Zosteraexploitation by Brent Geese and Wigeon on the Exe Estuary, southern England

Abstract

The Exe Estuary (southern England) was recolonized by the expanding wintering population of dark-bellied Brent Geese during the 1960s; Wigeon numbers have decreased since their return, but numbers of both species have been stable since 1982. Brent Goose and Wigeon feeding behaviour was studied at two Exe intertidal Zostera beds during 1990–1992, to assess the overlap in time and space of these two herbivorous waterfowl, apparently feeding upon the same food resource. Results showed less than 5.5% overlap between the two species using either Zostera bed. From arrival in autumn, Brent Geese extracted rhizomes in softest substrates and consumed whole plants, mostly in an area with low-tide pools. Previous studies have shown that Brent Geese only probed for Zostera rhizomes after shoot biomass has been much reduced. Wigeon fed elsewhere on the same flats by dabbling the floating above-ground parts of Zostera only in water retained by the physical characteristics of the Zostera beds. Despite this spatial separation, some exploitative competition must occur through the removal by Brent Geese of potential feeding resources for Wigeon. Brent Goose foraging may also modify sediment characters to their own benefit. As Wigeon numbers continue to increase throughout Britain and because the Exe Estuary was the only site in a series of 18 major coastal areas showing any inverse relationship between wintering Wigeon and Brent Geese in southern England, it is considered that any competitive interaction that may occur or may have occurred in the past has not stopped an increase in numbers at the national level.