Abstract
Life histories, annual production, and genetic structure for two insect species were examined for two adjacent streams that differ in flow regime. Birch Creek had a mesic groundwater (seasonally constant) flow regime; Little Lost River had a snowmelt (seasonally cyclic) flow regime. Baetis tricaudatus was multivoltine, whereas Hesperoperla pacifica had a life cycle longer than one year. The life histories of B. tricaudatus were similar between streams, but mean biomass was greater in Birch Creek. Gene frequencies of B. tricaudatus also were similar between streams, Nei's genetic distance being only 0.011. Expected heterozygosities and population polymorphisms were higher in the Little Lost River. In contrast, the life histories of H. pacifica were different between streams, emergence being later in the Little Lost River. This difference could be explained by temperature. However, the genetic structure of H. pacifica showed the same pattern as B. tricaudatus with more polymorphisms and higher heterozygosities occurring in the Little Lost River. Nei's genetic distance between the H. pacifica populations was 0.054. These data support the idea that populations exhibit greater genetic variability in more variable environments than populations in relatively more constant environments.
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