Bears and Plant Communities

A Grizzly digs for a ground squirrel in Denali National Park, just after the first snow fall of the season. This bear took just a few minutes to excavate the squirrel, which he promptly caught and devoured.

A Grizzly digs for a ground squirrel in Denali National Park, just after the first snow fall of the season. This bear took just a few minutes to excavate the squirrel, which he promptly caught and devoured.

Last week I finished up my final guiding trip of the summer season. It was a long one, two weeks, with just two clients, a great (and hilarious) couple from Germany. At some point, I’ll get around to telling some more about this challenging trip, but suffice to say we got hammered by all kinds of weather, trekked over 50 miles and packrafted some 40 more.

But I’ll save the trip report for another post, so you’ll just have to hold your horses for a few days until I get around to writing about it.

On this trip, like most trips to the Brooks Range, we encountered bears. The first bears we saw were a sow with three cubs. We were hiking up on a ridge, avoiding an enormous, and nasty-looking tussock field, from which one of my clients spotted the bears. They were foraging in a small sedge meadow near the top of a low pass. A few hundred meters away and upwind, we were able to drop our packs and watch them for awhile.

It was a beautiful sunny afternoon, and we leaned back with our binoculars, ate some lunch and watched. The cubs were young, fluffy little things, that spent more time wrestling with each other, or tackling the thick legs of their mother than they did foraging for roots and berries. The adult however, was all business, first working her way through the sedge meadow, digging as she went, before moving up onto the tundra to rake blueberries from the bushes with her teeth.

When digging through the meadow, she was a 4-legged rototiller, sending clumps of dirt and tundra flying through the air. Bear digs are common. Any meadow that holds species of arctic vetch is likely to be dug up by bears at some point. Huge swaths of tundra are often dotted with shallow digs where the top layer of vegetation and duff has been pulled aside to expose the roots below.

In areas with lots of Ground Squirrels, bears will dig deep to extract the fat rodents, leaving behind pits a meter deep and a couple meters across. They perform this feat surprisingly quickly. I once watched a Grizzly in Denali National Park extract a ground squirrel from its hole in just a few minutes of enthusiastic digging.

Eventually, the wind shifted and carried our scent down to the sow with her cubs on the tundra below. She stood up, glanced around and spotted us watching her from the rocks above (not to worry, we were 400m away). She dropped onto all fours and, with her cubs trailing along behind, took off across the tundra in the opposite direction. They didn’t stop running until they were out of sight behind a low rise a half mile away.

In their wake they left behind an acre of tundra, practically upturned.

I figured this kind of impact must play a role in tundra ecosystems. Turns out, I was right.

A fairly recent, shallow, dig where a bear was searching for roots.

A fairly recent, shallow, dig where a bear was searching for roots.

I was able to dig up two studies that looked into this. The first took place in Alaska’s Chugach and Wrangell Mountains and looked at how the species richness of tundra plants is affected by bear digs, how that changes over the life of the dig, and how the plant community structure is affected by bears digging for specific species of plants. Specifically, they looked at squirrel digs (the deeper holes) rather than the holes dug in search of roots.

The authors compared tundra containing bear digs with mature, un-dug tundra. They found that tundra containing digs held significantly (statistically) more species of vascular plants than intact tundra. Disappointingly, that number was a mere one species per plot (13 species in intact tundra vs 14 in areas with digs). The authors appropriately concluded that though bears digging for squirrels do seem to have an impact on tundra plant diversity, that impact is relatively small. They point out that tundra often sees impacts that can change plant cover (frost heaves, erosion, avalanches, etc.) and the plant community is structured around such changes.

Still digging. This was just moments before the squirrel tried to make a last, futile, bolt for freedom.

Still digging. This was just moments before the squirrel tried to make a last, futile, bolt for freedom.

Interesting, but the study failed to get to the point I was really curious about: areas with a high density of digs resulting from bears digging for vegetation.

Findings in the second study, which covered research conducted in Montana’s Glacier National Park, were less vague. The authors looked at how bears digging for glacier lily bulbs impacted nitrogen availability (an important nutrient for plants), and the impact on the lily growth and reproduction.

The researchers found that bear digs significantly (statistically, and biologically) increased the nitrogen in the soil. Counterintuitively, despite the impact from the bears digging the lily’s roots, the same plants grew more prolifically in areas that had been dug. Too, lilies growing in digs produced more seeds and contained more carbohydrates than than those in intact tundra. The bears were so important to the subalpine plant community, that the authors even referred to them as “ecosystem engineers”.

Sadly, I couldn’t find a comparable paper that covered the area where I work in the Brooks Range. As interesting as the first paper was, outlining how squirrel dig sites impact plant communities, it didn’t really get at the meat of my question. I was much more curious about how areas with a high density of shallow root-digs affected the meadows. I guess that leaves me to a do a little critical thinking/hand-waving about my query.

I’ve spent a fair amount of time wandering around these areas and though I don’t know exact lengths of time, it strikes me that it doesn’t take too long for plants to reestablish in the shallow dig sites (deep squirrel digs are something else entirely). Pioneer plants, those that are best adapted to growing in newly exposed soils, are certainly the first species to appear. The surrounding, intact, vegetation is dominated by plants favoring a more established habitat. This, I’m certain, will result in a greater richness of plants in a meadow dug by bears a year or two before.

A meadow with a couple of recent bear digs. Some areas have 30-40% of the soil upturned.

A meadow with a couple of recent bear digs. Some areas have 30-40% of the soil upturned.

If the increased nitrogen found in bear digs in Montana is also true for Alaska, then I’m doubly certain of my conclusion. Plants growing in the bear digs will have access to superior nutrients, resulting in faster growth and greater reproduction. Presumably this will lead to tundra quickly growing over the exposed soils of the digs, which dove-tails nicely with what I’ve observed on my backcountry adventures.

Anyone have thoughts on this? I’d love to hear from some plant ecologists if they think my conclusions are growing in fertile soils, or if I’m off in the gravel, so to speak.

The papers noted in this article are:
Doak, Daniel F. and Michael G. Loso. Effects of Grizzly Bear digging on alpine plant community structure. Arctic Vol. 35(4):421-428.
Tardiff, Sandra E, and Jack A. Stanford. 1998. Grizzly Bear digging effects on subalpine meadow plants in relation to mineral nitrogen availability. Ecology Vol 79(7): 2219-2228.