Rapid Northward Expansion of the Blacklegged Tick, Ixodes scapularis, in Response to Climate Change.

Climate change rapidly drives species range dynamics, prompting many terrestrial organisms to shift northward to higher latitudes and forcing new species-species and species-environment interactions. The blacklegged tick, Ixodes scapularis, a biological vector of human pathogens including Borrelia burgdorferi (the bacteria causing Lyme disease), is undergoing rapid and persistent expansion into Canada, exposing new human populations to zoonotic diseases. Here, we used an ensembled forecasting approach to construct niche models of I. scapularis' current and future distribution and to identify the environmental drivers of habitat range. Georeferenced occurrence points were acquired from community science programs (eTick and iNaturalist) between 2017 and 2022 in Canada and the United States. We also collected high-resolution environmental data using a spacing of approximately 1 km. We carried out 4704 model iterations across two datasets, 12 algorithms, and 10 climate profiles using 40 environmental variables. We extrapolated select models over three time periods, 2011-2040, 2041-2070, and 2071-2100, across two projected climate scenarios, SSP5-8.5 and SSP3-7.0, incorporating 2094 future outcomes of I. scapularis distribution. Our ensembles (AUC: 0.9565 ± 0.0065; TSS: 0.8435 ± 0.0155; Kappa: 0.819 ± 0.014) identified temperature, precipitation, biomass production (NPP), length of the growing season, climate moisture index, and number of yearly degree days as the variables that best explained I. scapularis distribution. Further changes to these climate conditions will result in continued I. scapularis range expansion, with, at the highest estimate, an increased niche area of ~248% (447,532 km2 to 1,556,760 km2) and, at the lowest estimate, by ~205% (409,475 km2 to 1,247,689 km2) before the turn of the century. These distributional niche changes coincide with a northern latitude limit reaching as far as ~48° N by 2040, ~50° N by 2070, and ~52° N by 2100. These findings highlight the invasive potential of I. scapularis, with implications for public health and changing ecosystem dynamics.