Wood-poppy (Stylophorum diphyllum) COSEWIC assessment and status report: chapter 7
Population Sizes and Trends
Search effort
Historical records suggest that Stylophorum diphyllum was very rare in Ontario, even before the habitat destruction that followed European settlement. Nearly a century elapsed between the sparse earliest records and more recent discoveries. The striking appearance of the species in early spring makes it unlikely that it has been seriously overlooked. Sites within the known historic range are quite well botanized.
Since 1987, the senior author has made searches in the known historic range of Stylophorum diphyllum, with additional searches by Brinker, Oldham and Stephenson. Early (1947) air photographs were used to concentrate areas of search by identifying sites within the known historic range that had mature trees in 1947, and may have had continuous tree cover. Candidate sites were walked during the flowering season. Search time accumulates to more than 70 person-hours.
The only known populations of Stylophorum diphyllum in Canada are at least 15 km apart and some 200 km from the nearest wild populations of the species in the United States. Each Canadian population consists of a few to several hundred closely spaced plants with a few outlying clumps and individuals. The London and Ilderton populations are spread over an area of 100-150 m² with outliers up to 40 m and 150 m away respectively. The Fanshawe population is much smaller. Until 2005 it consisted of between 5-8 mature plants in an area of less than 1 m². During 2005 there was some recruitment and in 2006 all of the original plants had died, but 24 new plants were censused in an area of about 19.5 m².
Abundance
Descriptions of population sizes at the three known sites in 2006 are given below. All three sites have experienced an increase in population numbers in the last 2-3 years, but this may represent short-term natural fluctuations in response to weather or other factors rather than a trend of increasing population.
London
This site, discovered in 1987, is now in the City of London. The original population size was estimated to consist of about 740 individuals in the main patch, approximately 10 x 15 m in size, with a few smaller stands and isolated individuals, bringing the total to about 800 plants (Bowles, 1997). The report of 5000 individuals (Gosnell and Stephenson, 1996) is considered erroneous because of the way that individuals were recognized and density counts were multiplied up over the stand area.
In 1993 logging and filling occurred at the site and the main population was buried. Gosnell and Stephenson (1996) considered the population to be reduced to about 160 remaining plants in 6 stands, plus some additional recruitment.
In 1997 a more thorough mapping and population census revealed approximately 235 mature plants and about 50 recruits. Problems confounding counting individual plants at this site include a clumped distribution of multi-stemmed individuals and very steep slopes.
The population was monitored through 1997-2005, although another detailed census was not done. Populations in individual stands remained more or less stable at about 250 plants, with some recruitment and some turnover. In 2006 landowner permission to access the site for a more accurate census was denied.
Fanshawe
This site was rediscovered in 1994 when it consisted of a small patch about 1.5 m across containing about 8 individuals. Several of these plants died, apparently from crown rot, over the next 10 years. No recruitment was seen until 2005. In 2006 none of the original plants remained, but 24 additional recruits were mapped over an area of about 5 x 6 m.
Ilderton
This site was rediscovered in 1996, when it consisted of approximately 140 individuals in the main patch, with an additional 20 outlying individuals. When the site was surveyed in 2006 the main stand consisted of 180 plants, two additional loose patches of about 35 and 28 individuals respectively, and a number of outlying plants totalling about 255 individuals. An additional survey in May 2006 located two additional stands, of 5 and 12 individuals respectively, separated by about 150 m from the main site. Thus the population appears to have increased slightly over this time.
Rescue effect
Recruitment of plants from the next nearest population in Michigan and Ohio is unlikely. The distances are large, and the intervening territory includes extensive areas of unsuitable habitat including agricultural and urban land as well as significant stretches of water (Lake Erie, Lake St. Clair and the St. Clair and Detroit Rivers). The species appears to be locally dispersed by ants, and no long-distance dispersers are known. The continued existence of Stylophorum diphyllum in Canada depends, therefore, on the maintenance of the three known populations.
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