Altitudinal zonation of Plecoptera in a Rocky Mountain Stream1

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Abstract
Thirty‐three species of Plecoptera in 8 families were collected from rubble riffles in a Colorado stream, from alpine tundra to the plains. Maximum diversity (19 species) occurred in middle reaches. Only two species occupied the alpine tundra stream (3414 m a.s.1.); four were collected at the plains location (1544 m a.s.1.). Mean density was lowest at the plains site (54 nymphs per m2) with maxima in the montane (527 per m2) and foothills (448 per m2) limnological zones. Although some species were restricted to middle (e.g., Paraleuctra occidentalis, Cultus pilatus) or lower (e.g., Claassenia sabulosa, Skwala parallela) reaches, none were restricted to the plains or headwater sites. The two stoneflies occurring in the alpine tundra (Zapada oregonensis and Megarcys signala) were among the most euryzonal species. The few stoneflies collected at the plains location primarily represent marginal populations of foothills species near the lower limits of their altitudinal distributions. The observed altitudinal zonation patterns are attributed largely to variations in the thermal regime and trophic conditions.