Lemming grazing and structure of a snowbed plant community - A long-term experiment at Kilpisjarvi, Finnish Lapland

Eight years after establishing exclosure fences on an alpine snowbed, the habitat preferred by wintering Norwegian lemming (Lemmus lemmus), the vegetation had profoundly changed. The moss-dominated vegetation had expanded in the exclosures and haircap mosses (Polytrichaceae spp.) had formed about a three times thicker bryophyte carpet than in the open plots. This was due to the vertical growth of the mosses in undisturbed conditions. Some graminoid species able to grow through the moss carpet were also more abundant in the exclosures. The low-statured snowbed species (hepatics, some prostrate vascular plants) were virtually absent in the exclosures. Bryophyte species with good colonizing ability were only found on open plots. The number of plant species in open plots (19) was higher than that within the exclosures (11). This was most likely due to increased utilization of space by dominant species in the absence of grazing. These changes took place even though lemming density was low throughout the study period, which emphasizes the importance of lemming grazing in snowbed habitats. The results conform with the predictions of exploitation ecosystems hypothesis which stresses the importance of natural grazing in unproductive habitats.