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

Nutrient-poor, dry calcareous grasslands in Central Europe harbour an extraordinary
high diversity of plants and invertebrates. Consequently, they are of high
conservation value. However, changes in agriculture (intensification or abandonment)
have resulted in a dramatic reduction of semi-natural grasslands in the twentieth
century. Today, dry grasslands are among the most endangered habitats.
Furthermore, these grasslands are frequently fragmented and surrounded by forest
or intensively cultivated agricultural areas.
Semi-natural grasslands are fragile because their maintenance depends on
traditional farming techniques. In order to avoid any loss of species by inappropriate
land use, it is important to assess the responses of threatened species to particular
types of grassland management. Although different types of present and past pasture
management are known to affect the species richness and composition of plant
communities, knowledge of the effects on invertebrates is limited. In particular, no
studies exist on the influence of different types of pasture management on animals
with limited mobility, such as gastropods.
In the present thesis, I examined the effects of different pasture management
practices on the snail community in dry, nutrient-poor grasslands of the Swiss Jura
mountains, where extensive grazing with low stocking rate and without use of
fertilizers is a traditional form of grassland management. I assessed the snail
communities in extensive pastures grazed by horses, cattle or sheep, in cattle
pastures with different management intensity and in extensive pastures with different
management history in the last 55 years. Furthermore, gastropod species richness
and abundance were examined in transects running from extensive pastures through
gradual or abrupt forest edges into the forest interior.
Grazing by different livestock species did not affect the species richness,
abundance and species composition of land snails. However, independent of
livestock species, snail species richness, abundance and number of red-listed
species decreased with increasing grazing intensity. Furthermore, cattle pastures
without fertilizer application and with low grazing intensity harboured more snail
species and more threatened snails than pastures with annual addition of fertilizer
and higher grazing intensity. Management intensity had also a negative influence on
individual snail species (Cochlicopa lubricella, Truncatellina cylindrica, Vitrina
pellucida, Helicella itala and Helix pomatia). Former changes in pasture use for a
period of 10–40 years altered the present-day snail fauna. Past shrub cover had a
negative effect on the total number of snail species and individuals, the number of
open-land species and individuals and the number of red-listed individuals. Former
use of fertilizer and higher grazing intensity reduced red-listed species and
individuals and altered the snail community. The grassland snail communities of the
pastures changed distinctly to forest communities at the first bushes or trees of
edges towards forest interior irrespective of the type of forest edge. In pastures, at a
distance of 10 m from gradual forest edges, more open-land snail species were
found than at the corresponding distance from abrupt forest edges. Furthermore,
ecotones of gradual forest edges harboured more open-land individuals than those of
abrupt forest edges.
For the conservation of grassland land snail communities, it does not matter
whether pastures are stocked with horses, cattle or sheep, provided the grazing
intensity is low. To preserve the threatened snail species in dry, nutrient-poor
grasslands, a network of pastures should be managed without fertilization and
grazing intensity should not exceed 180 LU.ha-1.d (product of livestock units per
hectare and grazing days). Furthermore, to recover the typical grassland snail
community in shrub cleared pastures or former fertilized pastures, the connection
between intact pastures and grasslands under restoration should be improved by
creating and maintaining new semi-natural areas and by exchanging livestock among
these areas during the grazing season. Since shadowing of trees and alterations of
the microclimate close to the forest edge may reduce the actual size of small
grassland fragments, encroaching shrubs should be regularly removed and gradual
forest edges created and maintained.