The Slyest Seed Predator: Interactions between Red Fox and Whitebark Pine in the Greater Yellowstone Area
By Patrick Cross, Yellowstone Ecological Research Center
Originally appeared in Nutcracker Notes, Summer 2015
"Aha, there's a fox scat for you, Joel!" I proclaimed, pointing ahead with my ski pole across the otherwise unblemished snow.
It was a crisp January day, with alpine sunshine sparkling off ice crystals suspended in the mountain air, high on the Beartooth Plateau near Top of the World, Wyoming. Field technician Joel Forrest and I were there to conduct snow tracking surveys, collecting habitat use data that could explain why the red foxes that live there seem different from those at lower elevations. With kit-rearing dens up to 9,400' (2,820 m) and year-round occupation of elevations as high as 11,000' (3,300 m), this is the highest known fox population in North America. It is also distinguished by unique physical and genetic characteristics: visitors to nearby Yellowstone National Park have long noticed the lighter blond coat colors and gray underfur of its foxes living at high elevations compared to the rich red found at lower elevations, and recent genetic studies have revealed significant differences between foxes across an elevational gradient within the ecosystem3. Suspecting that behavioral differences could be contributing to these observations, we wanted to compare genetic and habitat use patterns high in the Beartooths to those lower down in Yellowstone4, hoping to identify the mechanisms driving this diversity.
But as we skied closer, I noticed that there was something odd about this scat. It was not composed of the fine gray hair and tiny bones one would expect from a predator of small rodents; instead it prickled with rigid, angular, broken bits of brown shells. If it wasn't for its small size and its being found in the wrong season, it could have been mistaken for a late-summer grizzly bear scat. This fox scat was packed with crunched whitebark pine nuts.
Later, over beers at the Miners Saloon in Cooke City, Montana, we reported our discovery to Jesse Logan, an expert on the area's whitebark ecology as well as its trout fishing and powder skiing.
"I think you are on to something here," Jesse said, encouraging us to continue documenting this apparently novel behavior, which was easy to do since the whitebark pine nuts would have a major effect on fox activity throughout the rest of the winter.
In the following months, we found pine nuts in nearly half of the 30 scats collected across the territories of multiple foxes, often in large quantities accounting for most or all of the scat's content. And on several occassions, our snow tracking surveys even led us deep into the forests, far from the edge territory that foxes generally prefer, to the raided red squirrel middens that had yielded the nutritious food. The snow around these sites, which were usually at the base of a grand old spruce or whitebark, would be packed down from so many fox tracks and have cone bracts, bark, needles, and other debris from the excavated midden strewn all over its surface. There would also be short trails leading away from the midden to smaller packed down rest sites where the fox would carry a whole cone, pull off its waxy, purple bracts, pluck out the seeds, and drop the empty husk before returning to the midden for another. Surely this was a more effective way to obtain calories than by diving through the deep snow after a small, scurrying vole. And foxes were not the only carnivores enjoying pine nuts that winter: we also observed several American marten scats that were obviously loaded with pine nuts.
The following winter, we did not find any pine nuts in the fox scats collected, but this was expected since we had not heard as many raucous Clark's nutcrackers that summer, nor did we see the overloaded tree tops like we had the summer before. Whitebark pine often exhibit the cyclical reproduction strategy known as 'mast seeding' in which cone production is high in some years and low in others so as to discourage seed predators from settling in. Although there is substantial variation in these cycles from whitebark stand to stand given their site-specific environmental conditions, researchers from the Interagency Grizzly Bear Study Team conducting cone count transects throughout the ecosystem rated the first year of this study (2012-2013) a "generally good cone production year" for the ecosystem, while the following year was rated a "generally poor cone production year." That second winter, the foxes consumed more snowshoe hare, which seems like a formidable advisary for a predator that, at about 9 lbs (4 kg), is no bigger than the average house cat ̶ they may look bigger, but it is all fluff. Nevertheless, we recovered snowshoe hare remains from over 70% of fox scats that winter, compared to less than 25% the winter before. Also that winter's field tech, Jake Kay, and I both recorded numerous kill sites while snow tracking, yet neither of us found any excavated squirrel middens.
Almost everyone who has seen these foxes, or even just their tracks, in such an extreme environment has wondered, 'what do they eat up here all winter?' and clearly the answer is whatever they can find. Surviving in the subalpine requires a fair deal of adaptability, which is something that the red fox, the most widely distributed terrestrial carnivore in the world, excels at.
So it was not surprising when, at the completion of its metamorphosis from raw field data to spreadsheets to statistical analyses, we found significant variance in the food items consumed between the two winters of this study. But we were surprised when we likewise analyzed the habitats used between the two winters and again found statistically significant variance. During the first winter when whitebark seeds were available, there was a significant spike in the usage of mature spruce-fir cover types, while the following winter saw a more even distribution of habitat useage as the spike in mature spruce-fir leveled off and more mid-successional forest stages were used. To understand why, we turn to red squirrel ecology.
Because of interannual variance in whitebark cone production due to mast seeding, pure whitebark stands are generally considered poor squirrel habitat since they lack the diversity of food types needed to sustain squirrels during low cone production years. And where there are no squirrels, there are no squirrel middens. Spruce-fir cover types, on the other hand, often have a significant whitebark component in addition to more consistent but less nutritious food sources. This makes them better squirrel habitat and the most likely places where whitebark seeds would be available to foxes, thus explaining the significant spike in spruce-fir habitat use corresponding with the significant spike in whitebark pine nut consumption.
Ever since we found that first nutty scat high on the Beartooth Plateau, we were excited since it was, to the best of our knowledge, the first time foxes were recorded using whitebark pine nuts, adding them to the long list of animals that directly benefit from whitebark pine. But the combined results of these statistical analyses are far more profound since they suggest that, beyond simply using this novel food source, the foxes were actually changing their habitat use behavior in response to its availability. Perhaps this is a clue pointing to why these high elevation foxes are distinct; perhaps they have evolved in this landscape where whitebark pine play such an important role; perhaps the foxes themselves, like the Clark's nutcrackers, red squirrels, and grizzly bears, play an interactive role in this particular system. In reality, these findings do more to raise new questions than they do to answer our original questions, which may just be the result of good science, but we can be confident that whitebark pine nuts are an important resource facilitating the persistence of this fox population in such an extreme environment.
This past spring, I met up with Jesse again at the Miners, and this time he had questions for me: how cold did it get up there last winter? what is the snowpack like right now? did you see any blood red brood trees infested with mountain pine beetles? When he later returned from a day of bark chipping on the Beartooth Plateau in the very area where the fox-excavated squirrel middens were located, his report was grim: wriggling beetle larvae were thriving in those high forests unaccustomed to the epidemic pest. Should the Beartooth Plateau experience the dire whitebark declines that have happened in other parts of the Greater Yellowstone Ecosystem, the adaptability of its remarkable population of red foxes will be put to the test.