Grizzly bears (Ursus arctos horribilis) affect plant distributions and mineral nitrogen availability when they forage by digging for the bulbs of glacier lilies (Erythronium grandiflorum) growing in subalpine meadows of Glacier National Park, Montana, United States. Our working hypothesis is that grizzly bears structure plant communities and influence nitrogen availability when they selectively dig for preferred plants. In this paper, we report on differences found in recently disturbed digs (<5 yr old) when compared to adjacent, undisturbed meadow.
We used ion exchange resin bags to determine the availability of mineral nitrogen in grizzly bear digs compared to undisturbed meadow. Soil in digs contained significantly more ammonium-N and nitrate-N than adjacent, intact meadow. Glacier lily bulbs revegetating bear digs had higher tissue nitrogen and water-soluble carbohydrate concentrations than lilies in undisturbed meadow. Mature glacier lilies in digs produced twice as many seeds as did those in adjacent meadow. Glacier lily seedlings establish best on bare mineral soil, which in these meadows is found primarily in bear digs. Therefore, grizzly bear digging may benefit old, deeply seated plants that survive digging and reproduce. Digs overlapped spatially, meaning that grizzly bears were returning to dig in patches disturbed in previous years, perhaps in response to easier digging conditions and more nutritious glacier lily bulbs.
To test the idea that the observed increase in mineral nitrogen was due to the physical disturbance of grizzly bear digging and not bear choice of sites already high in nitrogen or bear excretion, we created experimental digs. Ammonium-N and nitrate-N levels increased significantly following our digging treatment, just as we had observed in the natural bear digs. Although we do not know how a bear chooses an initial digging site, this disturbance has the potential for influencing long- and short-term plant community structure.
Copyright 1998 by the Ecological Society of America. Sandra E. Tardiff and Jack A. Stanford 1998. GRIZZLY BEAR DIGGING: EFFECTS ON SUBALPINE MEADOW PLANTS IN RELATION TO MINERAL NITROGEN AVAILABILITY. Ecology 79:2219–2228. http://dx.doi.org/10.1890/0012-9658(1998)079[2219:GBDEOS]2.0.CO;2.