Extreme climatic events alter the aquatic insect community in a pristine German stream

Abstract

As a result of ongoing climate change, extreme climatic events (ECEs) are expected to become more frequent and severe. The high biodiversity of riverine ecosystems is susceptible to ECEs, especially to water temperature (extreme heat and extreme cold) and discharge-related (flood and drought) events. Long time series are needed to unravel the effects of ECEs on ecological communities. Here, we used 20 years (1986–2005) of unusually high-resolution data from a pristine first-order stream in Germany. Daily recordings of species-level identified aquatic insect (Ephemeroptera, Plecoptera, Trichoptera: EPT) emergence, water temperature and discharge data were used to examine the effects of four types of ECEs (extreme heat, extreme cold, flood, and drought events) on insect abundance, common taxonomic diversity metrics, and selected traits after five different time lags (2 weeks, 1, 3, 6, and 12 months). Extreme heat events increased from 1.8 ± 1.9 SE events per year before 2000 to 5.3 ± 1.9 SE events per year after 2000. Water temperature-related ECEs restructured the EPT community in abundance, species richness, and traits (community temperature index: CTI, and dispersal capacity metric: DCM). The strongest effects on the EPT community were found when it was exposed to multiple ECEs and 1 and 3 months after an ECE. The changing frequencies and durations of ECEs, especially the increasing frequency of extreme heat events and the negative cumulative effects of ECEs, paint a worrisome picture for the future of EPT communities in headwater streams. High-resolution, long-term data across sites is needed to further disentangle the effects of different ECE stressors.