. The Canadian field-naturalist. 1999 Notes 297 800 700 600 o 500 - 400 300 200 100. GOCOCdOOGOOOOOCOQOO}03d>d> Year Figure 1. Cirsium pitcheri populations, 1981-1997, Pukaskwa National Park, Ontario. Descriptive statistics such as mean and standard deviation have been applied to the results to reveal trends in the data over the 17-year period. Results Over 17 years the data indicates a mean total pop- ulation of plants along the two beaches within Pukaskwa National Park (Figure 1). There is a stan- dard deviation of There was a high of 760 individuals in 1985 and a low of 1
. The Canadian field-naturalist. 1999 Notes 297 800 700 600 o 500 - 400 300 200 100. GOCOCdOOGOOOOOCOQOO}03d>d> Year Figure 1. Cirsium pitcheri populations, 1981-1997, Pukaskwa National Park, Ontario. Descriptive statistics such as mean and standard deviation have been applied to the results to reveal trends in the data over the 17-year period. Results Over 17 years the data indicates a mean total pop- ulation of plants along the two beaches within Pukaskwa National Park (Figure 1). There is a stan- dard deviation of There was a high of 760 individuals in 1985 and a low of 153 in 1991. In 1986, a beaver dam upstream from the thistle colony burst releasing a substantial volume of water (Sahanatien 1985*). This forced the re-routing of the creek near its mouth clearing out 63% of the thistle population. Between 1986-1991, the population remained low (< 300 total population) until 1991 with a population of 153 individuals. Since 1991 the population has rebounded to a high of 497 in 1996. Discussion The C. pitcheri population appears to have sur- vived the catastrophic impact to its habitat in 1986. As suggested by Mosquin (1990*), periodic distur- bance may actually assist the survival of the plant by reducing the encroachment of later successional species. Since 1986, however, the re-routing of the creek into Oiseau Bay has reduced the annual level of dis- turbance at the site (Mosquin 1990*). The population near Creek Beach (the larger of the two concentra- tions), is no longer influenced by deposition or peri- odic water fluctuations. With this lack of distur- bance, Mosquin suggested that the population would have difficulty competing with other encroaching vegetation (Mosquin 1990*). However, the total plant population has actually increased since 1991, initiated by a higher number of seedlings in 1992. Reside (1992*) suggests that the cool weather and moist conditions that were prevalent during the sum- mer months of 1992 may have contribute
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