Voles on the Range

Josef K. Schmutz

What are voles? Ranchers David Kingcott and Keith MacDonald called them 'field mice.' Naturalists call them voles, these vegetarians of the mouse world. Actual mice eat some juicy plants too but mostly eat seeds, fruits and insects. The voles, with help from Mother Nature, have opted for small eyes, short ears and short tails. Small eyes work fine in their shadowy world at ground level. Big ears and long tails just get in the way when one turns around in the grass/snow tunnels. Mice, on the other hand, use their big eyes and long tails for balance when climbing for a rosehip fruit at night. The picture shows a sagebrush vole (Lemmiscus curtatus). The vole shown here had fallen prey to a ferruginous hawk -- but more about that later.

The below-snow world. In the winter of 2010-11, as David told the story, the field mice ran in the packed snow of his snowmobile track. They were disoriented because the snowmobile squashed their nests and tunnels in the grass and shrub under the snow. Naturalists have a fancy name for this layer between earth and snow, the 'subnivean' space, in recognition of this unique habitat. Here, voles are insulated from the cold air and have access to food plants. Sometimes they even have an early litter of young in this remarkably comfortable subnivean world. This space the voles call home, or did, until David came along.

David was checking his cows in the deep snow of this unusually late, climate-change winter. David and Keith run their cattle in the mixed-grass prairie of the northern Great Plains near Hanna, Alberta. Their neigbours also noticed the unusual number of voles. Karen Gordon was most concerned about her newly planted fruit trees.

Finding food is hard for the voles in winter. However, they've learned that when the wheat-, spear and blue grama grasses have dried and lost food value, there is bark on every rosehip-, snowberry- and silver willow bush, and even small trees. The bark is right there at ground level, within easy reach of their monstrous rodent teeth. The 'vascular' layer, below the thin outer bark and plain wood, is loaded with nutrients. The shrub plants hold these nutrients there waiting for the first good sign of spring. Maple-syrup farmers need to wait for the sugar-containing sap to flow in spring. Voles know where to find tasty sap all winter long.

Two- and four-footed plant ecologists. Ranchers plan carefully to put up enough cattle feed for the winter, and keep an eye on the beef markets to chart their strategy for the year. David, Karen and Keith also think like plant ecologists when they plan cattle-grazing rotations to maintain the grass supply for the coming year, and the years beyond. This is where the voles come in.

Plant ecologists use the term decreasers for those groups of plants that the cows, or the bison before them, love to eat most. These plants decrease in vigour if they are pushed too hard, by leaving a herd to graze too long at a time, or at the wrong time of year. Grasses need a rest to recover and to restore energy in their roots for survival. June-, wheat- and spear grasses are in this tasty-to-cattle decreaser category. Other plants, especially shrubs, qualify as increasers because the reverse is true. Cows might nibble on the odd leaf of a snowberry bush, but mostly leave all the shrubs alone. This can give shrubs the upper hand in a grassland community where everyone is competing for space, nutrients and light.

Voles to the rescue! The snowberry stem above was nibbled by a vole but the plant survived. The photo at the right shows several snowberry stems with the bark totally eaten off and the plant died as a result.

Just by doing what they do naturally, the sagebrush voles, and their prairie- and meadow vole cousins, have become key participants in keeping the prairie landscape in balance. This is an 'ecological service' which the voles provide. The benefit is huge, yet this service, like many others, does not get tallied on the ranch's end-of-the-year income and expense ledger. It is taken for granted as long as it works.

The photo at the left shows a mix of live and dead wild rose and snowberry bushes in the prairie landscape. About half the shrubs are dead. The leafless stems let light penetrate to the ground and the grasses begin to sprout anew from their seeds and roots. The grasses will gain a good footing, especially in year two. They will turn back the clock on the increasers, and also provide new forage for cattle.

Layers of interactions in the system. If the voles ever thought they had it made - they should think again. A neat feature of close-to naturally functioning systems, is their resilience to disturbance. Resilient systems are hard to knock off their rocker. A key feature of resilient systems is that when a factor changes, say voles increase in number, this trend is rarely purely linear. There comes a point when vole numbers level off in a 'non-linear way', or change direction suddenly and unpredictably in a 'chaotic' way. It is believed that resilience in a system increases in line with the complexity of a system, with layer upon layer of interactions. Tinkering with systems, which ranchers, voles, you and I do daily, is best done by making sure we keep all the pieces of a system when we try to rearrange them - intelligent tinkering.

For the voles, non-linearity came from the raptors. Swainson's hawks have always been fond of voles, likely even depend on voles in most years. Their response to the rise in vole numbers was to eat more of them. The graph shows that the number of voles we found in Swainson's hawk nests had more than doubled in 2011, compared to the previous ten-year period.

More than Swainson's hawks, ferruginous hawks, which normally have their predatory eye on ground squirrels, shifted their focus and added voles to the smorgasboard that fed their young.

The Swainson's hawks' response was one primarily of numbers, a 'numerical response' as ecologists call it. It was just plain linear, it seems. On the other hand, the ferruginous hawks responded more categorically. While taking primarily ground squirrels and only negligible numbers of voles from 2000 to 2010, in 2011, voles played a more signicifant and 'functional' role in their diet.

The voles had become David's, Karen's and Keith's allies in managing the stocks and flows in the grassland ecosystem that sustains the ranch. Not only the raptors, but owls, weasels, coyotes, diease organisms and parasites, together, will determine the voles' future. David, Karen and Keith have always lived with this prairie system. They've benefitted from the ecological services and the layers of interactions that make the system work.

Cattle can be farmed, as can grass be farmed, without the help of voles, by paying for the required inputs and tractor time. Sharing space with the Swainson's hawk, the sagebrush vole, the snowberry bush and the spear grass, means that David, Karen and Keith have been a part of the system, and not outside of it. They've been a part of the layers upon layers of prairie ecological services....... that's what makes them ranchers.

for further information contact:

Joe Schmutz, Ph.D.

Saskatoon, Canada

phone: 306-382-8964

e-mail: joe.schmutz@usask.ca

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This webpage was last updated March 7, 2012 by Sheila Schmutz