vorticulus (Gastropoda, Planorbidae) in its natural and
Fig. 1. Anisus vorticulus. Picture by G. Faulkner from Terrier et al. (2006).
Since 1st May 2004, the planorbid gastropod Anisus vorticulus
(Troschel, 1834) (Fig. 1) has been listed in the EU Habitats and
Species Directive as a species of community interest, requiring
special areas for conservation (Annex II) and strict protection
(Annex IV). Conservation actions, surveillance, as well as research
programmes have to be undertaken by the member states where the
species occurs. Moreover, since 1991, the species is listed in
the UK Red Data Book (RDB) and the UK ‘Biodiversity Action
Plan’ (BAP), pursuant to conservation and management research
A. vorticulus inhabits both water bodies of natural and of anthropogenic origin (Terrier et al., 2006). Recently, A. vorticulus was the subject of two studies carried out separately in water bodies of the French Rhône floodplain, which can be regarded as a natural habitat for the species (Fig. 2) and in drainage ditches in English humid grasslands (a secondary man-made habitat) (Fig. 3).
2 (near right). A cut-off channel of the French Rhône
floodplain inhabited by Anisus vorticulus. From
Fig. 3 (far right). A ditch on Pulborough Brooks, West Sussex, where Anisus vorticulus occurs. From Watson (2002).
The floodplain study was undertaken as a Master project of the University of Geneva (Terrier, 2006). The English study was undertaken as a training project of the University of Carfiff, following several research projects about Anisus vorticulus, Segmentina nitida or Valvata macrostoma (Watson, 2002; Watson and Ormerod 2004a, b, Nieggerbrugge et al., in press).
The aim of this short note is to summarize both studies that are being prepared for publication and to highlight their findings relevant to the understanding and conservation of A.vorticulus.
Anisus vorticulus in floodplain
The study was carried out in the French Upper-Rhone floodplain where the species was shown to be frequent and locally very abundant (Castella et al., 2005). It comprised an attempt to model its occurrence along gradients of environmental variables using Generalized Additive Models (GAMs) and a comparison of densities and size structure (using shell diameter) of the populations between the different sites occupied.
GAMs showed that the occurrence of the species in this floodplain environment could be predicted by the season and four environmental variables: the percentage of organic matter of the sediment, the aquatic vegetation richness and the cover of Elodea canadensis, Hydrocharis morsus-ranae and Lemna minor. This model could be used for example to identify candidate locations for re-introduction. The size-structure of A. vorticulus populations appeared to be heterogeneous across the floodplain habitats suggesting that the timing of reproduction and growth could depend upon local conditions such as temperature or flooding regime that remain to be identified. The monitoring of the species in a cut-off channel that underwent severe modifications (dredging and removal of riparian tree cover) revealed that A. vorticulus could survive – at least during the short time scale of the study – strong modifications of its habitat.
Anisus vorticulus in humid grassland ditches
Based upon previous analyses by Watson (2002), which revealed that gastropod’s conservation value and richness were significantly greater for ditches occupied by Anisus vorticulus, Segmentina nitida or Valvata macrostoma, three Red Data Books (RDB) gastropods, and that these three species could be considered as “umbrella species”, the research in English humid grassland ditches was principally set out to answer the following questions: Do the three RDB species indicate ditches with wider conservation value? How constant is the composition of mollusc assemblages associated with the RDB species? Are RDB gastropods also indicators of ditch conservation value for macrophytes? Finally, which factors (e.g. dispersal, distance, water quality, and vegetation structure) best explain the drop out of both RDB species and commoner ones (non-RDB species) from typical assemblages?
Ordination and classification techniques were used to assess how species were associated across ditches and to relate gastropod assemblages to their environment. Analysis of variance was used to compare the composition and richness of gastropods, bivalves, molluscs (gastropods and bivalves together) and vegetation (marginal and aquatic) between ditch clusters, and also their conservation value. Ditches were classified in three groups characterized by specific gastropod assemblages. Each gastropod assemblage was associated by specific vegetation and water chemistry conditions. More interesting, each RDB species was representative of one of the three assemblages. Analysis of variance revealed that each ditch cluster was characterized by a distinct composition and richness of gastropods, molluscs and vegetation, while representing the same importance in terms of conservation value. This survey also confirmed the ‘umbrella’ role of the three RDB species A.vorticulus, S.nitida and V. macrostoma that indicated, first, high conservation value in the ditches where they occur and, secondly, distinct ecological conditions. Moreover, each RDB species represented wider an assemblage of associated gastropod molluscs and vegetation.
As an additional question, the project also assessed whether non-RDB species might indicate locations that hold the RDB species, or could be suitable following reintroduction. Chi-square test calculated the statistical significance of the association between each RDB species and the more common gastropods of their respective cluster. Segmentina nitida could be indicated by the presence of Bathyomphalus contortus, Stagnicola (Lymnaea) palustris and Planorbarius corneus, and Valvata macrostoma locations by the presence of Hippeutis complanatus and Bithynia leachii. A. vorticulus could not be indicated by the presence of any gastropods species.
Concerning A. vorticulus in particular, this study showed that it can be used as an umbrella species that indicates typical environmental conditions. It occurred, with its associated species assemblage, in deeper ditches, with more open water and submergent vegetation and less alkaline water with less emergent vegetation cover than the other assemblages.
The juxtaposition of these two sets of studies shows at the same time the capacity of Anisus vorticulus to adapt to different types of environments (a semi-natural large river floodplain vs. man-made ditches), and that the species occurs at sites with particular environmental conditions in both cases. As shown in previous studies, the nature and quantity of aquatic vegetation is a key factor controlling the occurrence of the species in both environments. Conversely, water chemistry appears to be more determinant in the English ditches than in the French floodplain. This might be because the ditches are more directly influenced by agricultural runoff, than the cut-off channels, generally buffered by riparian forest.
Continuation of these studies could develop in a more direct comparison between the two systems where A. vorticulus occurs. In this respect, it could be important to look for the species in man-made ditches adjacent to the French floodplain and compare its requirements there with the results obtained in England. In both sectors, it would also be important to investigate the connectivity between populations given the highly fluctuating character of the floodplain condition, relative to the more stable hydrology in the ditches.
I am sincerely grateful to Prof. Steve Ormerod, Dr. Isabelle Durance and Dr. Emmanuel Castella for their supervision, patience, and guidance. I extent my gratitude to the Malacological Society of London and to the Lucie and Ernst Schmidheiny Foundation (Switzerland) for their financial support in the research about English grazing marshlands.
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