Investigation on the fundamental niche of a rare species an experiment on establishment of Luronium natans

The rare aquatic plant Luronium natans (L.) Rafin. is a representative of t he Littorelletea, i.e. plant communities growing typically in unproductive habitats with oligo- to meso-trophic, slightly acid to circumneutral soft w aters. Apart from morphological and physiological adaptations to these part icular habitat conditions, Littorelletea communities are characterised by t heir narrow ecological amplitude. Despite the fact that L. natans is report ed to be limited to habitats with low calcium content, it is abundant in tw o of the numerous potential yet calcareous habitats in the floodplains of t he Upper Rhone, and its tributary, the Ain River (France). The present stud y examines whether the restriction to such a low number of habitats is link ed to particular abiotic conditions therein. It also aims to identify the r ange of floodplain habitats that belong to the species' fundamental niche, i.e. those where, after arrival, it would be able to establish in the absen ce of interspecific competition. L. natans was therefore transplanted to si tes along a gradient of sediment composition, and allowed to establish duri ng a competition-free period. Extensive physico-chemical analyses were carr ied out on sediment, surface and interstitial water of the transplantation sites. The species' fundamental niche appeared very large, since establishm ent was successful in 3/4 of the transplantation sites. The magnitude of es tablishment success differed greatly, but only redox-potential was signific antly correlated with establishment success. Abiotic parameters could not p rovide a general explanation of complete establishment failure either. The experiment demonstrated that the restriction of L. natans distribution in t hese floodplains is not caused by a lack of habitats with suitable water an d sediment chemistry. (C)2000 Elsevier Science B.V. All rights reserved.