Ixodes ricinus demonstrates a high adaptability to European environments and has a tendency to expand its geographic range towards northern altitudes, which is mostly explained by climate change (Materna et al., 2008; Jaenson and Lindgren, 2011; Martello et al., 2014; Hvidsten et al., 2020; Garcia-Vozmediano et al., 2020). This brings to the attention whether this expansion is likely to give rise to new, distant foci for Lyme and TBE diseases. Environmental parameters play a significant role in influencing the distribution of I. ricinus. Factors such as climatic conditions, habitat characteristics, and landscape heterogeneity have been identified as key determinants affecting the abundance and prevalence of this tick species (Krasnov et al., 2007; Ruiz-Fons et al., 2012; Hauser et al., 2018).
The Microclimate model identified temperature seasonality as the primary environmental predictor, while the Macroclimate model indicated the annual temperature range as the most significant contributor. These findings are consistent with previous studies. Annual temperature range, annual mean temperature, and annual precipitation were identified as the most influential factors by Alkishe et al., (2017). Similarly, temperature seasonality is considered a relevant factor according to Cunze et al., (2022). The second influential factors are vapor pressure and growth season precipitation according to Microclimate and Macroclimate models respectively. The off-host survival of ticks depend strongly on water availability since desiccation is one of the most prominent causes for tick mortality and decreasing questing activity of ticks (Perret et al., 2004; Tagliapietra et al., 2011; Nolzen et al., 2022; Van Gestel et al., 2022). Factors such as higher soil moisture content and increased cloud cover have been linked to increased questing activity of I. ricinus (Medlock et al., 2008; Lauterbach et al., 2013). Additionally, the distribution of I. ricinus has expanded in the past three decades due to more favorable biotic and abiotic conditions, which can be influenced by changes in precipitation and an increase in mean winter temperatures (Gray and Ogden, 2021). Other modeling studies have underscored the importance of precipitation as a significant factor in the distribution of this species in both current and future projections (Alkishe et al., 2017; Cunze et al., 2022). Temperature has been recognized in various studies as a crucial determinant in the completion of the life cycle and duration of host-seeking activity of I. ricinus in northern latitudes (Lindgren et al., 2000; Schwarz et al., 2012; Hauser et al., 2018). However Gst (growth season temperature) had a very small contribution in the final Macroclimate models and Gdd5 (degree days above 5 ⁰C) was not included in the final selected models. Another discrepancy with these previous studies is that minimum temperature of the coldest month (SBio6) contributed less compared to other variables in the Microclimate model and in the final selected Models for Macroclimate, Bio6 was not included due to correlation thresholds. However, when we included Bio6 in the side-models out of curiosity, we saw that Bio6 was behind other parameters with a contribution smaller than 3%. This might be due to the difference in the CHELSA and Soiltemp datasets we used in creating the models instead of Worldclim. Additionally, the relation between minimum temperatures and tick survival might be more complicated due to the buffering effect of microclimatic factors such as snow cover and leaf litter on tick survival and activity (Van Gestel et al., 2022). For example, in situ measurements have shown that humidity and temperature values in the understorey of forests where I. ricinus resides are indeed much more tepid and stable compared to that gathered from standard weather stations (Boehnke et al., 2015). Also, the increased winter mortality effect of removing leaf litter and snow cover on Ixodes scapularis was documented by experimental field plots (Volk et al., 2022).
The models indicate that the region climatically suitable for I. ricinus covers a great portion of Europe except southern areas in Spain and Greece and the coastal regions of the Black Sea in West Asia. This outcome is consistent with the results of previous models on I. ricinus. On the other hand, model predictions showed a more constrained distribution in the Southern regions of Europe especially in southern and western Anatolia, this is an expected result as we know that presently there is no record of I. ricinus in this region (Hekimoğlu, 2022). The model also revealed a small area of suitability on the North African coast, however the suitability of this region is not reliable due to the high uncertainty. Although some of the previous studies included I. ricinus from North Africa, there is high probability that some of these records are I. inopinatus as recent genomic analysis clearly demonstrated the presence of I. inopinatus in North Africa (Younsi et al., 2020; Rollins et al., 2023). Nevertheless, the absence of current and future distribution of I. ricinus in the Mediterranean region can not solely be ascribed to I. inopinatus. Although the distribution of I. inopinatus in different European countries, such as Germany, Spain, and Portugal, has been suggested (Estrada-Peña et al., 2014; Chitimia-Dobler et al., 2018; Hauck et al., 2019), genomic data has shown that German samples are in fact I. ricinus (Rollins et al., 2023). In this case, the taxonomic status of Mediterranean populations should be reevaluated, which could influence the projections of I. ricinus in these areas. Considering the taxonomic reevaluation mentioned above, many recent studies excluded North African populations from their dataset (Cunze et al., 2022; Noll et al., 2023). The findings of these studies largely align with our current and future predictions. A large part of Europe constitutes the distribution area of the species, with the most suitable areas located in Western Europe. While Mediterranean countries such as France and Italy include suitable areas for the species' distribution, Portugal and the northern part of Spain also appears suitable in the west Mediterranean. Along the Black Sea coast, the most suitable areas include coastal regions encompassing the northern part of Turkey.
