. The Canadian field-naturalist. Natural history. 240 The Canadian Field-Naturalist Vol. 109. Figure 3. Distribution of Hydrocharis morsus-ranae in North America up to 1994 based on examination of specimens at CAN, DAO, MX, NYS, OAC, QFA, SFS and TRT. The shaded area represents the Canadian Shield region of largely granitic upland. absent from much of the Canadian Shield region to the north, east and west of the current distribution and are also absent in the Appalachian uplands to the east. The apparent obstacle presented by relative scarcity of habitat in much of the Canadian Shield and Appa
. The Canadian field-naturalist. Natural history. 240 The Canadian Field-Naturalist Vol. 109. Figure 3. Distribution of Hydrocharis morsus-ranae in North America up to 1994 based on examination of specimens at CAN, DAO, MX, NYS, OAC, QFA, SFS and TRT. The shaded area represents the Canadian Shield region of largely granitic upland. absent from much of the Canadian Shield region to the north, east and west of the current distribution and are also absent in the Appalachian uplands to the east. The apparent obstacle presented by relative scarcity of habitat in much of the Canadian Shield and Appalachian upland is supported by the limited spread into this region (Figure 3), and resultant con- finement to the northeast - southwest axis of the lower Great Lakes - St. Lawrence system. However, Frogbit has recently been found at the top of a water- shed in the granitic barrens north of Tamworth in eastern Ontario {Brownell & Catling 20603, DAO), so that the extent to which the Canadian Shield will be a complete barrier is uncertain. Since it occurs from Point Pelee to Quebec City, the genotype(s) of European Frogbit established in North America appear to have a broad climatic toler- ance. It has now colonized portions of two major ecozones of Canada and four ecoregions (cf. Canada Terrestrial Ecoregions, National Atlas Information Service 1993: Wiken 1986). Evidently, appropriate habitat for European Frogbit occurs throughout much of southern New York, all of southwestern Ontario and generally throughout much of the southern portion of the Great Lakes basin including portions of the states of Illinois, Indiana, Michigan, Ohio and Wisconsin. It is anticipated that European Frogbit will spread throughout much of this area over the next 10 years based on its recent rate of spread in this direction (Figure 2), distribution of appropriate habitat and evident climatic tolerance of the introduced geno- type. It is quite possible that within 20 years it will have become prevale
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