Swamp rose-mallow (Hibiscus moscheutos) COSEWIC assessment and status report: chapter 8

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Limiting Factors and Threats

The greatest threats to the survival of Hibiscus moscheutos in Canada derive from two sources – the continued degradation of its coastal wetland habitat, and the invasion of this habitat by invasive species, in particular Phragmites australis. Although large populations are currently being ‘protected’ in Ontario, as yet there is no successful control of Phragmites at any wetland in the province, and the threat which this highly invasive grass ultimately poses to Hibiscus is unknown. Observations by the senior author at many of the Hibiscus stations in 2002 would suggest the prognosis is not good.

Due to the showy appearance of this plant, private landowners are aware of its presence on their property but are generally unaware of its significance.

1) Habitat Degradation

As mentioned previously, Hibiscus appears to do best under a management regime of controlled water levels with partial yearly drawdowns (Farney and Bookhout, 1982). Eleven of the Ontario populations occur in dyked marshes, the best examples being those at sites # 8 and 39. This artificially maintained regime serves to greatly reduce the competition from not only encroaching shrubs and trees, but also from Phragmites. In pre-settlement southern Ontario, fires and fluctuating water levels would have in large part served to control the succession of the open meadow marshes. Today there are few examples of prescribed burning in our coastal marshes, the annual management of the Walpole marshes being one of the exceptions. The majority of the Hibiscus stations are thus subjected to a gradual decline in quality of habitat, the result of continuing nutrient loading, successional change, and general lack of natural disturbance. The resulting proliferation of Phragmites and Typha X glauca is a symptom of this degradation, as these species exploit the compromised environment.

During the 2002 field surveys for the update, habitat degradation was noted at several sites. The station at site # 16 on the south shore of Lake St. Clair, has only ever been documented as consisting of a couple of plants, but it really has nowhere to expand to. About 50% of the habitat is occupied by wooden docks, and mowed lawns are maintained right up to the edge of the two extant plants. The Hibiscus actually grows in fill deposited along the edge of a creek. The shore and mouth of Pike Creek are intensely developed with the high-end homes of Pilot’s Cove Estates on the west tip. Concrete and steel retaining walls, a marina, and manicured lawns and gardens, preclude any coastal wetland habitat greater than the tiny remnant occupied by the two plants of Hibiscus. Proximal to the population, Phragmites dominates along the banks of the backwater stretch of the creek. Impacts from adjacent homes directly on Hibiscus stands were noted at sites # 17 and 27, where lawn waste and brush were observed as being dumped onto the Hibiscus plants. Several populations are in very close proximity to agricultural operations. The two largest populations, sites # 8 and 39, both face the lake, but are surrounded on the other three sides by cultivated land. At site #12, for the most part, the Hibiscus plants are in remnant wetlands immediately downslope from intensively farmed soybean fields, and are thus prone to sediment and nutrient loading, and pesticide and herbicide drift. Several stations are subjected to roadside maintenance, such as at site # 20, where a 3 metre swath was observed as being freshly cut into the Hibiscus stand by the municipal roads crew. Similar maintenance would occur along the railway verges, and it is difficult to say what the net impacts are, since such cutting does at least serve to retain the open character of the habitat. With the receding Great Lakes water levels over the past few years, Cottonwood (Populus deltoides) was observed as succeeding into several of the coastal stations, as at site # 23, where it is actually invading a solid sea of Phragmites.

2) Shoreline Development

A few of the historical Hibiscus stations are believed to have been extirpated as a result of the development of coastal wetland for industrial or recreational use, e.g. Sarnia, Niagara-on-the-Lake, Queenston, and City of Welland. Ford indicated the continuation of such problems in the 1985 report, citing examples of marina development on the Canard River close to the existing population of H. moscheutos; wetland loss due to housing development at site # 7; and the longstanding reclamation of wetland which continued into the 1980s at site # 38, which threatened to eliminate the population of H. moscheutos at that site. This outright loss of wetland habitat is probably not as pronounced today as it was 20 years ago, at least in part due to improved legislative protection for wetlands via the Provincial Policy Statement and perhaps an enlightened attitude toward wetlands by owners, and of the benefits they accrue.

Hibiscus populations are still impacted by shoreline development, as observed by the senior author in 2002. For example, the south shore of Lake St. Clair is undergoing rapid development of subdivisions. At site # 17 sub-populations have been lost in the urbanizing landscape, and the shoreline through this area is now entirely developed with estate homes, manicured lawns, docks, and retaining walls. The only habitat exists along the railway rights-of-way. Other Hibiscus stations are being impacted by adjacent development of housing units. The population at site # 25 was lost when the railway verges were ‘improved’ as part of the general upgrade of that site during the development of the abutting subdivision. At site # 15, a sewer line constructed in a berm, dissects the coastal wetland, and compromises the hydrology. A 149-lot single family subdivision is planned, which will surround the wetland, and could further imapct this already degraded habitat. Similarly, several lots immediately adjacent to site # 7 are approved for development, where presently the Hibiscus is only subject to lawn waste being dumped at the edge of mowed lawns of vacant lots. Shoreline development has also been on the rise along the Detroit River, and this results in virtually a continuous steel retaining wall of a couple of metres in height, with no transitional edge for Hibiscus. Phragmites tends to occupy what little habitat remains along the developed shore of the river.

The only case of direct impact from agricultural operations observed in 2002 was at site # 20 where several clumps of Hibiscus were noted as growing in the open, moist edge of a soybean field.

