In this episode, I’m taking a deep dive into the impacts that grizzly bears have on subalpine and alpine meadows as they tear up the turf in their constant quest for roots, tubers, and ground squirrels.
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Now Listed as one of the Top 15 Ecology Podcasts on the Internet
Before we get started I just wanted to share the news that the Mountain Nature and Culture Podcast was recently selected as one of the top 15 podcasts focussing on ecology by Feedspot. I’ve put a link to the list in the show notes for this episode, but if you want to head straight there, you can visit https://blog.feedspot.com/ecology_podcasts. If you like this show, you may be able to find additional shows to subscribe to. One example is a show called Naturally Speaking produced by the Institute of Biodiversity, Animal Health and Comparative Medicine at the University of Glasgow.
The Mountain Nature and Culture Podcast is now one of the top 15 podcasts on ecology. Click To TweetAlternatively, you may enjoy the show Future Ecologies which looks at the ecological processes that define planet Earth, how they affect us, how we’ve affected them, and how we can align with them to create vibrant, biodiverse, and resilient societies.
If you’re like me, I’ve always struggled to find ecology-focused programs so this list might just be the place to go to expand your subscription list…next up grizzlies tearin’ up the turf.
How do Grizzlies Impact the Alpine Landscape when they dig?
Grizzly bears are designed to dig. It's what they do. They dig for roots and tubers. They dig for ground squirrels and marmots. In fact, digging is so important to brown and grizzly bears that their entire physiology has evolved to make them the ultimate excavator.Click To TweetGrizzly bears are designed to dig. It’s what they do. They dig for roots and tubers. They dig for ground squirrels and marmots. In fact, digging is so important to brown and grizzly bears that their entire physiology has evolved to make them the ultimate excavator.
If you’re lucky enough to get a good look at a grizzly bear, you’ll notice their claws are incredibly long, up to 8 cm, making them longer than your longest finger. In addition, they sport a huge shoulder hump made of dense muscle to power those sharp claws as they tear the turf apart.
Do you hike in the Rocky Mountains? If so, you’ve probably come across grizzly bear diggings when wandering through the subalpine and alpine. When a grizzly tears into the turf, it’s like a commercial excavator has worked over the meadow.
In other episodes, I’ve examined the many seasonal foods that drive the annual movements of grizzly bears in the central Rockies. The seasonal availability of specific foods motivates bears to be in the right place at the right time to maximize their annual calorie count.
While these seasonal movements have been a key part of the annual food hunt of bears, there has been very little research to look at the impacts of these digs on sensitive alpine and subalpine landscapes. After all, we’re always told how sensitive the alpine is to disturbance and reminded to stay on the main trails when possible to avoid unnecessarily damaging delicate plant communities.
However, there’s an interesting thing about nature; any time there is a regular and inevitable disturbance to the landscape, nature will learn to adapt to it, and in many cases thrive because of it.
Take forest fires for example, and I’ve spent a great deal of time talking about the fire ecology of the mountain west over the past few years, but most of our local forest ecosystems have adapted to thrive through regular, low-intensity fires.
As it turns out, the mountain landscape has also adapted to the digging of bears. In this episode, I want to look at two particular studies, one that looked into the effects of shallow digs for the roots of glacier lilies, and another that investigated deeper digs when the bears were excavating ground squirrel and marmot dens. Surprisingly, each type of digging had different impacts on the meadow.
we observed at some distance the appearance of ... a field, and riding up towards it, found a ploughed large piece of ground more than four acres in extent, dug up and turned over. On getting to the spot, we observed no less than nine...grizzly bears at work, rooting away. Alexander Ross, 1824Click To TweetIn September of 1824, Hudson’s Bay Company trader, Alexander Ross was in the Stanley Basin in Idaho. He described a scene he came across:
…we observed at some distance the appearance of … a field, and riding up towards it, found a ploughed large piece of ground more than four acres in extent, dug up and turned over. On getting to the spot, we observed no less than nine…grizzly bears at work, rooting away.
