Hill's pondweed (Potamogeton hillii) COSEWIC assessment and status report: chapter 4

Distribution

Global range

Hill’s pondweed has been found in scattered streams, ditches, ponds, and wetlands in southern Ontario, Michigan, Wisconsin, Ohio, Pennsylvania, New York, Connecticut, Massachusetts, and Vermont (Figure 2). 

Figure 2. North American distribution of Hill’s pondweed (after Hellquist (1984) and state rare plant atlases).

Figure 2. North American distribution of Hill’s pondweed (after Hellquist (1984) and state rare plant atlases).

Canadian range

In Canada, Hill’s pondweed is found only in Ontario. The earliest collection recorded was made in 1901 (Appendix 1). The species is found mostly in a 400 km² area of Manitoulin Island and on about 700 km² of Bruce County with an additional 100 km² in each of Wellington County and Peel Region, for a very conservative total extent of occurrence of about 1300 km² (Figure 3).  Most reports are from Bruce County and Manitoulin District. However, if all historic and known extant localities are included in two convex polygons, one for southern Ontario mainland and a second for Manitoulin Island, then the total extent of occurrence (EO), based on a Geographic Information System (GIS) calculation, is in the order of < 20,000 km². If the extirpated site 3 is removed from the estimation, the EO is <10,000 km². This latter value is likely more representative of the potential extent of occurrence for this species (E. Haber, GIS calculations of EO).

Given its unremarkable appearance, it is likely that Hill’s pondweed has been long overlooked, and could be more widespread than is currently reported.  It is associated with dolomitic limestone (Hellquist, 1984) so potential habitat might be found along the Niagara Escarpment and the Precambrian contact line (Brownell, 1986). More surveys would be needed to determine the full extent of its distribution.

Figure 3. Distribution of Hill’s pondweed in Canada. Solid dots represent extant or possibly extant populations; open circles, 03 & 22, represent known extirpated populations.

Figure 3. Distribution of Hill’s pondweed in Canada. Solid dots represent extant or possibly extant populations; open circles, 03 & 22, represent known extirpated populations.

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