Effects of exotic submerged aquatic vegetation on waterfowl in the Mobile-Tensaw Delta

Abstract

Survey conducted in the Mobile-Tensaw Delta, located in the Northern Gulf of Mexico, have documented a 96% decline in waterfowl populations from over 100,000 birds in 1939 to around 4,000 birds in 1999. Coincident with this decline has been the introduction and spread of nonnative Eurasian watermilfoil (Myriophyllum spicatum). Six surveys have documented the replacement of native wild celery (Vallisneria americana), the perceived preferred food for waterfowl, by M. spicatum as the dominant species of submerged aquatic vegetation (SAV) in this setting. Simple comparisons of SAV coverage and waterfowl surveys indicate that declines in waterfowl populations are not strongly related to invasion of M. spicatum. Stable isotope analysis of three species of waterfowl (Anas strepera, Anas fulvigula, and Aix sponsa) and their food sources show these waterfowl feed on both wild celery and milfoil. Isotopic signatures of animals living on these SAV were also in waterfowl tissues. Based on these two lines of evidence, it is unlikely that the invasion of milfoil, by itself, is responsible for waterfowl declines in this delta.