Does environmental adaptation or dispersal history explain the geographical distribution of Ixodes ricinus and Ixodes persulcatus ticks in Finland?

Abstract

In Finland, the distribution area of the taiga tick, Ixodes persulcatus (Schulze, 1930), is nested within a broader area of distribution of a congeneric species, the sheep tick, Ixodes ricinus (Linnaeus, 1758) (Acari: Ixodidae). We assess whether distinct environmental adaptations or dispersal history provides a more parsimonious explanation for the differences in the distributions of the two common and medically important ixodids in Finland. We used an innovative spatially constrained randomization procedure to analyze whether crowdsourced occurrence data points of the two tick species had statistically different associations with any of the 28 environmental variables. Using points of presence in a region of species co‐occurrence, we built Maxent models to examine whether environmental factors or dispersal history could explain the absence of I. persulcatus in a part of the range of I. ricinus in Finland. Five environmental variables—number of inhabitants, road length, elevation above sea level, proportion of barren bedrock and boulders, and proportion of unsorted glacial deposits—were significant at p ≤ .05, indicating greater between‐species difference in original than in the randomized data. Of these variables, only the optimum value for unsorted glacial deposits was higher for I. persulcatus than for I. ricinus . Maxent models also predicted high relative habitat suitability (suitability >80%) for I. persulcatus south of its current, sharply bounded distribution range, suggesting that the species has not fulfilled its distribution potential in Finland. The two most common and medically relevant ixodids in Finland may colonize habitats with different environmental conditions. On the contrary, the recent establishment and ongoing dispersion of I. persulcatus in Fennoscandia rather than environmental conditions cause the southernmost distribution limit of the species in Finland.

Crowdsourced tick observation data from across 60 years reveals major increases and northwards shifts in tick contact areas in Finland

“…The expansion of the geographical distribution of I. persulcatus appears to still be ongoing, as highlighted by the first reports of the species from the coast of Ostrobothnia in Sweden in 2015, close to the border of Finland 24 . Based on studies regarding habitat suitability 28 , 29 and observations of I. persulcatus further south in Estonia and Latvia 30 , 31 , there are likely no abiotic factors limiting their further spread south in Finland or along the Bothnian Bay in Sweden.…”

For the people by the people: Citizen science web interface for real‐time monitoring of tick risk areas in Finland

“…Due to the high correlation between population density and observations, assessing tick abundance based on such data is difficult—and indeed often not even attempted. The relationship between observers and observations is further complicated by several factors, such as spatial structures in the data that need to be preserved in spatially explicit analysis (Kulha et al., 2022), and different patterns of human movement, activity, and willingness to participate in online crowdsourcing (Brown & Kyttä, 2014).…”

“…These observations conform to previously made studies regarding the impacts of climate change on tick populations and TBDs in northern Europe, which have predicted that new climatic patterns will be beneficial for Ixodes ricinus (Linnaeus, 1758), the main vector for Lyme borreliosis and TBE in Europe, leading to range expansion and increases in densities (Jaenson et al, 2012;Jaenson & Lindgren, 2011;Laaksonen et al, 2017). The same is likely true regarding another tick species of medical interest, Ixodes persulcatus (Schulze, 1930), which appears to be continuing their range expansion in Finland (Kulha et al, 2022).…”

“…Consequently, Finland is one of the few countries in Europe with considerable populations of both these medically important tick species (Capligina et al, 2020;Katargina et al, 2015). While both species transmit the causative agents of TBE and Lyme borreliosis, some differences in their behaviour, phenology and habitat preferences have been observed (Kulha et al, 2022). For example, the activity period of I. persulcatus appears to be mostly limited to April-June in Finland, quickly diminishing thereafter (Laaksonen et al, 2017;Pakanen et al, 2020; hope that the success of Punkkilive serves to highlight the usefulness of citizen science in the prevention of vector-borne diseases.…”

Genomic signatures of hybridization between Ixodes ricinus and Ixodes persulcatus in natural populations

“…Multiple pathogens in an individual tick can result in the transmission of multiple pathogens to humans or other animals after just one bite. In Finland, the castor bean tick Ixodes ricinus (Linnaeus, 1758) (Figure 1a ) and taiga tick Ixodes persulcatus (Schulze,1930) (Figure 1b ) are the two main vectors for tick‐borne pathogens and their geographical distributions are well known (Kulha et al., 2022 ; Laaksonen et al., 2018 ; Sormunen et al., 2020 ). Overlapping geographical ranges of these tick species create sympatric zones, which might lead to increased sharing of pathogens or pathogen strains between the tick species (when feeding on the same hosts), increased pathogen loads within each tick species (when ticks are exposed to more hosts carrying pathogens) and to higher genetic diversity of the pathogens due to increased population sizes (when there are more ticks and host species to infest) (Ogden et al., 2013 ).…”