Expansion of seagrass habitat by the exotic Zostera lapontca. and its use by dabbling ducks and brant in Boundary Bay, British Columbia

Abstract

The exotic seagrass Zostera japonica was first documented on the Pacific Coast of North America in the late 1950s, and has extensively colonized formerly unvegetated tidal flats and dramatically altered the habitat structure. In Boundary Bay, British Columbia, Canada, there was an almost 17-fold increase in Z. japonica coverage between 1970 and 1991. In Boundary Bay's 6385 ha of intertidal and shallow subtidal area (tidal range 4.7 m), Z. japonica occurred mostly from 0 to -1.8 m Mean Water Level (MWL) and covered 3845 ha in October 1991 The native 2. marina occurred mostly from -0.9 to -5.5 m MWL, and covered 3444 ha including 1684 ha of overlap with Z. japonica. Standing stock of Z. japonica, mostly in formerly unvegetated areas, measured 520 metric tonnes (t) above ground and 235 t below ground in October 1991 Thls introduced species provides an important feedlng habitat for many migratory waterfowl. Percent dry mass of Z. japonica in esophagus contents of birds collected in Boundary Bay was 57.2 % (n = 62) in brant Branta bernicla, 84.8 % (n = 45) in American wigeon Anas arnencana, 72.3 % (n = 20) in mallard A. platyrhynchos, 48.3 % (n = 54) in northern pintail A. acuta, and 1.7 % (n = 14) in green-winged teal A. crecca. Percent dry mass of the native Z. marina was 41.2% in esophagi of brant but only 0.1 to 4.6% in the other species. Grazing by brant and dabbling ducks, with peak numbers of about 80 000 in early December, removed 50% (262 t) of the above-ground biomass and 43 % (100 t) of the below-ground biomass of Z. japonica. This exotic seagrass thereby supported almost 4.6 million use days by these birds.

Seagrass ( Zostera spp.) as food for brent geese ( Branta bernicla ): an overview

“…The observation made by Nacken (1998) that Z. noltii plots where brent geese were excluded from feeding in autumn had sparser vegetation cover in the following spring than grazed plots had seems to support this speculation. Although Baldwin and Lovvorn (1994b) suggest that "the trophic role of avian herbivores in temperate seagrass food webs may often be underemphasized", restricted accessibility of Zostera and relatively short staging periods of the birds during the least productive times of the year still make it seem unlikely that they have a major impact on Zostera stocks in most of the areas used.…”

“…Because Z. japonica occupies a previously unused niche in the middle to upper zone of tidal estuaries, vast areas of seagrass habitat accessible at low tide have been added to the previously present subtidal and lower intertidal stands of Z. marina (Baldwin and Lovvorn 1994b). This has enormously increased the feeding opportunities for herbivorous water birds, although the changes have not been quantified for black brant alone.…”

“…This has enormously increased the feeding opportunities for herbivorous water birds, although the changes have not been quantified for black brant alone. In contrast to dabbling duck species, which use the intertidal stands of Z. japonica exclusively, brant still have a high proportion of Z. marina in their diet because they can reach down into the shallow subtidal zone (Baldwin and Lovvorn 1994b). …”

“…These behaviours have been reported especially in periods of severe food shortage (Cottam et al 1944;Einarsen 1965). Frequently, brent geese also glean fresh leaves of Zostera from shore drifts at high tide (Cottam et al 1944;Baldwin and Lovvorn 1994b).…”

“…In a number of studies the decline of Zostera biomass or leaf cover during the presence of the birds was determined without measuring how much of the decline was due to grazing activities (e.g. Baldwin and Lovvorn 1994b;Fox 1996), while only a few studies estimated more closely the amount consumed by birds using exclosures which were inaccessible to grazing birds. Most of these studies relate to dark-bellied brent geese in autumn and winter.…”