How long is long-term? Comparing decadal- and centennial-scale insect diversity dynamics

Anthropogenic changes to the environment are leading to a decrease in global biodiversity, with the aquatic insect fauna being particularly affected. Information on insect loss trends is mainly based on decadal-scale monitoring data, which does not allow natural baselines to be established, or for the impacts of multiple anthropogenic drivers to be separated. Here, we present decadal- and centennial-scale datasets of chironomid diversity change and quantitatively compare changes in chironomid diversity resulting from climate change and anthropogenic impacts. Our decadal-scale data show changes in community composition for a set of lakes near Bergen (Norway) between 1996 and 2019, but trends in diversity change were not unidirectional across the twenty study sites. We additionally analysed temporal trends in chironomid communities in sediment records obtained from two of the study sites in order to provide a centennial-scale perspective to the observed recent changes. The sediment records date back to ca. 800 CE, and on these longer timescales the effects of climate on insect diversity are clearly identifiable and quantifiable. By using a palaeoecological approach, we show that the decadal-scale data did not capture the full extent of human-induced insect dynamics in this region. We conclude that palaeoecological records contribute unique insights on baseline conditions, effects of individual drivers and long-term diversity trends. Such information is key for effective ecosystem restoration and insect conservation.