That would have been a remarkable sight and would be amazing in any place in the world where grizzlies still thrive. Unfortunately, the passage of time has not been kind to bears. Where Ross could see nine bears at one time in 1824, today there are only around 40 bears in the entire state of Idaho.
In a 1998 study published in The Ecological Society of America, Sandra Tardiff, and Jack Stanford looked at grizzly digs in Glacier National Park in Montana. They wanted to explore how digging impacted the availability of nitrogen to the roots of glacier lily plants. They believed that tearing up the turf might impact nutrient availability in meadows right down to the molecular level.
They wanted to look into whether the act of churning meadows might add additional nitrogen to the soil, and thus improve the growth of nitrogen loving plants like the glacier lily, a key fall food for grizzly bears.
Just about anywhere species interact, there are always impacts, some positive and others negative. When we talk about sensitive high elevation landscapes, it would seem that a large bear tearing up the surface might not be great for the ecology of the meadow.
Plant-eating animals by their very definition are always impacting the landscape as they feed. Beavers harvest aspen and willow trees and build dams that alter entire ecosystems.
Elk and deer browse grasses and shrubs, and if not controlled by strong predator populations, can seriously impact entire ecosystems. Back in episode 45, I described the many changes that happened in the Yellowstone ecosystem when wolves were returned to the landscape and for the first time in decades, elk were finally back on the menu as the wolves put the entire ecosystem back into balance.
Interactions between species also alter nutrient cycles present in every ecosystem. What’s a nutrient cycle? Here’s a good definition from Brittanica.com:
“The cells of all organisms are made up primarily of six major elements that occur in similar proportions in all life-forms. These elements—hydrogen, oxygen, carbon, nitrogen, phosphorus, and sulfur—form the core protoplasm of organisms, and the first four of these elements make up about 99 percent of the mass of most cells. Additional elements, however, are also essential to the growth of organisms. Calcium and other elements help to form cellular support structures such as shells, internal or external skeletons, and cell walls. Chlorophyll molecules, which allow photosynthetic plants to convert solar energy into chemical energy, are chains of carbon, hydrogen, and oxygen compounds built around a magnesium ion. Altogether, 16 elements are found in all organisms; another eight elements are found in some organisms but not in others.”
Essentially, we’re all composed of at least 16 different elements and they all occur in both organic and inorganic forms. The nitrogen cycle is just one of the important cycles impacting all global ecosystems. Nitrogen is the most common element in the atmosphere, comprising 78% of the air we breathe.
Nitrogen is often converted to nitrates by lightning, making it available to plants as it falls as rain during storms. In the soil, nitrogen occurs in numerous forms. Nitrogen-fixing bacteria and decomposing fungi and bacteria create ammonium, which is then converted into Nitrates by nitrifying bacteria. These nitrates are then absorbed by the roots of plants.
Essentially, nitrogen is constantly cycling through various forms. Plants absorb nitrogen through their roots. Animals eat the plants, and occasionally carnivores eat the herbivores. Eventually, all things die and the action of bacteria and fungi help to return those nutrients to the soil.
An important aspect of the nutrient cycle is the ratio between carbon and nitrogen in soil. A ratio of 10:1 indicates that there are 10 units of carbon for every unit of nitrogen. The ratio has a strong impact on how the nitrogen is absorbed by plants and animals.
Since the nitrogen cycle begins with microbes, it’s important to understand their metabolic requirements. According to Agriculture.com: “microbes need a C:N ratio near 24:1: Around 16 parts of carbon are used for energy and eight parts for maintenance.”
Just like we need to eat the right foods to enable efficient digestion, microbes need the right ratio of carbon to nitrogen in the food they consume. Every plant has a different ratio of carbon to nitrogen, which then impacts the metabolic needs of these tiny microbes. As it turns out, the availability of nitrogen is the most important aspect of this ratio.
The quicker microbes are able to absorb nitrogen, the faster plants can grow. If microbes are digesting high ratio plants like wheat with its 80:1 ratio, it doesn’t provide enough nitrogen for the amount of carbon it contains.
