Millipedes step up: species extend their upper elevational limit in the Alps in response to climate warming

Climate warming is predicted to result in changes to phenology, behaviour and abundance as well as poleward shifts or upslope displacements of the distribution of species. However, climate-warming induced changes in distribution patterns have rarely been studied in ground-dwelling arthropods. We investigated changes in the upper elevational limit and relative abundance of 11 millipede species by repeating historical surveys from 1917 to 1919 in five valleys in the Swiss National Park (Eastern Alps) in 2018–2019. We found that the upper elevational limit for 10 out of 11 millipede species has risen, on average, by 161 m in 100 years, accompanying a 1.5 °C rise in mean annual temperature in the investigation area. The 10 millipede species differed in upslope expansion of their distribution with the shift ranging from 50 m to 363 m. The relative abundances of species (percentage of individuals sampled for each of the 11 millipede species) were very similar in 1917–1919 and 2018–2019. Only three species showed a change in frequency >6% over 100 years: two species had reduced abundances by 13.7% and 12.5% in the recent survey, while one species increased in abundance by 12.2%. We provide evidence showing that soil-dwelling millipede species with low dispersal ability have raised their upper distribution limit to higher elevation between the surveys done in 1917–1919 and 2018–2019. This upslope shift in distribution is most probably due to the significant increase in temperature recorded in the investigation area in past decades.