are inaccessible by roadway.

So we’ve flown in and gotten dropped off and picked up a month later. And we’ve been sampling at various areas.

So the areas where we identified lots of change in the satellite imagery in areas where we identified very little change, we’re comparing those kinds of areas to see what’s on the ground in those areas and how the plants have grown using tree ring analysis in areas where there are trees. And in areas where there are only shrubs, we can also look at their annual growth rings. And we can infer a fair bit about growth of plants.

And we call that ground validation of our imagery. So sort of matching up what’s seen from outer space with what we can tell on the ground.

DBD: I think I envy you more for your fieldwork than staring at the computer screens with satellite imagery. One of the things that I read in, again, your paper, was complexities upon complexities and one of the things that that wouldn’t have struck me before, but now seems obvious, is the fact that the declining herd itself would have an impact on the vegetation because you lose this large herbivore. And it’s not going to be eating as much and the other thing, which I think may become evident in the field work, is that you see a growth in vegetation or increase in productivity, but it may be the wrong kind of vegetation.

KD: Yeah, I mean, I guess you would maybe think that an increase in vegetation productivity would be good for a herbivore. And it certainly could be in some cases. But there have been quite a few now studies in the tundra that have shown that tall deciduous shrubs, specifically birch, in the case of the range of the Bathurst herd, are increasing in abundance in the tundra.

And there’s a little bit of evidence starting to come out that suggests that actually might be detrimental to caribou. It’s not their preferred forage, they tend to prefer more graminoids species. So sedges and things like that.

And there’s a bit of evidence to suggest that maybe those tall deciduous shrubs, which are really responding to climate warming and an increasing in abundance. are actually out competing some of the caribou’s preferred forage species.

DBD: One thing that I was curious about, is there any information about the decline in the herd? Is it adults dying, or is it their inability to calve or reproduce?

RD: The demographic parameters, I’m actually not as familiar with Dick, I don’t know that it’s one particular demographic metric that that has been the result here, because you’re right, ultimately, we can come down to population dynamics being a balance or sort of an equation, if you will, between births and deaths, and immigration and emigration.

Right. And then we can add things on to that. There’s some thought that right now as the herd is really small, that the remaining individuals and there’s actually some evidence of this are actually emigrating to other herds. We have some evidence so that the government Northwest Territories has collars, satellite collars on some animals. And I think it was two winters ago, there was a number of animals that moved to another herd. They don’t think that that was the cause of the decline. But it’s an effect of the decline because caribou are very gregarious animals, they like to hang out with each other. And when the herd declines to a certain size, it gets too small, and they can’t find other individuals so they start moving to find where those are.

So there was a few collars – that data from some of these collars that showed these animals moving to join another herd, at least for part of the year.

DBD: So, Ryan, Katherine, there are clearly many issues to explore and ideas to build upon with this research.  Thank you for sharing it.

Banner and in-text images: provided by Dr. Ryan Danby and Dr. Katherine Dearborn

Podcast music is “Spring Cleaning” by Jay Man, www.Our-Music-Box.com

Canadian Science Publishing

Canada’s largest publisher of scientific journals and not-for-profit leader in mobilizing science.