Going on a snail hunt...

...Going to catch a tiny one! NatureBureau had a lovely, but damp, staff day out at Thatcham reedbeds the other day, hunting for the nationally rare Desmoulin's whorl snail. This diminutive invertebrate is only 2mm long, so it's not an easy one to spot!

It is considered to be an Atlantic-Mediterranean species with a range extending far and wide from Ireland to Russia and south to North Africa! In Thatcham, new wetland habitat was created from mitigation measures on the Newbury by-pass, and the reedbeds, which form part of the internationally important Kennet and Lambourn Floodplain Special Area of Conservation (SAC - a European designation), were designated in 2005 solely for Desmoulin’s snail. While no one is certain whether today’s population arose from snails present on introduced clumps of sedge only, or was supplemented by waterborne individuals from nearby populations, there is no doubt regarding the species’ capacity to colonise if the conditions are right.

However, across its global range the Desmoulin’s whorl snail has experienced widespread decline, due to wetland drainage (particularly during the 20th Century), change in agricultural and land management practice, scrub encroachment onto fen, and change of land use and development. In many countries the status ranges from Vulnerable to Endangered or Presumed Extinct. Drainage of wetlands has been the principal cause of the snail’s decline throughout its European range. However, there are many additional factors, applicable at any site, that could adversely affect Desmoulin’s whorl snail populations:

• Changes in hydrology, such as water abstraction and general drainage of wetlands leading to reduced water tables and loss of periodic flushes.
• Canalisation of rivers, deepening of drainage channels, and creation of vertical profiles to riverbanks, eliminating wetland marsh habitat.
• Regular cutting of riparian margins of rivers and tidying of riverside paths, often associated with fisheries.
• Changes in land use – for example, from rough pasture or meadow to improved grassland.
• Encroachment by scrub or alien plant species, which may result in too much shade and/or drying out of the habitat.
• Intensive grazing of fens or excessive poaching of ditch margins.
• Introduction of cutting or burning programmes at sites where there is no history of these activities.
• Use of pesticides and herbicides, and the effects of eutrophication.
• Decline of alder coppicing routines, leading to increased shading.

As the snail’s basic requirement is swampy, usually unshaded ground with tall plants (which helps it survive winter floods), management should be directed at maintaining these conditions.

It is a bit of a goldilocks’ snail in that the water level must remain close to the surface so that the ground remains at least moist for most of the summer with a high humidity in the vegetation, but  conditions must not become so wet that aquatic plants such as watercress (Rorippa nasturtium-aquaticum) and fool’s watercress (Apium nodiflorum) become dominant, and permanent flooding may also be detrimental as there will be no litter layer in which the snails can over-winter, and no sites for laying eggs – it’s got to be ‘just right’.

The distribution of Demoulin’s whorl snail in the UK is so strongly linked to sites with high groundwater levels that research is needed into the critical factors and threats to these sites. This research should help to both protect the best existing sites, and enable the restoration or recovery of others.

Unfortunately, we weren't lucky enough to see the snail on this occasion, but we did spot lots of other wildlife, including longhorn beetles, amber snails, doli flies, cormorants, grebes, and some very fluffy ducklings. We dried off in the Nature Discovery Centre afterwards, enjoying a coffee and a cake and some chat about the wildlife we have been lucky enough to see around the world.