Twenty years of zebra mussels: lessons from the mollusk that made headlines

© The Ecological Society of America www.frontiersinecology.org S zebra mussels (Dreissena polymorpha) first appeared in North America in 1988 (Hebert et al. 1989; see Carlton [in press] for a precise chronology), the mollusk has become the iconic alien species. The appearance of zebra mussels, together with many other invaders, highlighted the absence of effective controls on alien species in North America, and helped to usher in an era of heightened awareness about alien species. Now that we have had two decades to observe zebra mussels in North America, this seems a good time to reflect on what we have learned from the invasion and how this illuminates advances and limitations in scientific understanding, management, and policy about alien species in general. Here, I will consider four important questions that were raised at the outset of the invasion (Roberts 1990): (1) How far would the species spread in North America? (2) What ecological and economic impacts would result? (3) Might this high-profile invader raise public concern about alien species? (4) Would we develop effective controls and policies to manage this and other alien species? Range and spread