Field Guide
The Canadian population of slender mouse-ear-cress is slender and becoming slenderer. Blame the historic bison slaughter for that
Is it the mouse that is slender, or the mouse’s ear? It’s not clear. We do know that the Canadian population of slender mouse-ear-cress is slender and becoming slenderer, limited to a shrinking patchwork on the Prairies. For the last two decades, it has been listed as threatened in Canada. As to why it is disappearing, one intriguing theory involves an iconic Canadian megafauna species whose number also dwindled almost to nothing.
Halimolobos is a flowering plant genus in the Brassicaceae family, which includes mustards, cabbages and crucifers such as broccoli. This branch of the family sticks out like a straggly weed. A forb as opposed to a grass, it grows from a slender taproot and woody base, with a solitary stem that’s sometimes branched and always covered with fine grey hairs. Narrow spoon-like basal leaves (also hairy) contrast with alternating, stalkless stem leaves, often with attached lobes that really do resemble mouse ears. Charming tiny flowers with pink-veined white petals and purplish sepals create its attractive if weedy look. Preferring silty or sandy soil amid grazed grassland, this biennial herb once proliferated throughout the North American plains. Now it is limited in Canada to a handful of locations in Alberta and Saskatchewan and is prone to sudden disappearance. (One grouping near Canadian Forces Base Suffield, Canada’s largest military base, 50 kilometres from Medicine Hat, went from 400 plants to zero in only six years, with no obvious signs of trauma — as a paramedic might say). Numerous experts have speculated why. The most compelling explanation to date links this cress to the fortunes of the mighty bison that roamed the vast prairie for millennia before the arrival of European settlers and their bloodthirst.
In February 2019, a biogeographer in Alberta named Cheryl Hendrickson, articulated a fascinating theory in Iris, the newsletter of the Alberta Native Plant Council. Attempting to account for the great distances between slender mouse-ear-cress colonies, Hendrickson blamed the decline of the bison.
Bison love wallowing, rolling on the ground and kicking up masses of dust as they writhe on their backs. It looks playful, like an excited labradoodle in the local park, except an adult bison weighs a ton. The scrubbing helps them shed winter hair, soothes insect wounds and creates a protective coating. When massive numbers (28 million, says one estimate) of bison roamed the continental grasslands, the land would have been covered with wallows as common as gopher holes.
Epizoochory — when seeds such as burrs attach to the exterior of an animal for dispersal — is the least common of the five ways plants spread their seeds. As a means of propagation, it is a sketchy one, compared with the efficacy of the seed being eaten, moistened and later excreted in a fertilizer and growth medium. But bison don’t eat forbs like this cress. Our plant shows an adaptation: in spring, despite snow loads and desiccation, the previous year’s stems stand tall amid the flattened grasses, retaining a healthy number of seeds. Passing bison would pick up the seeds in their hair and hooves. The much-travelled beasts dispersed these seeds some distance along, planting the seeds as they wallowed. Without bison, seeds fall only centimetres from the plant base and struggle to grow.
Biologists today look to that ancient pattern of wallows as a lost archipelago of biodiversity, tiny rain-filled reservoirs that supported multiple species of creatures and were rich with pioneer plant species. With the disappearance of bison, this plant's primary dispersal vector collapsed and with it many species that relied on it for propagation. Slender mouse-ear-cress is a casualty of the slaughter of bison a century ago, as is our country’s biodiversity.