Future projections indicate a spread in northern and eastern Europe, confirming other projections with different bioclimatic datasets (Cunze et al., 2022; Noll et al., 2023). A consensus is also observed with areas that will not be suitable in the future, particularly the predicted habitat loss in Spain, Greece, and the Balkans (Cunze et al., 2022). Another outcome is that in the near future scenarios (2011–2040) the midway scenario (SSP370) estimates a slightly wider new distributional area for I. ricinus in the northeast Europe compared to SSP585, a worse case scenario. This outcome is compatible with the previous future projections by Porretta et al., (2013) and also Alkishe et al., (2017), where midway scenarios also predicted a wider new region of suitability in near future. While the spread in the 2070 scenarios is similar between SSP370 and SSP585 scenarios, the decline in the southern distributional areas including Balkans and Mediterranean is much more prominent in SSP585, which is most probably due to the predictions of great declines in precipitation in this region. Several climatic studies in past decades since the early 2000’s deemed the Mediterranean basin one of the climate change hot spots, with projections showing increased temperature and aridity, heightened vulnerability to drought and high temperatures and a higher frequency of heat waves (Ulbrich et al., 2006; Giorgi and Lionello, 2008; Naumann et al., 2018). Furthermore the impacts of recent climate change on increased drought frequency and magnitude is already documented in Mediterranean type climates (Hoerling et al., 2012; Feng et al., 2019). Current projections indicate the most suitable distribution areas for the species in Turkey are the Black Sea coasts and the Thrace region. However, while smaller suitable areas are also indicated along the Mediterranean coast of the country, the uncertainty in these predicted areas is again quite high. Future projections suggest a loss of suitability in the southern coast and, additionally, declines in certain localities, especially in the inland areas of Thrace in the north. This pattern indicates that expected climate changes in the Mediterranean Basin in the future will result in a decrease in the distribution areas of the species in Turkey, similar to the projections for Spain.
Ecological Niche Models are valuable tools for assessing the possible present and future distribution of parasite vectors, However there are several caveats and special care is needed when building distribution models. Major challenges are the potential bias and limitations in the outputs of niche models, especially when projecting to future scenarios (Peterson et al., 2018) so it is crucial to provide necessary uncertainty values associated with these models. Additionally, ectoparasites including ticks partially depend on their hosts for survival and transportation pose other risks in modeling. Host species and its abundance is one of the most important factors in the distribution of ticks to new areas and establishing populations. Moreover, the vast numbers of migrating birds increase the probabilities for the geographic spread of I. ricinus and related diseases, emphasizing the importance of considering avian hosts in disease epidemiology (Waldenström et al., 2007; Ciebiera et al., 2019). Although hosts play a crucial role due to the transmission dynamics of ticks, there are some limitations associated with including hosts in modeling studies. For instance, I. ricinus is a three-host species feeding on different hosts during its life cycle (Hofmeester et al., 2016), which can introduce complexity when using multiple hosts in modeling studies. Recent papers have shown contrasting hypotheses regarding tick-host associations. Some suggest that ticks select hosts based on the environment, while others propose that ticks select environments and feed on any available host within those environments (Nava and Guglielmone, 2013; Zhang et al., 2019; Ginsberg et al., 2022; Estrada-Peña et al., 2023). More to the point, while it has been suggested that hosts can influence tick distribution at a smaller geographical scale, the distribution of ticks over a wide spatial scale is primarily determined by direct climatic effects rather than host presence (Cumming, 2002). Considering all these factors, the importance of understanding the interactions between hosts, environment, and ticks is essential for developing effective strategies to manage tick-borne diseases and control their spread.
In conclusion, this study aimed to update previous projections by reviewing coordinates and utilizing new datasets and additionally implementing microclimatic parameters to build assistive models that would complement other distribution models that are reliant on macroclimatic parameters. For species distribution models, we still depend on macroclimatic datasets since predictions of very fine grained parameters like microclimate for future are not yet available and macroclimate data would still provide good predictions in large scales. Therefore, it is crucial to continuously replicate, validate and update previous models for better predictions of the possible future distributions of disease transmitting ticks.