The spring range of barnacle geese Branta leucopsis in relation to changes in land management and climate

This study examines recent changes in the distribution of spring staging barnacle geese Branta leucopsis in Norway. From the early 1980s onwards, two major changes have taken place. First, increasing numbers of geese moved from the traditional islands with a semi-natural vegetation towards agricultural fields on larger islands close to the mainland. Secondly, geese expanded their range by exploring more northerly located islands. Data on food exploitation by geese on the traditional islands were collected to interpret these changes. The density of shoots of edible grasses explained the spatial variation in grazing pressure over the traditional islands. This pattern was caused by intensive grazing of available habitat, where apparently a lower threshold of the intake rate set a limit to the amount of food cropped. It is concluded that the carrying capacity of the traditional islands had been reached. On many of the traditional islands, capacity declined, however, as the vegetation type with the highest density of grasses and most preferred by geese disappeared. This negative development seems to have started when people abandoned the islands and summer grazing by sheep stopped. The declining number of geese that can find sufficient food within the traditional range, in conjunction with the rapidly growing size of the total population, are the main causes for the observed expansion of the population. Increasing spring temperatures and a change in agriculture may have allowed the geese to move to the north, and to invade agricultural areas. To lessen conflicts between geese and farmers on the large islands, two lines of management are available: restoration of management on islands within the traditional range, and enhancing the attractiveness to geese of saltmarshes on the larger islands. (C) 1998 Elsevier Science Ltd. All rights reserved.