Current and Future Habitat Suitability Models for Four Ticks of Medical Concern in Illinois, USA

The greater U.S. Midwest is on the leading edge of tick and tick-borne disease (TBD) expansion, and tick and TBD encroachment into Illinois is occurring from both the northern and the southern regions. To assess historical and future habitat suitability of four ticks of medical concern within the state, we fit individual and mean-weighted ensemble species distribution models for Ixodes scapularis, Amblyomma americanum, Dermacentor variabilis, and a newly invading species, Amblyomma maculatum using a variety of landscape and mean climate variables for the periods of 1970-2000, 2041-2060, and 2061-2080. Ensemble models for the historical climate were consistent with known distributions of each species but predicted the habitat suitability of A. maculatum to be much greater throughout Illinois than what known distributions demonstrate. Proximity to wetlands and water bodies was important in predicting both I. scapularis and A. americanum presence. A. americanum occurrence was highly dependent on increasing forest cover, while A. maculatum habitat was more strongly predicted by open habitats. As the climate warmed, the expected distribution of all species became more strongly impacted by precipitation and temperature variables, particularly mean temperature of the wettest quarter and mean temperature of the driest quarter. By 2070, I. scapularis was expected to retract by as much as 60% from southern and central regions of the state as compared to historical climate distribution but remained concentrated in the Chicago metropolitan area. A. americanum was predicted to initially expand across parts of east- and west-central Illinois by 2050, but then largely retract in distribution to along rivers and water bodies by 2070. The ranges of D. variabilis and A. maculatum, however, were predicted to contract in the 2050 climate scenario, but then expand in the 2070 scenario. Predicting where ticks may invade and concentrate as the climate changes will be important to anticipate, prevent, and treat TBD in Illinois.