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Are Snails Too Slow for their Own Good?
25 Sep, 2007 10:10 am
Recent research in south-east England shows how constraints on the dispersal of threatened and declining snails has been overlooked as a constraint on their survival and persistence.
The Shining ramís horn (Segmentina nitida), the Little whirlpool ramís horn (Anisus vorticulus) and the Large-mouthed valve snail (Valvata macrostoma) are three of Europeís rarest snails, and occupy drainage ditches on wet grasslands. All are listed in the UK Red Data Book and figure in the UK Biodiversity Action Plan. Anisus vorticulus has been listed on Appendix II of the EU Habitats Directive since 2003 (92/43/EEC). Smaller than a little-finger nail, all three have declined dramatically over recent decades as their habitat has been lost, degraded or polluted, largely by the runnoff of agricultural fertilisers. While potential limits on dispersal have been postulated, they have never previously been evaluated. Previous workers have surmised that passive transport during marshland flooding may now be prevented on the remaining occupied marshes because flow patterns are increasingly controlled.
Working extensively on several protected marshlands in south-eastern England, including Pevensey Levels and the Arun Valley, we used spatial analysis in combination with surveys of habitat quality to analyse site occupancy in relation to habitat suitability. We found unexpected gaps in the range of all three snails that were more frequent at larger distances from occupied sites. Sites that held each of the Red-data Book snails were usuially 200-600 m apart, but occupany then declined with increasing distances. Even the best habitats were sometimes unoccupied, with evidence of constraints on dispersal both within marshes and between them. The clear implication of the work is that assisted dispersal and re-introduction to suitable sites must now be considered as part of the conservation strategy for these species. We also advocate work to better understand the genetic and population implications of the current fragmented range.
Dr Isabelle Durance, responsible for the spatial analysis, summed up the wider implications: "Natural habitats increasingly form a tattered and changing patchwork, and animals must reach habitats in which they can survive. With their snail-paced movement in a fast-changing world, these species epitomise, better than many others, the larger conservation problems of dispersal and fragmentation."
Karla Niggebrugge, Isabelle Durance, Alisa M Watson, Rob S.E.W. Leuven and S.J Ormerod (2007) Applying landscape ecology to conservation biology: spatially explicit analysis reveals dispersal limits on threatened wetland gastropods. Biological Conservation, 139, 286-296
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The article should mention just what the dispersal ability of these species is, in m or similar, to give readers an idea of the scale being described. Also, how do they disperse? Just a line would do here. Otherwise, I thought it was clear and interesting.
[Response] Thankyou, Dr Gange: both are pertinent points that have been included in a revised version. Kind regards, Steve Ormerod