Testing for seasonality in alpine streams: How does altitude affect freshwater insect life cycles?

Abstract Altitudinal offset in adult emergence has been well characterised for terrestrial insects, but remains poorly understood for freshwater insect assemblages. Stream insects, like their terrestrial counterparts, are expected to exhibit phenology gradients, with delayed emergence associated with increasing altitude (Hopkins' Law). We analysed the collection records of the entire endemic stonefly fauna from New Zealand's mainland (99 species, 19 genera, four families) to test for intraspecific and interspecific correlations between altitude and adult emergence. Intraspecific analyses of widespread New Zealand plecopteran taxa indicate that high‐altitude populations emerge substantially later than those at lower elevations, with 25 of 31 widespread stonefly taxa yielding significant positive associations between altitude and emergence date. Similar patterns were detected for interspecific comparisons, even when phylogenetic relationships were considered, with high‐altitude taxa emerging substantially later than lowland counterparts. In a number of species, delays in emergence (mean: 5.3 ± 2.0 days/100 m) exceeded those previously reported for terrestrial insect taxa and also exceeded values originally predicted under Hopkins' Law. These findings suggest that stream‐insect life cycles have potential to be highly sensitive to temperature shifts and thus raise questions about their response to anthropogenic climate impacts.