3) Competition from Invasive Species

The most significant change with respect to invasive species in the fifteen year span since the original status report has been with the Common Reed (Phragmites australis). This species has undergone a dramatic increase in wetlands in Essex County in particular (Pratt, pers. comm., 2003), at Long Point (Wilcox and Petrie, undated c), and throughout the lower Great Lakes (Haggeman, pers. comm. 1999, in Wilcox and Petrie, undated b).  A number of Hibiscus stations are dominated by Phragmites, including those on the Canard River, the Detroit River, the south shore of Lake St. Clair, and the Big Creek (Amherstburg) system. Some stations are seemingly bordering on exclusivity by this grass, for example at sites # 21 and 23. From the almost total dominance of Phragmites observed by the senior author at numerous stations in 2002, it is believed the species has probably resulted in the extirpation of populations or subpopulations at the following stations: Canard River, Fox Creek, Fighting Island, LaSalle Marina, Belle River, and the mouth of the Thames River.

In a study at Long Point NWA (Wilcox and Petrie, undated c), researchers have detected a moderate increase in Phragmites cover between 1985 (4ha) and 1995 (18ha), and an exponential increase between 1995 and 1999, when it expanded to 142 hectares, or 1% of the entire study area. Perhaps most critical to the survival of Hibiscus was the finding that the primary communities that were replaced by Phragmites were meadow marsh (33%) and Typha marsh (32%). The study concluded that Phragmites abundance is negatively correlated with Lake Erie water depth, and positively correlated with ambient temperature, causing the researchers to suggest that, if global warming predictions are realized, Phragmites will continue to expand on the lower Great Lakes.

Phragmites australis is reported to be the most widely distributed flowering plant in the world (Tucker, 1990, in Rice et al. 2000). It has been so successful in its 30-year spread along the Atlantic Coast of the United States that it is now considered a noxious weed (Rice et al., 2000; Tucker, 1990; Chambers et al., 1999; and in Wilcox and Petrie, undated c) and there is wide agreement that it should be controlled (Phragmites Bio-control Workshop, 1999; in Wilcox and Petrie, undated c). Of 22 US National Wildlife Refuges surveyed, 18 attribute a great deal of time and resources annually to controlling Phragmites (Phragmites Bio-control Workshop, 1999; in Wilcox and Petrie, undated c).

One of the characteristics of Phragmites likely impacting Hibiscus, is that its growth pattern produces homogenous clones with up to 200 culms/m² (Tewksbury et al., 2002). Its dead canes remain standing for 3 to 4 years before becoming part of the slowly decomposing litter layer. The sheer biomass of these dead canes may exceed that of living shoots, resulting in thick mats which even new Phragmites shoots can not penetrate (Haslam, 1971; in Wilcox and Petrie, undated a). Another, as alluded to above, is that invasions typically follow disturbances or stresses such as an altered hydrologic regime, dredging, increased nutrient availability, or development (Wilcox and Petrie, undated a, Rice et al., 2000)). Such competitive advantages have seen the replacement of diverse wetland vegetation by Phragmites monocultures and decreases in plant diversity and alterations in nutrient cycling and hydrologic regimes (Marks et al., 1994; and Chambers, 1997, in Tewksbury et al., 2002).

The authors are unaware of any sites in Ontario where effective control has been implemented for Phragmites, and none of the Hibiscus stations are being monitored for impacts from Phragmites. Biological controls are not yet available in North America (Wilcox and Petrie, undated b).

The second-most problematic invasive species impacting Hibiscus is Hybrid Cattail (Typha X glauca). It is dominant or co-dominant (invariably with Phragmites) at a number of the Hibiscus stations, for example throughout the Big Creek (Amherstburg) wetland, Point Pelee, and the wetlands around Lake St. Clair. At site # 37 Haggeman (pers. comm. 2002) has observed that the hybrid cattail has been effectively out-competing the Hibiscus over the last few years. Interestingly, Typha X glauca is quite common in the marshes at both site # 8 and site # 39 where Phragmites appears to be excluded. Phragmites occurs immediately outside the dyke walls at site # 39 in the coastal marsh open to the lake. As with the Phragmites, the authors are unaware of any proven control sites for Typha X glauca in Ontario, and we are not aware of any monitoring of the effects of Typha X glauca on populations of Hibiscus.

Although purple loosestrife (Lythrum salicaria) is certainly present at a number of the Hibiscus stations, the threat which it posed to Hibiscus through the 1980s, as noted by Ford in the original status report, has been downgraded. Whether this is due to a natural levelling off of its own exponential rise, or to biological controls, is unknown. At site # 8, it was noted by Allen in 2002 that a large stand of Lythrum in the northwest corner had died out, and Impatiens capensis was moving into the standing dead colony.

Several other invasive species were noted within Hibiscus populations by Allen during 2002 field surveys, including flowering rush, Butomus umbellatus, (sites # 4, 8, 27); common thistle, Cirsium vulgare, (sites # 15, 27, 19); Scot’s pine, Pinus sylvestris, (site # 25 Matchette Rd. sub-pop); teasel, Dipsacus fullonum, (site # 19), and black alder (Alnus glutinosa). The potential negative effects of these non-natives on Hibiscus moscheutos is unknown.

One other species undergoing a dramatic rise in its populations, this one indigenous, should be noted. Double-crested Cormorants now occur in extremely high numbers in southern Ontario, where they can overlap directly with Hibiscus, probably resulting in negative effects on populations. Hibiscus populations potentially affected are those in the western basin of Lake Erie: sites # 26, 29, 30, 32, 33, and 31. Again, no studies to monitor such impacts are known.