Abstract1 Theory suggests that any given rise in temperature resulting from climate change will have its greatest effect on high Arctic ecosystems where growing seasons are short and temperatures low.2 A small temperature rise, similar to that predicted for the middle of the next century, has profound effects on a population of the high Arctic, Dryas‐feeding aphidAcyrthosiphon svalbardicumon Spitsbergen (Strathdeeet al.1993a).3 Here comparative experiments on a closely related Dryas‐feeding species,A. brevicorne, at two contrasting sub‐Arctic sites are described. Together with the results from Spitsbergen these sites represent two colder sites (high Arctic and upland sub‐Arctic) and one warmer site (lowland sub‐Arctic).4 Differential responses in aphid population density and overwintering egg production to temperature elevation support the hypothesis that the ecological effects are greatest at sites with the most severe climates; however, there is no similar gradient in advancement of host plant pheno
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