Desmoulin's Whorl Snail Vertigo moulinsiana (Dupuy) in Essex
The areas of sedge bed searched were felt to be structurally and floristically very similar to the
known sites further south, although the groundwater regime may be the critical factor, especially
the degree of winter flooding. The River Cam locality is likely to suffer from winter flooding and
this may be detrimental to the survival of Vertigo species in general.
It is perhaps a little surprising that the snail has not been recorded from Rushy Mead, given its
close proximity to the other known sites. There may be some credence in the observation that the
early survey period made sampling more difficult, so that a very low-density population might go
un-noticed. Further survey work during the autumn, using the tall vegetation beating technique to
sample a larger number of places might bear fruit. Many of the immediate ditch margins are
dominated by nettles rather than sedge, leaving more isolated areas of sedge that might provide
islands of suitable vegetation that arc too small to provide the necessary micro-climate.
The perceived distribution of V. moulinsiana at Thorley Flood Pound would appear to fit in with
the broad habitat and micro-habitat requirements: during the summer of 2004 at least, much of the
site was too dry to produce the critical humid micro-climate and thus be able to support populations
of this species. It was noted that a good deal of new stock fencing has been erected here. Whilst
grazing by livestock is known to have a detrimental effect on populations (Killeen 2003), the use of
livestock to get sites into a better condition is likely to be a useful tool. Cattle grazing might break
up the ground litter layer and ultimately produce stronger, wetter marsh vegetation that would be
favourable to the spread of the snail at Thorley.
At Little Hallingbury Marsh, the very wet ground conditions at the south-eastern end of the site
only supported V. antivertigo, a species that tends to favour the wettest leaf litter in which to live.
V. moulinsiana was found in the more species-rich and generally denser vegetation in the centre of
the site. The marsh is totally unmanaged and this would be expected to help create favourable
habitat conditions. In the long term, the spread of willow scrub should be monitored, to prevent the
loss of marsh habitat to scrub.
Field observations during the survey of Sawbridgeworth Marsh tended to suggest that V. moulinsiana
was more abundant in areas where the sedge had not been cut recently. This would fit in with the
known response to mowing. Killeen (2003) states that,
"...at sites where there has been a very long history of management, the snail is not
significantly affected by mowing....However, it appears to be more affected when mowing
is introduced at sites where there is no previous history of the practice.....Regular mowing
can produce a denser sward over time, and this is why such practices may be compatible
with the snail in the longer term but not in the short term-".
The mowing regime of the reserve may need to be modified to take account of this habitat response.
The discovery of Parndon Mill as a new site for V. moulinsiana is of some significance, it being
only the second main location for this species in the modem county of Essex. The Essex Biodiversity
Action Plan replicated a generally held belief that this snail was restricted to old, long-established
wetlands with a very poor ability to disperse to new sites (e.g. Bratton 1991). However, Killeen
(2003) summarises extensive modern research which has shown that it is well able to colonise new
sites and even newly created habitat. This colonisation is aided by its presumed ability to float on
the water surface or to drift on mats of floating vegetation. The snail's ability to self-fertilise is also
Essex Naturalist (New Series) 21 (2004)
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