Impact of past and present management practices on the land snail community of nutrient-poor calcareous grasslands

摘要

Nutrient-poor, dry calcareous grasslands in Central Europe harbour an extraordinaryudhigh diversity of plants and invertebrates. Consequently, they are of highudconservation value. However, changes in agriculture (intensification or abandonment)udhave resulted in a dramatic reduction of semi-natural grasslands in the twentiethudcentury. Today, dry grasslands are among the most endangered habitats.udFurthermore, these grasslands are frequently fragmented and surrounded by forestudor intensively cultivated agricultural areas.udSemi-natural grasslands are fragile because their maintenance depends onudtraditional farming techniques. In order to avoid any loss of species by inappropriateudland use, it is important to assess the responses of threatened species to particularudtypes of grassland management. Although different types of present and past pastureudmanagement are known to affect the species richness and composition of plantudcommunities, knowledge of the effects on invertebrates is limited. In particular, noudstudies exist on the influence of different types of pasture management on animalsudwith limited mobility, such as gastropods.udIn the present thesis, I examined the effects of different pasture managementudpractices on the snail community in dry, nutrient-poor grasslands of the Swiss Juraudmountains, where extensive grazing with low stocking rate and without use ofudfertilizers is a traditional form of grassland management. I assessed the snailudcommunities in extensive pastures grazed by horses, cattle or sheep, in cattleudpastures with different management intensity and in extensive pastures with differentudmanagement history in the last 55 years. Furthermore, gastropod species richnessudand abundance were examined in transects running from extensive pastures throughudgradual or abrupt forest edges into the forest interior.udGrazing by different livestock species did not affect the species richness,udabundance and species composition of land snails. However, independent ofudlivestock species, snail species richness, abundance and number of red-listedudspecies decreased with increasing grazing intensity. Furthermore, cattle pasturesudwithout fertilizer application and with low grazing intensity harboured more snailudspecies and more threatened snails than pastures with annual addition of fertilizerudand higher grazing intensity. Management intensity had also a negative influence onudindividual snail species (Cochlicopa lubricella, Truncatellina cylindrica, Vitrinaudpellucida, Helicella itala and Helix pomatia). Former changes in pasture use for audperiod of 10–40 years altered the present-day snail fauna. Past shrub cover had aududnegative effect on the total number of snail species and individuals, the number ofudopen-land species and individuals and the number of red-listed individuals. Formeruduse of fertilizer and higher grazing intensity reduced red-listed species andudindividuals and altered the snail community. The grassland snail communities of theudpastures changed distinctly to forest communities at the first bushes or trees ofudedges towards forest interior irrespective of the type of forest edge. In pastures, at auddistance of 10 m from gradual forest edges, more open-land snail species wereudfound than at the corresponding distance from abrupt forest edges. Furthermore,udecotones of gradual forest edges harboured more open-land individuals than those ofudabrupt forest edges.udFor the conservation of grassland land snail communities, it does not matterudwhether pastures are stocked with horses, cattle or sheep, provided the grazingudintensity is low. To preserve the threatened snail species in dry, nutrient-poorudgrasslands, a network of pastures should be managed without fertilization andudgrazing intensity should not exceed 180 LU.ha-1.d (product of livestock units perudhectare and grazing days). Furthermore, to recover the typical grassland snailudcommunity in shrub cleared pastures or former fertilized pastures, the connectionudbetween intact pastures and grasslands under restoration should be improved byudcreating and maintaining new semi-natural areas and by exchanging livestock amongudthese areas during the grazing season. Since shadowing of trees and alterations ofudthe microclimate close to the forest edge may reduce the actual size of smalludgrassland fragments, encroaching shrubs should be regularly removed and gradualudforest edges created and maintained.