ENGELMOER M, TAAL J, WYMENGA E & KUIPERS R (2001) Decreasing numbers of Brent Geese Branta bernicla along the Frisian Wadden Sea coast. LIMOSA 74 (2): 41-56.

Until recently, the Frisian Wadden Sea coast (Fig, 1) harboured the large st concentrations of Brent Geese during spring migration in The Netherlands, The area is also one of the principal wintering sites for Barnacle Geese in The Netherlands, and since recently the most important spring staging site for this species in the country , However, the spring staging numbers of Brent Geese along the Frisian Wadden Sea coast have decreased since May 1995, This population decline is also recorded from other areas along the migration route, and is probably caused by a lower productivity of the Brent Geese population, In this study, we investigated whether the population decline was solely responsible for the decline in numbers along the Frisian Wadden Sea coast, or whether the increased use of the Frisian Wadden Sea coast by Barnacle Geese during spring caused extra reductions in the numbers of staging Brent Geese due to interspecific competition

Since the winter of 1994/95, Barnacle Geese stay longer and with larger numbers along the Frisian Wadden Sea coast (Figs, 2 & 3) This development started in April 1995 with about 60 000 birds and it involved about 90 000 birds five years later. In the first spring seasons until April 1998, they were nearly exclusively grazing on meadows, but in that very month they shifted to the salt-marshes as well. In the following spring seasons the grazing pressure of Barnacies on the salt-marshes increased importantly (Tab, 1) Since May 1998 also the May-numbers increased, Nowadays about 20 000 Barnacle Geese are still present in the first weeks of May, Both Barnacle and Brent Geese favour the same short salt-marsh vegetation with Agrostis stolomfera, Plantago maritima and Trichlogin marilima when feeding on the salt-marshes (Tab, 2) resulting in positive regressions between the grazing pressures of Barnacle and Brent Geese per site during spring 1997 and spring 1998 (Fig, 4),

The decrease in Brent Geese numbers occurred occurred throughout the Dutch Wadden Sea area (Tab, 3), but the decrease appeared to be somewhat larger in the areas with salt-marsh grazing by Barnacle Geese (66% between spring 1995 and spring 1999) than in areas without grazing Barnaeles (50% decrease over the same period).

Since there is a long series of spring counts of Brent Geese, it was possible to investigate the statistical relationships between the spring numbers of Brent Geese along the Frisian Wadden Sea coast on the one side, and the total numbers of Brent Geese in the Dutch Wadden Sea and/or the total population size on the other side, during the years that no spring-grazing on the North-Frisian salt-marshes ocurreded (Fig, 5, Tab, 4) The derived equations were used to predict both the total numbers present in the Dutch Wadden Sea and the total population based upon the spring numbers of Brent Geese along the Frisian Wadden Sea coast from spring 1995 onwards (Tab , 5) These spring staging numbers predicted significantly lower numbers in the whole Dutch Wadden Sea and/or the total population size as was found in reality, This discrepancy was espeeially evident during April and less so during May and depended upon the number of Barnacle Geese present in the area at the same time (Fig, 6) It is thus concluded that the prolonged stay of Barnacle Geese along the Frisian Wad den Sea coast has resultsed in an additional decline of the spring staging numbers of Brent Geese,

The population numbers of Brent Geese in the East-Atlantic Flyway have reduced drastically from about 300000 in the early 1990's to less than 200000 nowadays, This decline is linked with a reduced productivity in the population as a whole, This article shows, that the competition with Barnacle Geese on the spring staging grounds has increased, This increase might be one of the factors causing the reduced productivity, since an increase of the completion for food might result in lower departure masses to the breeding grounds, worse arrival conditions and a worse breeding performance.Dark-bellied Brent Goose Branta bernicla

[free pdf] [dutch summary]