. The Canadian field-naturalist. 82 The Canadian Field-Naturalist Vol. 99. 1. Cincinnatia cincinnatiensis 2. Amnicola limosa stagnalis megasoma gyrina anceps campanulatum pilsbryi trivolvis armigera exacuous parvus deflectus Figure 2. Significant positive associations between gastropods in the study area. Distances represent x " units. from the small number of occurrences of both species (7), or to restrictions operating with respect to other, unmonitored, parameters. The greatest number o
. The Canadian field-naturalist. 82 The Canadian Field-Naturalist Vol. 99. 1. Cincinnatia cincinnatiensis 2. Amnicola limosa stagnalis megasoma gyrina anceps campanulatum pilsbryi trivolvis armigera exacuous parvus deflectus Figure 2. Significant positive associations between gastropods in the study area. Distances represent x " units. from the small number of occurrences of both species (7), or to restrictions operating with respect to other, unmonitored, parameters. The greatest number of associations, a total of five, was shown by Lymnaea stagnalis (3), which thus appears to occupy a central position in the cluster. The most closely associated species were Bulimnea megasoma (4) and Planorbula armigera (10). The linear relationship between Helisoma campa- nulatum (7), Gyraulus deflectus (13), Promenetus exacuous (11) and, to a lesser extent, /I m/7/fo/fl limosa (2), may reflect the strong tendencies of these species to occur in lakes and rivers, where seasonal variation is less pronounced. Chi-square tests were also used to identify snail- macrophyte associations. Values were again adjusted for apparent tolerance ranges of both snails and plants. The results showed a total of 49 significant positive and 3 negative associations (Table 3). Heli- soma campanulatum showed the greatest number (11) of positive associations, followed by Bulimnea mega- soma, Planorbula armigera and Gyraulus deflectus, with 6 associations each. The plants most frequently associated with gastropods in the region were Elodea canadensis (Common Elodea), Spirodela polyrhiza (Big Duckweed), Zizania aquatica (Wild Rice) and Slum suave (Water Parsnip). Discussion The southwestern edge of the Precambrian Shield region is the most diverse area of central Canada in terms of both gastropods and macrophytes (Pip 1978). This region has thus far escaped much of the effects of acid rain which have ra
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