. Die kanadische Feld - naturforscher. 232 Die kanadische Field-Naturalist Vol.100 140 130^^ • K'o 100 Sb^ yXjb'o 70 60 Opuntia fragiljs in Kanada k m Kolonie. Abbildung 3. Verteilung der Opuntia fragilis in Kanada, von UWIN, UWPG, MMMN, DAO, WNRE, CAN, UBC, USAS, UVIC, V, ALTA, LEA, UAC, SASK, und SCS. Die sieben Kolonien untersucht mehr als 100 pads Jede enthalten. Kolonie Abmessungen, pad Zahlen und die Anzahl der Klumpen pro Bergspitze waren nicht korreliert (a = 0,05) mit einem der Boden gemessenen Variablen (d. h. Macronutrient Inhalt, organische Inhalte, pH-Wert, Dichte oder Leitfähigkeit). Auf der Grundlage der Kolonie

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. The Canadian field-naturalist. 232 The Canadian Field-Naturalist Vol. 100 140 130 ^^ •K'o 100 sb^yXjb'o 70 60 Opuntia fragiljs in Canada k m colony. Figure 3. Distribution of Opuntia fragilis in Canada, from UWIN, UWPG, MMMN, DAO, WNRE, CAN, UBC, USAS, UVIC, V, ALTA, LEA, UAC, SASK, and SCS. the seven colonies examined contained more than 100 pads each. Colony dimensions, pad numbers, and the number of clumps per outcrop were not correlated (a =0.05) with any of the soil variables measured (i.e. macronutrient content, organic content, pH, bulk density or conductivity). On the basis of colony and pad numbers, we feel that White and Johnson (1980) are justified in excluding this species from the rare and endangered species list for Manitoba. Habitat All colonies occurred on sloping (rarely, horizon- tal) south-facing rock outcrops between 0.9 and 15.0 m above and from 10.5 and 32.0 m away from the shores of rivers and lakes. Exposed rocks are often heated several degrees above the temperature of the surrounding substrate by radiant energy and they have the capacity to retain heat for longer periods of time (Rejmanek 1971). These effects would be increased by the southern exposure of the Prickly- pear sites which would therefore experience early spring snow melt, heat retention during cool spring and autumn nights, and extended growing seasons. Brittle Prickly-pear colonies were absent from rock outcrops away from the water's edge, even though these outcrops appeared similar and were often less than 50 m away from riparian outcrops with colonies. It is unlikely that the species has dispersal limitations which would account for this distribution because the dispersal units (pads) are dispersed by animals as well as by flotation (Frego and Staniforth 1985), and hence have a means of transport away from riparian sites. Two other theories may account for the absence of the species on inland outcrops. Firstly, fires may have eliminated the cacti from non-riparian sites

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