Loss of genetic variation due to population bottlenecks may be a severe threat for the survival of endangered species. Assessment and maintenance of genetic variability are thus crucial for conservation programs related to endangered populations. Scandinavian beavers went through an extensive bottleneck during the last century due to overhunting. In Sweden the species became extirpated but in Norway extinction was avoided by legal protection. Following reintroductions of small numbers of remaining Norwegian animals in 1922-1939, the Swedish population has increased tremendously, now harboring 100,000 animals. We show here that this viable population of beavers possesses extremely low levels of genetic variability at DNA fingerprinting loci and monomorphism at major histocompatibility complex (MHC) class I and class II loci. A similar pattern was also evident among Norwegian beavers but low levels of genetic variabilitywere not a characteristic of the species since Russian conspecifics displayed substantial DNA fingerprinting polymorphism. However, the Russian animals were monomorphic at MHC loci, indicating that the European beaver is exceptional in its low level of MHC variability. The results demonstrate that a conservation program can be successful despite low levels of genetic variation in the founder population.

Deliberate reintroductions of locally extermi. hated animal species to areas within their former ranges is an increasingly important conservation tool Most reintroductions are fairly recent and still in an initial phase of population development There are few long-term studies of reintroduced population~ The aim of this study was to see if the population development of the reintroduced European beaver (Castor fiber)population in Sumden exhibits the same pattern of population development as other introduction~ in accordance with general theorie~ and to discuss possible management consequence£ Since the European beaver was reintroduced to Sweden 70 years ago, the population has developed in a way predicted by the Riney-Caughley model for introduced ungulate~ exhibiting an irruption and a subsequent decline. In two study area~ the rate of population increase (r) turned negative after 34 and 25 years and at densities of 0.25 and 0.20 colonies/km 2, respectively. The data suggest that management policy for an irruptive species should allow hunting during the rapid-increase phase thus maintaining food resources and avoiding uncontrolled pop. ulation decline DesarroUo a largo plazo de unq pobaci6n reintroducida de Castor (Castor fiber) en Suecia Resumen: Las reintroducciones deliberadas de especies de animales exterratnados en forma local en dreas dentro de st, antigua zona de distribuci6n es una manera de conserva. ci6n de tmportancia crecient¢ La mayor parte de las reintroducciones son bastante recienteg y todavta estdn en una fase iniclal del desarrollo poblacionaL Ya que estudios realizados durante una larga temporada son escaso~ no se sabe si se comportardn del mismo modo que otras especies reintroducidax El prop6sito de este estudio fue comprobar si el desarrollo poblacional de la poblaci6n de castor europeo (Castor fiber) reintroducido en Suecla exhibe el mismo pa-tr6n de desarrollo poblacional de otras introduccione~ de acuerdo con teorias generale~ y tambi~n discutir las posibies consecuencias para el manejo de especlas reintrodacidax E1 castor europeo rue reintroducido en Suecia hace 70 aao~ En este estudio mostramos que la poblaci6n se ha desarrollado segt~n la predicci6n del modelo Riney-Caughley para ungulados inroducido~ mostrando una irrupct6n y rods tarde una disminuci~ Si se pone r observa~ de dos areas estudlada~ en relaci6n con la temporada despu~s de ia colonizaci6n y se ajustan las curvas a los dato~ se obtienen modelos donde r pasa a set negativa despuds de 34 y 25 a~o~ y con densidades de 0.25 y 0.20 colonias/km 2. Los datos sugieren que la$ estratg]las de manejo de especie irruptivas deberlan permitir la caza durante la fase del crecieraento rdpido para mantener los recursos alimentario~ y tratar de evitar una declinacion problacional descontrolada 713

Globally, artificial river impoundment, nutrient enrichment and biodiversity loss impair freshwater ecosystem integrity. Concurrently, beavers, ecosystem engineers recognized for their ability to construct dams and create ponds, are colonizing sites across the Holarctic after widespread extirpation in the 19th century, including areas outside their historical range. This has the potential to profoundly alter hydrology, hydrochemistry and aquatic ecology in both newly colonized and recolonized areas. To further our knowledge of the effects of beaver dams on aquatic environments, we extracted 1366 effect sizes from 89 studies on the impoundment of streams and lakes. Effects were assessed for 16 factors related to hydrogeomorphology, biogeochemistry, ecosystem functioning and biodiversity. Beaver dams affected concentrations of organic carbon in water, mercury in water and biota, sediment conditions and hydrological properties. There were no overall adverse effects caused by beaver dams or ponds on salmonid fish. Age was an important determinant of effect magnitude. While young ponds were a source of phosphorus, there was a tendency for phosphorus retention in older systems. Young ponds were a source methylmercury in water, but old ponds were not. To provide additional context, we also evaluated similarities and differences between environmental effects of beaver-constructed and artificial dams (767 effect sizes from 75 studies). Both are comparable in terms of effects on, for example, biodiversity, but have contrasting effects on nutrient retention and mercury. These results are important for assessing the role of beavers in enhancing and/or degrading ecological integrity in changing Holarctic freshwater systems.

No abstract

Dam-building behaviour is a distinctive feature of beavers; yet they often occupy streams or sections of streams where they do not construct dams. This study was designed to determine how stream depth and stream width influence dam-building behaviour. Stream depth and stream width were measured at 39 beaver lodges, which did not have dams associated with them and at 74 dams. The mean water depth at the lodges was 1.16 m (range = 0.55-2.0 m), while the mean water depth at the dam sites was 0.36 m (range = 0.10-0.85 m). The stream width was greater at lodge sites (mean = 11.15 m, range= 1.0-40 m) than at dam sites (mean = 2.5 m, range= 0.5-6.0 m) A discriminant analysis classified 93% of the sites correctly as either lodge or dam sites.