Loss of native aquatic plant species in a community dominated by Eurasian watermilfoil

Ecological evaluation of the impact of an exotic species upon native plant species is frequently a combination of historical data prior to introductio n and after full establishment with little observation in between. The intr oduction of Myriophyllum spicatum L. (Eurasian watermilfoil) into Lake Geor ge, New York, U.S.A. was first noted in 1985. In 1987, a few newly establis hed plants were allowed to grow unimpeded by human management to document t he rate of colonization of this species into a new habitat with its ultimat e dominance over, and systematic elimination of, native species. This chang ing community has been closely monitored over the past decade. Initially a 6 m(2) grid system composed of 144 0.5 m(2) plots with four radiating trans ect lines was laid out with the isolated M. spicatum plants as the center. The site was revisited between 1987 and 1997 to mark the extent of the deve loping M. spicatum bed and its effect on the composition of the native plan t community. Since 1987, the area of dense growth has expanded in all direc tions, impeded only where some physical barrier, such as upper or lower wat er depth limits or sediment type curtailed its growth. Concurrently, with t his expansion, a decline in species richness and abundance of native specie s was observed.