1.The genus Castor comprises two species: the Eurasian beaver Castor fibre , and the North American beaver Castor canadensis . Both species suffered from overexploitation, but have seen a revival since the 1920s due to increased protection and reintroduction programmes. Increases in the populations and distributions of species that are able to modify ecosystems have generated much scientific interest. Here we review the available literature concerning the possible ecological impact of beaver species in the Old and New World. 2. Beavers, being ecosystem engineers, are among the few species besides humans that can significantly change the geomorphology, and consequently the hydrological characteristics and biotic properties of the landscape. In so doing, beavers increase heterogeneity, and habitat and species diversity at the landscape scale. Beaver foraging also has a considerable impact on the course of ecological succession, species composition and structure of plant communities, making them a good example of ecologically dominant species (e.g. keystone species). 3. Nevertheless, the strength of beavers' impact varies from site to site, depending on the geographical location, relief and the impounded habitat type. Consequently, they may not be significant controlling agents of the ecosystem in all parts of their distribution, but have strong interactions only under certain circumstances. We suggest that beavers can create important management opportunities in the Holarctic, and this review will help land managers determine the likely outcome of beaver activity.

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Three separate effects on refuge use by signal crayfish Pacifastacus leniusculus and Atlantic salmon Salmo salar were examined: (1) the effect on Atlantic salmon of an addition of signal crayfish (doubling the total number of animals), (2) the effect on signal crayfish of an addition of Atlantic salmon and (3) intraspecific compared with interspecific competition, compared by holding total density of animals constant and varying the proportion of signal crayfish and Atlantic salmon in trials. Observations were made during winter, when both species are nocturnal. The proportion of Atlantic salmon sheltering was significantly lower in the presence than in the absence of signal crayfish when the interspecific treatment (Atlantic salmon plus signal crayfish) effected a doubling in density compared to the intraspecific treatment (Atlantic salmon alone). The proportion of signal crayfish sheltering was independent of the presence of Atlantic salmon. When total density was constant, the proportion of Atlantic salmon sheltering was significantly higher in intraspecific (52·8%) than interspecific trials (27·3%). Atlantic salmon out of shelter during the day in winter are believed to be very vulnerable to predators and the capacity for fish to share shelters with one another is known to be very low. Therefore, competition from crayfish for winter shelters may lead to detrimental effects on Atlantic salmon populations.

This trial aimed to eradicate illegally introduced signal crayfish Pacifastacus leniusculus in the North Esk catchment, Scotland. Sites treated were (1) an isolated gravel-pit (c. 9,000 m 3 ), with crayfish present for 6 years; (2) three dammed ponds, (c. 5,000 m 3 ) and (3) a leaking, offline pond (c. 6,000 m 3 ), with crayfish for two years. Preliminary toxicity tests with substrate present indicated doses. Treatment at sites (1) and (2) (in October 2004, water temperature 13°C) comprised deoxygenation with sodium sulphite to stimulate emergence, then application of natural pyrethrum (Pyblast). Exposed margins were sprayed with Pyblast to prevent escapes. Crayfish mortality was high, but one survivor was seen after 5 days. Pyblast was applied from a tank with Na 2 SO 3 residue, which subsequent investigation indicated reduced Pyblast below the target 0.1 mg l -1 . Site (1) was re-treated (end October, target 0.15 mg l -1 Pyblast, no Na 2 SO 3 , 9°C). Mortality was confirmed using caged crayfish. Prior to treatment at site (2), throughflow was stopped and fish removed. Biomonitoring was carried out with freshwater shrimps Gammarus in the adjacent watercourse. Treatment of site (3) (December, target 0.2 mg l -1 , 4°C) necessitated continuous back-pumping of leakage for a 2-week recovery period to avoid contamination of the river downstream. Caged crayfish took up to 5 days for 100% mortality. No crayfish were found in a summer survey after the treatments with Pyblast alone, but were caught in the ponds with Na 2 SO 3 pre-treatment. Monitoring is required for 2-5 years. Key-words:Pacifastacus leniusculus, eradication, biocide, ponds, natural pyrethrum. ÉRADICATION DE L'ÉCREVISSE SIGNAL DANS LES ÉTANGS ÉCOSSAIS PAR TRAITEMENTS BIOCIDIQUES RÉSUMÉCet essai avait pour but d'éliminer l'écrevisse signal (Pacifastacus leniusculus) introduites illégalement dans le nord du bassin de l'Esk, en Ecosse. Les sites d'expérimentation furent une sablière isolée (ca. 9 000 m 3 ), trois étangs de retenue (ca. 5 000 m 3 ) et un étang perméable (ca. 7 000 m 3 ), avec des écrevisses depuis moins de deux ans, à l'exception de celles présentes dans la sablière depuis 6 ans. Des tests l'eau 13 °C) a compris la désoxygénation avec du sulfate de sodium pour stimuler l'émergence, et ensuite l'application du pyrèthre naturel (Pyblast). Les berges exposées furent vaporisées à l'avance avec Pyblast pour empêcher la fuite des écrevisses. La mortalité des écrevisses fut élevée, mais un survivant a été vu 5 jours plus tard. Le Pyblast avait été pompé à partir d'un réservoir contenant encore des résidus de sulfate de sodium, ce qui a probablement réduit la concentration de Pyblast en dessous de 0,1 mg.l -1 . Un nouveau traitement a été conduit fin Octobre avec 0,15 mg.l -1 de concentration en Pyblast et 0 en Na 2 SO 3 (température de l'eau 9 °C). La mortalité fut confirmée par l'utilisation d'écrevisses en cage. Aux trois étangs, l'écoulement a été arrêté avant le traitement, les poissons ont été retirés, la végétation des berges a été tr...

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