The bacteria then need to find additional sources of nitrogen by absorbing it from the soil, resulting in a nitrogen deficit that will persist until the microbes in turn die and decompose, releasing the nitrogen stored in their bodies. This leaves the soil wanting for nitrogen leaving less for the roots of plants to absorb. Less nitrogen means slower growth of plants.
Alternatively, if microbes are feeding on plant residues with a low carbon-nitrogen ratio, like vetch with its 11:1 ratio, they digest it quickly leaving excess nitrogen available in the soil for growing plants.
Herbivores also prefer to eat plants with a low carbon-nitrogen ratio. They selectively crop the low ratio plants leaving behind harder to digest high ratio plants. Microbes are now limited to feeding on the remnant high carbon sources, resulting in the nitrogen in the soil decreasing.
When bacteria are consuming soil nitrogen as opposed to releasing it, plants requiring less carbon, meaning plants with a high carbon-nitrogen ratio have a competitive advantage over more nitrogen hungry plants.
This creates a negative spiral of soil nutrition. The more high ratio plants growing, the less nitrogen released to the soil, and so on. Moose are a good example. If a moose overbrowses an area, the amount of nitrogen in the soil also diminishes.
I know. This is a lot of sciency stuff, but gaining a good understanding of how nitrogen moves through the soil is critical to understanding how bears impact that ecosystem in their quest for glacier lily bulbs.
Curiously, when animals burrow into the ground, like grounds squirrels, the amount of nitrogen adjacent to the burrow often increases. An area with significant burrows can support a more diverse plant community simply because the act of burrowing improves nitrogen availability when compared to adjacent areas lacking burrows.
These changes don’t go unnoticed by animals like pronghorn or bison which will preferentially feed on areas with prairie dog (Cynomys ludovicianus) colonies.
Can you see where I’m going with this? Since burrowing animals have shown to have a beneficial impact on nitrogen availability and, in turn, plant growth, might grizzlies digging have a similarly positive impact?
Unlike ground squirrels and prairie dogs, grizzlies cover vast territories and thus impact a wide variety of habitats. According to the study:
“Disturbances caused by grizzly bears foraging for plant roots could have pervasive effects on the local plant assemblage, initially by changing species’ distributions and abundances and soil mineral nitrogen concentrations, and ultimately by changing successional patterns within disturbed patches.
If preferred forage species revegetate digs, they could be larger and or more nutritious as a consequence of disturbance effects that would minimize plant competition and potentially increase soil nutrient availability.
Competition would be minimized because of the decrease in the number of plants growing in a dig. Available nitrogen could increase for a variety of reasons, including the following: increased mineralization rates due to physical disturbance of the soil, decreased uptake by plants, and increased mineralization of organic material, such as grizzly bear excrement.
This scenario of larger and/or more nutritious plant foods might encourage grizzly bears to selectively dig in previously disturbed areas. Moreover, the patch mosaic created by grizzly bear digging over time would also affect other organisms living within the ecosystem.”
Here’s my English translation of this paragraph. Grizzlies digging for glacier lily roots may provide a local boost in available nitrogen to the soil for future generations of lilies. If this is true, the added nitrogen might mean that subsequent crops could be larger and potentially more nutritious in previously excavated sites.
The act of turning over the soil also may reduce competition in a dig site meaning more and larger lilies might be available. Digging up the soil interrupts plant growth, and increases soil mineralization, reduces the amount of nitrogen being absorbed by plants during the period of disturbance, and while feeding, bears may add additional nitrogen through their droppings. After all, manure is an excellent source of nitrogen.
The term soil mineralization simply refers to the process of decomposition where chemical compounds like nitrogen are released to the soil as organic matter decomposes. Digging up soil helps to increase this rate of decomposition.
If this is true, then bears may return to previous dig sites to find more nutritious and more plentiful glacier lilies. This study looked at recent digs, those less than 5 years old, and compared them to adjacent, undisturbed meadows.
Glacier lilies are perennial plants that live for many years, but often take up to 8 years to begin producing seeds. It also rarely reproduces vegetatively, meaning seeds are needed for new individuals to colonize a site. They’re an early bloomer, being one of the first plants to bloom on avalanche slopes in the spring.
