Replacement of sheep by chamois: a case study of grazing effects on the diversity of alpine grasslands

To study how the cessation of transhumant sheep grazing in alpine areas influences biodiversity, three study sites along a chronosequence (space-for-time substitution) were studied in the Canton of St. Gallen (Switzerland): (i) an intensively used sheep pasture (approx. 81 and 195 sheep ha(-1)) (SB0), (ii) a sheep pasture abandoned since five years and now used by approx. 1 chamois ha(-1) (SB5), and (iii) a sheep pasture abandoned since 19 years and now used by about 5 chamois and 1 ibex ha(-1) (SB19). Focussing on the two most frequent vegetation types, Poion alpinae and Nardion, we compared vascular plant diversity and floristic similarity among the three study sites at two grain sizes (10 m(2) and 200 m(2)). In addition, butterfly and orthopteran species were mapped. In SB0, Nardion was the most frequent vegetation type, whereas in SB5 and SB19 Poion alpinae prevailed. Mean plant species richness on 10 m(2) and 200 m(2) did not differ significantly between the three sites of the chronosequence for either Poion alpinae or Nardion. Only for the 200-m(2) plots of the Poion alpinae, we found a substantial but non-significant increase in plant species richness (46.8 vs. 40.8 species). The two extreme points of the chronosequence, SB0 and SB19, were most dissimilar in their floristic composition. In SB0, more butterfly and orthopteran species were recorded than in the areas no longer grazed by sheep, but the difference was significant only for the latter. The fact that the vascular plant species richness at the studied grain sizes did not differ significantly along the 19-year chronosequence suggests, that alpine grasslands respond very slowly to changes such as the replacement of sheep by chamois and ibex. However, it also suggests, that sheep have no major adverse effects on alpine grasslands, even though there seems to be a trend towards increased large-scale diversity when sheep are replaced by wild ungulates. In contrast, sheep grazing clearly influenced vegetation composition. For example, the higher frequency of Nardion grasslands and typical Nardion species in SB0 might be attributed to the effects of relatively intense sheep grazing.