During the summer of 2006, we collected snails at 4 to 6 sites in each of 44 focal lakes. Survey lakes ranged in surface area from 14 to 1400 ha (median = 130 ha), and were selected to span broad gradients of landscape position, water chemistry, human use, and other characteristics, and to maximize overlap with lakes where Morrison (1932) has previously described snail assemblages (see below). For each lake, site locations were chosen randomly within each compass quadrant of the shoreline, using Geographic Information System (GIS) software (ArcGIS 9.2; ESRI, Redlands, California). At each site we placed a 20 m transect line on the lake bottom along the 1 m depth contour. At 2 m intervals along the transect, two snorkelers collected all the snails from within 0.25 m2 quadrats (10 quadrats per site). They then searched the vicinity of the transect haphazardly for 5 minutes to reduce the likelihood that B. chinensis presence at a site escaped detection. Sampling ceased after the fourth site if at least 25 quadrats with non-zero snail abundances had been sampled; otherwise, sampling continued at alternate sites until that threshold was reached or 6 sites had been sampled. At some sites the entire 20-m transect fell in thick macrophyte beds, precluding effective snorkel surveys. In these cases, we sampled snails by vigorously sweeping a D-net (500 m mesh) through the macrophytes in two 1 m2 areas. Collected snails were preserved in 80% ethanol.
Identifications were made according to Burch (1989), following the revision of Hubendick (1951) for the Lymnaeidae, Hubendick (1955) for the Planorbidae, and Wethington and Lydeard (2007) for the Physidae. All samples are being curated into the Illinois Natural History Survey Mollusk Collection.
Rapid assessment protocol
We also tested the effectiveness of a rapid assessment protocol for detecting the presence of B. chinensis. Two observers snorkeled around the vicinity of the boat launch (if present) for up to 5 minutes each, or until B. chinensis was found. We conducted this rapid assessment at 27 of the focal survey lakes, as well as at 8 additional lakes where we did not conduct full quadrat surveys.