Blooming early in the season means they’ve finished flowering before grizzly bears become interested in them in the autumn. Like other root vegetables, they’re a fall crop, and the bears only dig them when they are at their most nutritious.
Grizzlies don’t have to dig deep to find these nutritious potato-like tubers, usually just 10 cm or so. The bulbs are subsequently munched down, while the rest of the plant is left behind. Keep in mind, that the lily has long since finished growing by the time the bears excavate them.
Other favourite roots, like Hedysarum, are often still growing when they’re dug up, but the bear still just chomps down the roots and leaves the stems and foliage behind.
The study showed that bears had a definite preference for previously excavated sites. If you hike up to Chester Lake in Alberta’s Kananaskis Country, you can see a perfect example of this. Right adjacent to the shoreline is a regularly recycled area of extensive digging. It’s obvious that bears return to the same dig site on regular occasions.
Grizzly bears digging up alpine and subalpine meadows give us more glacier lilies.Click To TweetThey also found that in areas where the bears excavated, glacier lilies were the first plants to reappear, resulting in dig sites having a different plant composition than adjacent sites. After excavation, glacier lilies encompassed 41% of the total plant cover while bare ground accounted for 65%.
In undisturbed areas, glacier lilies were limited to just 24% of the plant cover. This finding showed that glacier lilies thrived on the increased soil nitrogen in areas where grizzlies excavated. Plants in dig-sites had higher amounts of nitrogen in their tissues when compared to undisturbed sites.
While glacier lilies take a great deal of time to become established, bears never manage to eat every bulb in an excavation. In turn, the plants that remain produce twice as many seeds as plants growing in adjacent, undisturbed meadows.
Glacier lily seeds are also heavy, meaning they don’t disperse very far, with all of the seeds falling with a metre of the parent plant. On a dig site, most of those seeds fall on bare soil, which only enhances germination.
What we begin to see is a story where the glacier lilies in previously excavated sites have less competition from other plants, are larger and more nutritious and thus more easily digestible for grizzlies.
It also goes without saying that previously tilled sites would be easier to dig up in the future as well. While the actual number of bulbs in previously excavates sites may be lower than undisturbed sites, their higher nutritional value seems to be a strong attractant to return visits.
It’s always amazing how species become so intricately connected. When we look at any landscape, it’s a reflection of the endless number of interactions between species.
So if grizzly digs are beneficial when they’re relatively shallow, are there still benefits to deeper digs? A study in the publication: Arctic, Antarctic, and Alpine Research by Daniel Doak and Michael Loso looked at the impacts of grizzlies excavating deeper holes in search of ground squirrels.
This study differed in many ways from the previous one. They looked into a total of 43 bear digs to look at how they affected “species richness and diversity, recolonization patterns, and plants with different clonal growth strategies”.
For many high elevation plant communities, growing seasons are simply too short for widespread reproduction from seeds. Instead, most alpine plants spread out vegetatively to create clone plants. Many of these clonal plants grow as dense carpets low to the ground as in the very common moss campion (Silene acaulis).
Being able to spread vegetatively allows alpine plants to colonize harsh landscapes where the growing season is simply too short for most seeds to germinate. It also allows them to more efficiently utilize scarce resources, which in turn, makes them more competitive.
Despite the difficulty of seeds germinating through the short growing season, some plants do beat the odds and establish new plants which then send out stolons to expand a single plant into larger and larger clonal patches or carpets.
In addition, well established clonal patches can live very long lives. They also grow very slowly, with moss campion’s growth rate varying between .06 cm and 1.82 cm/year. This slow growth allows some of the largest campion cushions to live upwards of 350 years. In other mountain regions, such as the Andes of South America, some clonal plants can be as old as 3,000 years.
The slow growth of alpine plants is one of the reasons that we, as guides always try to reduce our impact when hiking off-trail above treeline. By spreading out, rather than walking in single-file, groups can reduce their impact on individual plants. It’s also important to try to step on rocks when possible and in doing so, save the plants from possible damage.
