by Patricia Robertson
Many of us have encountered wood bison on the Alaska Highway around Muncho Lake, B.C., looking intimidatingly large and unyielding.
Yet the largest land mammal in North America, which can weigh over 900 kilograms, is a grazing animal that eats only plants. Since bison eat roughly two per cent of their body weight per day, that’s a lot of plant material. So Lori Schroeder, a graduate student at the University of Alberta, decided to find out what effect the Aishihik bison herd is having on the area’s grasslands.
Grassland areas in the Yukon have historically been grazed by wood bison (Bison bison athabascae). Both wood bison and the better known plains bison (Bison bison bison) are descended from the steppe bison, which first came to North America from Asia via the Bering land bridge more than 90,000 years ago.
Radiocarbon-dated fossils indicate the presence of bison in the southern Yukon since the last ice age (roughly the last 10,000 years). However, as the climate warmed and became wetter, trees and shrubs expanded their range and grasslands shrank to pockets in the boreal forest. Then, sometime between 400 years ago and the 1800s, bison were wiped out in the Yukon. Loss of grassland habitat is likely an important factor.
Today’s wood bison were re-introduced in the late 1980s as part of a national bison recovery program. Between 1988 and 1992, 170 animals were released near their current range in southwestern Yukon, north of the Nisling River. With no natural predators (though wolves are learning how to hunt them), the herd has thrived and now numbers about 1,200. In an effort to keep numbers down, it was opened up to human hunting in 1998.
(Two other herds spend part of their time in southeastern Yukon: the Nahanni herd, from a wood bison restoration project in N.W.T., and the Nordquist herd, the animals we see near Muncho Lake and as far west as Watson Lake.)
Schroeder has spent the last two summers in the field, working with staff from the Canadian Wildlife Service and Environment Yukon. Her work involves measuring vegetation at grassland sites in the Aishihik-Sekulman lakes area.
“Bison need sources of graminoids – grasses and grasslike plants such as sedges,” explains Schroeder. They mainly find these plants in three different habitats: subalpine areas, which are a mixture of sedge-rich, moist meadows and higher, drier areas that are bug-free; wet lowlands where tall sedges grow; and grasslands. The grasslands house a number of rare plant species, some of which survived the last ice age north of the ice sheets and are now only found in Beringia.
In the southern Yukon, almost all grasslands are found on south-facing slopes. Because of the angle of the slopes and their aspect, or direction, these slopes stay dry – too dry for trees and shrubs to survive. In late winter and early spring, these areas are the first places to lose their snow and show signs of fresh plant growth, and that’s when the largest numbers of bison use them. “Where you find the crocuses in bloom is where you find the bison,” says Schroeder.
The team’s first priority was to revisit 14 grassland sites surveyed in 1981, before the introduction of the bison. “That’s your gold standard,” explains Schroeder. “If you want to look at the effects of bison, you want to compare what was there before with what’s there now.”
Schroeder and her colleagues carried out cover estimates for each 10-metre by 10-metre site. That involves looking down on the plot from above -“as if you were a bird,” says Schroeder – and then estimating how much of that square is covered by each plant species.
“This gives you an idea of the composition of the community and the relative abundance of the different species,” says Schroeder.
The team visited 26 sites in all in 2011 (14 of which were originally surveyed in 1981), and another 44 sites in 2012, for a total of 70 sites.
The team takes notes about the site, including the angle of the slope, the direction the slope faces, whether the site is mid-slope or top-slope, and the kinds of soils. They also do a bison turd count “in order to get an idea of how heavily the area is used by bison. We want to try and create a whole picture so that we can tease out the effects of bison relative to other environmental factors.”
Over the two summers, the team erected exclosures at five of the sites – fenced plots intended to keep bison out. Alongside the exclosure is a control plot so that researchers can compare what happens to the vegetation in each plot.
“We can already see a difference in the sites erected in 2011,” says Schroeder. “I wouldn’t expect there to be a difference in species yet, because it’s a very arid environment and not fast growing. But from photos, the amount of forage available to the bison has notably increased in those areas that have been protected from grazing.”
In 2012, the team also selected sites according to the level of use by bison, based on bison collar data (which records where bison have been). “Theoretically,” says Schroeder, “you would see differences in plant diversity based on the usage level.”
In more humid environments, grazing animals can open up the environment to other plant species, explains Schroeder. “Grazing makes space so that you have more of a mix of habitats available, and then you have more plant species – until the level of grazing becomes so high that only few species can survive the constant disturbance.”
However, in the dry grasslands of the Yukon, the main factor limiting plant diversity is probably the lack of moisture, not competition between plant species. Schroeder expects that grazing is decreasing the number of plant species rather than increasing them. “And where the level of bison use is high, you would expect the number of plant species to be lowest.”
The data analysis is just beginning, so it’s too early to tell whether Yukon grasslands follow this pattern or not. But as Schroeder works through the numbers, there are a lot of avenues to go down. “What the data doesn’t tell me yet is how the plant community has changed. For example, are we seeing shifts in which species are there, even if the total number of species is the same? Are we seeing an increase in the plants which do really well in an environment which is disturbed – grazed, in this case – on a regular basis?”
One surprise has already surfaced, however: Schroeder found almost no invasive plant species. She’s not aware of any other plant study in Canada with as many as 70 plots that does not have greater evidence of invasive plants. “I’m really surprised that the sweet clover we see all over the roadsides hasn’t made it up there, either with hunters or with the bison themselves carrying it in,” says Schroeder.
This column is co-ordinated by the Yukon Research Centre at Yukon College with major financial support from Environment Yukon and Yukon College. The articles are archived at www.taiga.net/yourYukon.