Figure 6.

Northward expansion of the zones of suitable climate, based on parasite species specific accumulated degree days, from 1980 through to 2017. (a–f) Maps are showing the geographic extent of the thermal niches (represented by red shaded areas, from above 1 to darker red for values above 3 units of PTI) of the northern range of UP and VE during three six-year periods 1980–1985, 1996–2001, and 2012–2017. Darker shades of red indicate a greater number of accumulated degree days. The maps are produced by modelling degree-day accumulation for the two parasites using the 2 m air temperature (3 hourly) North American Regional Reanalysis (NARR) dataset by the National Centers for Environmental Prediction (NCEP)³³. The average DD accumulated in six-year intervals, ten years apart, are displayed. Squares represent the known presence or absence of each parasite during the respective time frames. In maps a and b, no occurrence points are available as no surveys were done during that period. However, neither UP nor VE were detected on Victoria Island until 2008 and 2010, respectively, and neither have been found on any other arctic island despite targeted sampling since the late 1990s, thus they are both presumed to have been absent from the arctic archipelago in the 1980s. Occurrence points on maps c and d were obtained from published literature and our data. The maps were generated in ArcGIS software version 10.6 (ESRI 2011, ArcGIS Desktop: Release 10. Redlands, CA, Environmental Systems Research Institute, https://www.esri.com/en-us/home).