So, what does happen when a grizzly comes and digs deep holes in the hunt for ground squirrels and marmots? Well as our first study showed, disturbing soil had beneficial effects on glacier lily populations, but those were quite shallow excavations.
While this study took place in Alaska, the results are easily applicable to the mountain west as many of the species are similar to those found in alpine areas further north. This study looked specifically into how these digs impacted:
“mature alpine plant communities, with special reference to the differential effects of disturbance on plants with different growth forms. In particular, we asked the following questions:
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- How does the species richness and diversity of bear digs compare with those of neighbouring mature tundra?
- How does that relationship evolve over the life of the dig?
- Do bear digs influence community structure, at least in part, by differentially favouring plant species with particular clonal forms?”
OK, once more now in English! They wanted to see how digs changed the landscape and plant communities over time, as compared to undisturbed areas nearby. Which plants were more positively impacted, and were certain growth forms able to benefit more than others.
In the plots they studied, there were no annuals, or plants that reproduce by seed every year. The ground cover was limited to long-lived perennials that reproduced, at least in part, vegetatively. While most alpine plants reproduce vegetatively, there are numerous strategies that plants can take, and this is an important part of understanding this study.
They looked at two main strategies, each subdivided into two sub-strategies.
For plants that produce new clones vegetatively, there are two main strategies: those whose daughter clones grow beneath the parent’s plants canopy, and those whose daughters form away from the parent plant. Of these two main strategies, the expanding regions of the plant might be either above ground, or below the surface.
This gave them four main strategies:
- plants whose daughter clones remained close to the parent
- with their expanding horizontal stem, or stolon, remained above the ground such as heathers, mouse-ear chickweed, mountain avens, crowberry, moss campion, and purple saxifrage.
- with their expanding stems below the ground such as columbine, pussytoes, paintbrush, shooting star, fleabanes, gentians, and goldenrods.
- Plants whose clones were more distant from the parent and
- their stolons above the ground like willows and finally,
- stolons below the ground like yarrow, windflower, arnica, fireweed, and horsetail.
What I find interesting in this list, as complicated as it seems at first, each of the alpine and subalpine plants I’ve admired for decades each has a very different way of surviving at high elevations with short growing seasons.
Each of these strategies has proven very successful for the plants that employ them, but what was unknown at the time of this study was what would a grizzly tearing up these vegetative plant colonies do to the meadow.
Once they had divided plant strategies into 4-different groups, they further subdivided the landscape into two main habitats: moist snowbeds and warmer, drier meadows.
Snowbeds represented those areas where patches of snow persisted well into the summer, remained moist most of the season, and experienced the coldest temperatures. Meadows were warmer, had longer growing seasons and were significantly drier.
Snowbeds were dominated by saxifrage and willow, while meadows hosted lingonberry and pyrola, a type of wintergreen.
Ground squirrels and marmots burrow deeply into the ground. Not only did bears excavate equally deep holes, but they could also cover an area of up to twenty square metres. The simple act of tearing away at the vegetation meant they were ripping up, in some cases, centuries of slowly growing plant life.
As they shovelled downward, they left a pattern of holes separated by mounds of soil removed by their exploration. This study ignored the mounds and instead focussed on the impact the digs had on the ecology of the meadow. The mounds also recolonize vegetatively much quicker than those areas where the bear has removed the surface layers.
As a guide, I think of all the time I’ve gingerly crossed meadows hoping to have minimal impact, while bears leave a much deeper impression on the landscape.
At the end of the study, researchers found that 88 species of plants colonized the mature tundra, but only 62 were found in dig sites. When the bears put claws to tundra, they usually removed all the plants as their claws sought dinner.
The plants did recolonize the dig sites but, as the report states:
” Mature tundra plots have higher average plant species richness than bear dig plots, and very slightly lower mean species diversity than bear digs”
This sentence uses two very specific terms: richness and diversity. If two plots of land have the same number of species, say 5, then they both have the same richness. However, if in one plot the site is largely dominated by only one of those species and the other has a more even distribution of all 5 plants, than the second plot would have higher diversity.
