Development and viability of a translocated beaver Castor fiber population in The Netherlands

We monitored survival, reproduction and emigration of a translocated beaver Castor fiber population in the Netherlands for five years and used a stochastic model to assess its viability. Between 1988 and 1991, 42 beavers were released in the Biesbosch National Park. The mortality was initially high but gradually fell to normal rates. However, the breeding success was low, and we hypothesized that this was either a temporary phenomenon (the translocation hypothesis) or a permanent feature (the poor habitat hypothesis). According to the computer simulations, the isolated population was viable under the first but not under the second hypothesis. In the latter case, the prospects generally improved by the foundation of another population in the Gelderse Poort (100 km from the Biesbosch). However, this second habitat should be optimal for beavers in order to reduce the extinction probability of the Biesbosch population to below 10% in 100 years; the loss of genetic variability (1-2% per generation) was just above the applied tolerable risk (1%), but the effects of inbreeding are unknown in beavers. We conclude that the beaver population in the Biesbosch is not viable unless the reproductive success increases, either in the Biesbosch itself or in a nearby population. We recommend applying such viability analyses to evaluate the likely success of any translocation.