Effects of a mega wildfire on small mammals and ticks in a boreal forest in Sweden.

Ticks transmit pathogens among wildlife and humans, with small mammals acting as key reservoirs. Climate change may increase disease risk through expanding tick distributions and more frequent wildfires, which alter habitat conditions, influencing small mammal communities and tick populations. In 2014, the Sala Fire burned 131 km² of boreal forest in south-central Sweden. Fire intensity, winter conditions, and proximity to water may influence survival of small mammals and ticks. This study investigated the effects of a mega-fire on the dynamics of tick and small mammal populations, focusing on host-vector relationships. Eight years post-fire, we surveyed the Sala Fire Area (SFA) and used the nearby Grimsö Wildlife Research Area (GRWA) as a temporal reference area. In total 2023 small mammals were captured, finding 2366 ticks (Ixodes ricinus, Ixodes trianguliceps). Tick occurrence was defined as the probability that a small mammal carried at least one tick. In spring, small mammal abundance was highest in moderate fire severity areas, while in autumn moderate and high severity areas showed higher abundance. Abundance decreased with increasing distance to water in both study areas and with increasing snow cover the previous winter in the SFA. Tick occurrence was highest in high fire severity areas. In spring, tick occurrence increased with distance to water in both areas. Within the SFA, deeper previous-winter snow reduced probability of a small mammal carrying ticks. These results indicate that wildfire influences host-tick dynamics primarily through variation in fire severity and environmental conditions rather than causing persistent reductions in small mammal abundance.