With this in mind, while long-established sites had very rich plant growth, there were fewer species than an area of tundra with bear digs. The digs helped to reset the clocks in terms of plant colonization and allow a larger number of species to recolonize on in the newly tilled soil.
Once the turf was tossed, the future of the site was up for grabs, although most regrowth showed an affinity for plant species found nearest to the excavation.
This study can’t determine the point at which excavation would become a negative impact on species diversity however, it’s clear that a moderate level of grizzly digging enhances species richness over the long term.
Diversity isn’t the only thing that is impacted when the turf of a meadow is tossed in search of furry prey. It can also impact the mix of plants that are left behind as the excavation heals and new growth repairs the disturbance.
In this study, researchers compared 4-different growth strategies. Did bear digging offer advantages to one or more strategies? The results were dependant on whether the digs were in snowbeds or meadows.
The study found that:
“In snowbed areas, all clonal types except the most dominant one were favoured by digging, suggesting that the statistically significant reduction in aboveground guerrilla species may release species of other clonal types from competition, leading to the observed increases in species richness and diversity
This means that most plant groups benefitted from digs in snowbed zones with the exception of plants like willow whose stolons run far from the parent plant and remain above the ground.
Rather, it appears that tilling the soil, provides opportunities for plants of different clonal groups to make a play for the newly opened habitat by reducing the dominance of any one strategy that may have previously been well established on the site. This adds more species richness and diversity.
In meadow areas, digging favoured the plants with below ground stolons that remained close to the parent plant. It actually suppressed those with above-ground stolons like willows, heather, moss campion, and purple saxifrage. Plants benefitting from the dig included columbine, pussytoes, paintbrush, shooting star, fleabanes, gentians, and goldenrods.
The result is that regular digs help to prevent a single plant species from becoming dominant, even though the tundra may, at first glance, appear to be mature.
Some of the plants that benefited most by excavation, like fireweed, also occurred in mature sites, but usually in locations fractured by frost heaving. It can be argued that these sites disturbed by the constant freezing and thawing common in the high country are also disturbed sites. Fireweed is well known to prefer such sites. After all, it’s also one of the first plants to recolonize forest fire sites as well. Biological disturbances like digs often create ecological opportunities similar to non-biological changes like frost heaving.
The report speculates that those plants that spread slowly, with new clones forming very close to the parent are at a disadvantage in mature meadows. Disturbances like grizzly excavations may give them an advantage over more successful strategies like willows whose stolons spread to sites quite distant from the parent plant.
Basically, grizzly digs offer new opportunities for plants to colonize the disturbed site. Some plants are able to more effectively take advantage of individual digs depending on what the dominant vegetation was before the dig, whether the dig occurs in a snowbed or meadow, and what the species richness was before the bear arrived on the scene.
What does this study tell us…well it shows that bears are an important part of the alpine landscape, but at the same time, additional studies need to be done to determine just what their quest for ground squirrels has on the wider mountain ecology.
That’s the cool thing about science. It’s never done. Each study helps formulate questions for subsequent studies. As grizzlies disappear from portions of their historic range and expand into new habitats with warming climates, future research will need to look into how their changing range impacts landscapes in their absence from some areas and their advancement into new territories.
I recently returned from guiding visitors on polar bear viewing expeditions near Churchill, Manitoba. Few people are aware that Wapusk National Park, only a few kilometres from Churchill, is the only park in Canada that has black, grizzly, and polar bears. As habitats change, so do ranges. As ranges change, so do the impacts of bears altering the landscape in their constant quest for food.
And with that it’s time to wrap this episode up. I want to take this moment to say thank you for sharing your time with me. I know you’ve got a busy schedule and I appreciate your spending a little bit of it with me. Drop me a line if you’re coming to the mountains and are looking for a guide or guidance to help you make the most of your mountain experience.
If you’d like to reach out personally, you can hit me up on Twitter @wardcameron, or drop me an email at info@wardcameron.com, and with that, the snow’s falling and it’s time to go snowshoeing. I’ll talk to you next week.