Daniele Da Re, Gaelle Gilson, Quentin Dalaiden, Hugues Goosse, René Bødker, Lene Jung Kjær, Roberto Rosà, Nicholas Ogden, Sophie Vanwambeke

The tick Ixodes ricinus is the main pathogen vector in Europe. Many speculations have been made about the effect of past climate change on the potential distribution of this ectothermic organism, despite a poor understanding of how climate change has resulted in distribution changes to date.
In this study, we used a public cross-sectional dataset of I. ricinus abundance at the northern edge of its European distribution for 2016-2017 to identify a thermal limit for I. ricinus distributions.
We first modelled the nymphal tick abundance as a function of cumulative annual degree days (DD) > 0°C and biogeographical regions using observations for 2016-2017. We then identified the thermal limit for each biogeographical region as the minimum DD value where the predicted nymph abundance is greater than zero. Hindcasting the identified thermal limit suggested that I. ricinus has expanded its range by approximately 400 km in the Boreal biogeographical region between 1979 and 2020. Despite the lack of long-term data series on tick presence, this finding helps explain numerous observations of I. ricinus in areas presumed to be newly colonised.
While multiple other factors affect tick distribution and abundance at the local scale (e.g., host distribution, microhabitat), our approach appears promising for understanding species distribution changes driven by recent climate change. Accounting for biogeographic regions helped consider other dimensions of habitat at a broad scale. Our results underline the relevance of long-term time series data and the risk associated with short-time series for observing changes in distribution.