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Population changes in long-eared owls Asio otus, polecats Mustela putorius, red foxes Vulpes vulpes, stone martens Martes foina and badgers Meles meles were monitored during a water vole Arvicola terrestris scherman cycle in western Switzerland. Long-eared owls confirmed their status of highly mobile specialist predators in responding strongly and without time lag to water vole population changes. Even though polecats are considered generalists, they exhibited also a strong response to water vole fluctuations. Their numbers tracked water vole densities with a 1-yr time lag. Marked population changes were also recorded in red foxes and stone martens, but these changes were not related to water vole densities. Lastly, badgers did not show any significant population changes during the water vole cycle. We discuss the possible reasons for these differences and conclude that multi-factorial approach is clearly required to understand population processes in predator-prey systems.

Predation on the fossorial form of the water volc by the red fox was studied in western Switzerland from April 1988 to February 1991. As well as analysis of the ramains found in fox scats collected monthly, seasonal trapping was carried out to estimate water vole density. During this study, the water vole was the main prey of the fox, representing 54.5% of all items (n=1707). There was a significant correlation between availability of water voels and their consumption by the red fox.

Nous etudions la thermodynamique de KDP en ne considerant que les interactions entre les spins fictifs associes aux protons premiers voisins dans des models plans et spatiaux. On montre d'abord qu'on decouple completement le mouvement des spins fictifs du mouvement des ions en definissant une nouvelle energie d'interaction entre les premiers et en renormalisant les seconds. On etudie ensuite l'effet tunnel entre protons formant un cycle ferme, en regardant successivement son influence sur les etats a une et deux excitations elementaires. Ceci permet de penser qu'une transition du 2eme ordre n'est pas exclue. Nous notons enfin que ce modele conduit a un mode mou correspondant a une diffusion de spins fictifs et non a un phonon optique. A study is made of the thermodynamics of a two- and a three-dimensional model of KDP considering only nearest neighbour interactions between protons which are described here as fictive spins. It is shown that one may completely separate out the movement of the fictive spins from that of the ions by simultaneously redefining the interaction energy between the formers and renormalizing the latters. The tunneling effect between protons forming a closed ring is then studied by looking at its influence on one and two elementary excitation states. This allows to show that a second order phase transition is not a priori excluded. Finally a study is made of the soft mode associated with that model and it is shown that it corresponds to a fictive spin diffusion rather than to an optical phonon.