Density dependent population limitation in dark-bellied brent geese Branta b. bernicla

The dynamics were investigated of the world population of dark-bellied brent geese Branta b. bernicla, which winter in western Europe and reproduce on the Siberian tundra. The world population was fluctuating between approximately 220,000 and 315,000 individuals in the last decade of the 20th century, after recovering from an extremely low level of about 15,000 in 1955. Cyclical fluctuations are related to the population dynamics of lemmings on the Taymyr Peninsula in Siberia and have been present during the whole study period 1955-1998. We investigated whether, apart from these fluctuations, the population size will level off due to density dependence in reproduction or in survival. This being the case, we also aimed at predicting the approximate equilibrium population size. Data are provided on the world population since 1955, counted every January, giving details on adult survival and reproduction. A simple discrete-time model was formulated where the reproductive season in Siberia and the winter season in Europe are treated separately allowing for density dependence during either season. The nature of the density dependence for the model was estimated from the data. Density dependence was significant in reproduction, but not in adult survival. We argue that availability of suitable nesting habitat is an important factor influencing reproduction potential, but the possibility of other factors (e.g. competition for food during spring staging) cannot be excluded. Analysis of the simple model yields a relation between the predicted equilibrium population size and a constant adult death rate estimated at 0.15, which corresponds to a predicted maximum population size of 286.000-332,000 at the time of the January census. Although it is tempting to use such a generic model as a basis to gauge effects of habitat loss on migrating bird species, great care should be taken in formulating rules-of-thumb.