Swamp rose-mallow (Hibiscus moscheutos) COSEWIC assessment and status report: chapter 4

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Distribution

Global Range

Hibiscus moscheutos is one of the northernmost members of the largely tropical and subtropical family Malvaceae, and is the only native, extant member of this genus occurring in Canada. The global range of H. moscheutos encompasses most of the eastern United States north of Florida and east of the Mississippi River, with a narrowing coastal distribution evident north of Maryland to Massachusetts. The somewhat disjunct populations in southern Ontario, northern New York, Michigan, Wisconsin, Ohio, and Illinois are centred around the lower Great Lakes (Figure 2). Since this taxon was mapped by Ford and Keddy in 1987, it has expanded its range into the US west, and is now known in California, Utah, Texas, Kansas, and Oklahoma (NatureServe 2003).

Hibiscus moscheutos is also adventive in widely separated parts of western Eurasia such as northern Portugal, southwestern France, northern Italy and western Georgia (formerly the Georgian S.S.R.). It is also known from Africa along the Algerian coast (Blanchard 1976).

Figure 2. Global distribution of Hibiscus moscheutos (solid dots represent more recently identified state occurrences in NatureServe 2003).

Canadian Range

The Canadian range is restricted to southern Ontario (Figure 3), where H. moscheutos is confined to the coastal marshes and remnant wetlands of Lakes Erie, St. Clair, and Ontario, with a couple of inland stations (Figure 3). A total of 71 stations are known for Canada, and 51 of these are considered as extant. This species is most common in the western basin of Lake Erie, with particularly high numbers found in Essex County (#7 & #8). Extant stations are known from the following counties/municipalities: Essex (30), Municipality of Chatham-Kent (10), Niagara RM(3), Lambton (2), Norfolk (2), Elgin (2), Prince Edward County (1), and Frontenac (1).

Figure 3. Canadian distribution of Hibiscus moscheutos.

A total of 20 stations are either known or believed to be extirpated, 10 in Essex County, four in Niagara RM, and two each in Norfolk County, Municipality of Chatham-Kent, and Lambton County. Two stations are treated as historical populations of unknown status: one at Mitchell’s Bay in Lake St. Clair, the other at Long Point Biosphere Reserve. Given the distinctive nature of this species, no stations are considered as erroneous. Cultivated plants of H. moscheutos have been observed by the authors adjacent to homes in Belle River, Shrewsbury, Walpole Island, Amherstburg, Point Pelee NP. Although some of these were removed from natural populations, Hibiscus as a complex is extremely popular in the horticultural trade, and short of genetic analysis, it is probably impossible to discern native from horticultural stock.

Hibiscus moscheutos appears to have expanded its range northeastward in Ontario over the last 15-20 years. A similar phenomenon occurred in Ohio in the 1960s (Lowden, 1969) and a parallel expansion has been noted for the closely related H. laevis for some time in the US (Deam, 1940; Stuckey, 1968; Utech, 1970). As explanation for this ‘migration’, Blanchard (1976) has suggested that, “The increased erosion and sedimentation, which accompany lumbering and farming practices, have opened up new silted-in bottomlands and sand bars to colonization by Hibiscus.” The most notable expansion in Ontario has been on the north shore of Lake Ontario, where previously it was never recorded north of the Niagara Region. It is now known from Prince Edward County (site #51), and Frontenac County (site #52). This represents a northward extension of over 200 kms from the Niagara Region stations in the space of approximately 15 years. It is believed that this ‘movement’ has been achieved through ramets floating on the water from the New York populations and washing up in suitable habitat on the Ontario shoreline. Coincident with this northward expansion, the species seems to be making in-roads from its traditional shoreline habitats, and exploiting suitable habitat. Perhaps the best example being that at Kettle Creek (site #2) inland from Lake Erie. This, however, is believed to be the result of fill transported from coastal areas, rather than natural expansion.

The extent of occurrence in Canada is estimated at 22,000 km². The area of occupancy is about 9.5 km². This value is difficult to calculate for Hibiscus, since even though it occurs in extensive wetlands, such as at sites # 12, 26, 41, and 42, for the most part, the species is found in these marshes in a rather narrow linear band. Consequently, the entire wetland was not considered suitable habitat. Estimates for marshes at sites #26 and #42 and Rondeau, for example, were 1 km², for site # 41, 50 hectares, and for site # 12, 10 ha. The majority of stations (23) were less than or equal to 1 ha; 17 were between 1 and 10 ha; 6 were between 10 and 100 ha (sites # 4, 8, 9, 38, 39, 41), and 6 were estimated at 100 ha (sites # 37, 42, 44, 45, 49, and 50).  The largest meadow marsh habitat for H. moscheutos occurs at site # 8, where it is distributed over 30 ha. The area of occupancy is believed to be in decline, for the reasons noted in the Limiting Factors and Threats section.

The distribution pattern of H. moscheutos has some similarities to those of members of the Atlantic coastal plain flora that have a disjunct distribution in the southern Great Lakes (Blanchard, 1976). Peattie (1922) and MacLauglin (1932) have attempted to explain this distribution pattern in relation to the post-glacial development of the Great Lakes. They postulated that extensive wetlands existed along much of the shoreline. The coastal plain flora entered the Great Lakes during the Lake Algonquin stage when there was a blockage of the St. Lawrence Valley and drainage was through the Mohawk and Hudson River valleys. It was through this avenue that plants of the coast gained access to inland marshes. It seems probable that the populations of H. moscheutos currently found in Ontario migrated from the east coast, following the glacial retreat, using this corridor (Blanchard, 1976). The further recession of the glaciers after the Lake Algonquin stage reduced the Great Lakes in size and obliterated the Hudson-Mohawk migration route. Historically, H. moscheutos was probably more extensive in its distribution than it is today. In Ontario, it is only the coastal marshes of Lakes Erie, St. Clair, and Ontario that have continued to provide the required biotic and abiotic conditions necessary for its survival.