| Summary: | Selective foraging by cervids can affect vegetation. and that in turn car far-reaching effects on various components of the boreal forest ecosystem. Since terrestrial gastropods are sensitive to habitat alterations, they can be expected to respond to changes caused by grazing. This study is based on gastropod specimens from two data sets that were collected with pitfall traps in order to study the effects of moose and reindeer on ground-layer invertebrates. Invertebrates were trapped from 23 pairs of plots. where one plot was open to all animals while the other one was fenced to exclude large mammals. Half of the sires were in Finnish Lapland. where reindeer grazing was the main cause of differences between the plots; the rest were located in central Sweden and southern Finland, where moose was the most important cervid grazer. The composition of the gastropod fauna differed between grazed and ungrazed plots in both areas, and the abundance of gastropods in general and that of many species was higher in ungrazed plots (the number of gastropods caught was 17% lower in grazed than in ungrazed plots in moose sites. and 24% smaller in reindeer sites). Species richness differed significantly between grazed and ungrazed plots in the combined data of both sire types, and was 15% higher in ungrazed plots. In moose sites the relative diversity (H'/H'(max)) of gastropods was 17% higher in grazed plots. In reindeer sites, the means of richness and diversity (H') were considerably higher in ungrazed plots (35% and 40% respectively) but the differences were only marginally significant. By indirectly modifying the microclimate within the forest. cervids fulfil the criteria for a 'physical ecosystem engineer'. Since the population density and distribution of terrestrial gastropods depend largely on the physical conditions of the microhabitat, the engineering effects of cervids are the most likely cause of the changes observed in the density and composition of the gastropod fauna in the boreal forest.
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