identifying some new species and verifying their nonnative status, as well as uncertainties associated with sampling. As discussed later, confusion may also result over the inclusion or exclusion of microscopic organisms such as viruses, bacteria, parasites, or protozoans (Drake et al. 2007). The number of reported AIS established in the Great Lakes has, however, increased substantially from the early 19th century to modern times, with current estimates ranging from a low of 136 to more than 180 species of nonnative algae, fish, invertebrates, and plants (U.S. Geological Survey n.d.; Ricciardi 2006).
During the period of human-mediated biological invasions, a number of transitions have occurred with respect to both the types of AIS that have established and the mechanisms by which they entered the Great Lakes. Fish and plants were the most common invaders before the 20th century, with most introductions resulting from human releases (Mills et al. 1993). Algae and invertebrates became more common invaders after transoceanic shipping converted to use of liquid ballast around 1900. Shipping appears to have become the dominant means by which AIS have been transported into the Great Lakes during much of the 20th century, and notably since the opening of the St. Lawrence Seaway in 1959. The origins are less certain for invasive plants that now dominate coastal wetlands in highly urbanized areas, such as Lake Michigan’s lower Green Bay.
In invasion biology, a vector is defined as the physical means or agent by which a species is transported. There are multiple vectors by which AIS gain access to the Great Lakes, including commercial shipping, recreational boating, angling or bait fishing, aquaculture, commercial and home aquaria, water gardens, canals, and rivers.
For the purposes of this report, an invasion pathway is defined as the geographic path over which a species is transported from its origin (donor area) to its destination (target area). An analysis commissioned by the committee examined pathways of introduction for AIS reported as being established in the Great Lakes since the