A quantitative niche comparison of the Western Plains Garter Snake (Thamnophis radix haydeni) and the Red-Sided Garter Snake (Thamnophis sirtalis parietalis) in allopatric and sympatric regions of Manitoba's Interlake district

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Hart, Donald Ray
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(Thamnophis radix haydeni) and the red-sided garter snake (Thamnophis sirtalis parietalis) in the Interlake district of Manitoba were compared with respect to feeding habits and six environmental factors. Thamnophis sirtalis occuring in sympatry with T. radix altered its feeding habits, but not in such a way as to produce niche displacement. Its feeding habits did not differ significantly from those of T. radix either in allopatry or in sympatry. Changes in its feeding habits are attributed to changes in the relative availability of food items. Of the environmental factors examined, habitat, air temperature and substrate temperature contributed most strongly to niche discrimination in allopatry, T. radix occurring near meadow ponds at high air but low substrate temperatures and T. sirtalis occurring near fen-like marshes of the aspen forest at lower air but higher substrate temperatures. Thamnophis sirtalis was adapted to lower air temperatures than T. radix by virtue of its greater ability to absorb radiant energy. In sympatry the contributions of habitat and air temperature to niche discrimination were reduced whereas those of light intensity and substrate temperature were increased, T. radix selecting a higher value of both than T. sirtalis. This shift in the importance of environmental factors to niche discrimination need not be attributed to species interaction but may be explained in terms of behavioral compensation for concurrent changes in the available niche. The shift was produced, in part, by changes in the daily activity pattern, towards mid-day for T. radix and away from mid-day for T. sirtalis. The degree of bimodality in the activity pattern was directly proportional to temperature, available temperatures being lower in sympatry for T. radix and higher in sympatry for T. sirtalis.