. The Canadian field-naturalist. 284 The Canadian Field-Naturalist Vol. 91. Figure 3. A comparison of burnt versus unburnt cactus clones. The unburnt stem on the left was taken from a clone adjacent to the burn and placed next to a burnt and partially eaten clone. which occurred about one month previous. The only edible vegetation remaining was prickly pear cactus. The recent fire had removed most of the spines from the cactus plants (Figure 3) and the pronghorns had been actively pawing the cactus clones and foraging on the fleshy de- spined stems. Of about 30 cactus clumps observed within 1
. The Canadian field-naturalist. 284 The Canadian Field-Naturalist Vol. 91. Figure 3. A comparison of burnt versus unburnt cactus clones. The unburnt stem on the left was taken from a clone adjacent to the burn and placed next to a burnt and partially eaten clone. which occurred about one month previous. The only edible vegetation remaining was prickly pear cactus. The recent fire had removed most of the spines from the cactus plants (Figure 3) and the pronghorns had been actively pawing the cactus clones and foraging on the fleshy de- spined stems. Of about 30 cactus clumps observed within 1 hectare, all had been utilized. Close inspection of one large cactus clone approximately 1 m in diameter, revealed 13 dislodged and 9 intact stems. It was estimated that over 50% of the green stems had been utilized in this burn. Discussion Food habitat studies of pronghorn antelopes in Alberta have revealed a low incidence of cactus in the diet. Mitchell and Smoliak (1971) recorded 1% mean volume of prickly pear cactus in the late fall diet of pronghorns in the Manyberries district, 128 km south of Suffield, and 7% and 12% mean volume of ball cactus (Mamillaria vivipara) in spring and fall diets, respectively, of pronghorns in the Newell area, 32 k west of Suffield. Twenty years of range- pronghorn relationships (data unavailable) in a enclosure near Manyberries stocked with high densities of pronghorns suggested that cactus is more heavily utilized when other food sources are depleted (L. D. Gudmundson, Alberta Fish and Wildlife Division, personal communication). During the fall period, pronghorns tend to concentrate on areas having an abundance of green vegetation. They are commonly seen feeding on winter wheat (Triticum sativum) or alfalfa (Medicago falcata) fields in areas where this type of vegetation is available. The succulent nature of the vegetation promotes its use. Inasmuch as no cropland is available within the Suffield Reserve, the fall use of "spinel
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