Hill's pondweed (Potamogeton hillii) COSEWIC assessment and status report: chapter 7

Population Sizes and Trends

According to the Ontario Natural Heritage Information Centre (NHIC), Hill’s pondweed has been reported in 24 sites (Appendix 1), including nine new sites since the last status report (Brownell, 1986). It is believed to be extirpated from two sites: Little Eagle Harbour (site 22) on the Bruce Peninsula, and the St. Thomas site (site 03) in Elgin County. A field visit to Little Eagle Harbour confirmed the lack of suitable habitat in the area.

An erroneous report had come from Fish Lake in Lennox and Addington Counties. It was identified as Potamogeton hillii by M. Bristow in 1979, and was corrected to P. pusillus var. pusillus by Brownell and Catling in 1982 (Brownell, 1986).

Field visits were made to twenty of the sites reported in the NHIC database over a period of five field days from August 18th to 23rd, 2003. Sites were searched for a half hour to one hour depending on the size of the marsh or creek. A pondweed species that appeared to be Potamogeton hillii was found at 12 sites. Samples were collected in all cases, but eight did not bear fruit, which reduces the certainty of identification. Samples were examined by Dr. Paul Catling, Agriculture and Agri-Food Canada (DAO), Ottawa, and the fruit-bearing samples were deposited at the Agriculture and Agri-Food Canada vascular plant herbarium in Ottawa (official acronym, DAO).

Like many aquatic plants, Hill’s pondweed grows in thick patches of intertwined individuals, making it exceedingly difficult to distinguish the number of individuals without destructive sampling. As indicated on the web site of NatureServe (2005) for this species,

“Populations may be difficult to monitor because of their aquatic habit, particularly if populations are large. Individual clones can cover surface areas of 0.5 meters in diameter and if such clones are numerous, distinguishing between individual clones may be difficult or impossible.”

Population numbers provided are, therefore, only rough estimates of the number of individuals, especially since only one in situ sample was possible at a small 10 x 10 cm patch, where 20 distinct individuals could be counted (site 06). Extrapolating this number yields 2000 individuals per 1 patch. At all other sites where the species was found, population size was calculated by estimating the area in m². Population estimates are crude figures, however, since there could be as much as a 30-50% plus or minus variation in density. Uncertainty in numbers also exists since this species also grows in association with other similar pondweeds that would be difficult to distinguish unless every rooted specimen were in fruit.

Given that fruit-bearing plants were not found at some of the sites, numbers are given separately for confirmed populations, where identification is certain, and suspected populations, where identification is uncertain due to a lack of fruit. There are an estimated 55,000 individuals of Hill’s pondweed at four sites with confirmed identification (based on collections by Makkay) occupying an area of 17.7 ha, and 64,600 suspected individuals occupying an area of 2 ha. The total for all sites is 119,600 individuals occupying about 19.7 ha of marsh, stream and pond. For site-specific numbers, see Appendix 1.  Supplementary information on site 04 became available subsequent to the completion of this report, based on a Ministry of Natural Resources, Ontario, staff visit on 2 October 2003; numerous fruiting plants were observed in the eastern portion of the lake but no specific attempt to document numbers was made. On the basis of this additional report, there are, therefore, 5 verified sites known as of the year 2003. The additional area at site 04 where Ministry staff had observed fruiting specimens would add an additional 7.5 ha to give a revised total area of occupancy of about 27 ha.

Out of the eight sites where the species was not found, at three sites (15, 20, 21) there was some uncertainty that the location searched was the location where the species was previously reported. At the other five sites (01, 05, 08, 12, 13), previous reports were all at least 20 years old. Reasons for decline or extirpation may include competition by other species, particularly Elodea canadensis, or loss or change in habitat. Since little is known about the biology of this species, factors that could impact the population are unknown.

General long-term population trends for Hill’s pondweed are difficult to determine. The species has long been overlooked, as is the case for many aquatic plants, with most activity documenting its occurrence having taken place over the last several decades. Other than the Macoun collection of 1901, the next earliest record was from Elgin County in 1951 (site 03). Twenty-one locations have been discovered since 1974; few of these have been revisited regularly.

C.B. Hellquist undertook a systematic search for Hill’s Pondweed in 1983 (specimens cited in Brownell, 1986), and found ten sites on the Bruce Peninsula and Manitoulin Island (01, 02, 04, 05, 07, 09, 10, 11, 12, 13). These sites were all visited in 2003. Hill’s pondweed was confirmed at one site (10), and sterile samples collected from four more sites (02, 04, 07, 09). All sites except one had suitable habitat; the exception appeared to have been degraded by cattle (12).

Given the results of the field search, there is an evident decline in Hill’s pondweed, in spite of little change in habitat. Aquatic species, however, tend to be dynamic in their distribution (unpublished data based on a five-year project monitoring permanent plots of aquatics in the Rideau River, Ottawa, by the writer and Dr. Lynn Gillespie, Canadian Museum of Nature). It is not certain if Hill’s pondweed is inclined towards population fluctuations. Regular long-term monitoring would be needed to determine this.

Other sites that appeared to be suitable Hill’s pondweed habitat were briefly searched, particularly along the Niagara Escarpment. Two collections were made of a species of Potamogeton that resembled Potamogeton hillii, but neither had fruit so identification could not be confirmed. These were from the Styx River east of Highway 6, and the Mallet River north of Highway 9.

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