Table 1.
Overview of EURO-CORDEX simulations used as model input.
Fig 1.
State transition diagram of the IRIS model (Ixodes RIcinus Simulator).
The model simulates the development process of I. ricinus from larvae to nymph to adult stages forced by climate conditions. The white boxes represent the model cohorts. The black arrows indicate the transition between cohorts, which is controlled by the respective model process indicated by its label.
Fig 2.
Trigger values of the climate parameters and the induced change of behaviour state of the ticks.
Optimal conditions for tick activity is given in green, conditions leading to desiccation or freezing in red and blue, respectively. The overlap between the areas admitting activity and implying desiccation illustrates the trade off each individual tick faces when deciding whether to become active for host-seeking or to stay inactive to conserve humidity (indicated in yellow).
Fig 3.
Result of the model calibration for the year 2013.
The grey bars show the monthly observed nymphal densities (nymphs / 100 m2) at the sampling site in Haselmühl. The black connected dots show the monthly simulated nymphal densities with calibrated parameter values. The root-mean-square error (RMSE) is 11.29.
Fig 4.
Flowchart of the data processing and implementation of the German-wide tick density model.
Fig 5.
Relationship of the median temperature between January and June (x-axis) and the date of the maximum number of questing nymphs on the model landscape (y-axis). Each data point corresponds to a simulation run of a particular year. Orange crosses represent IRIS model runs with observed weather data leading to a correlation coefficient of r = -0.61. Blue circles represent IRIS model runs with projected climate data for the period 2021–2099 leading to a correlation coefficient of r = -0.65. There are 15 data points for each year simulated with data from climate models (blue dots). Each of them represents a different climate model.
Fig 6.
Relationship of the annual median temperature (x-axis) and the density of questing nymphs / 100 m2 (y-axis) for the seasons of a year.
Fig 7.
Minimum number of questing nymphs on the simulation landscape in the summer month (June–August) for the period 1949–2099.
Simulations were conducted with observed weather data between 1949 and 2020 (black crosses) and with weather data from 15 climate models between 1971 and 2099 (grey circles). The dashed vertical grey line between 2020 and 2021 indicates the border between past and future years.
Fig 8.
Ensemble means of nymphal I. ricinus density for the reference period (top left), the near-term scenario (top right), the mid-term scenario (bottom left), and the long-term scenario (bottom right). We set a threshold of 2,000 nymphs / 100 m2 which is twice the maximum currently observed annual nymph density in Germany (O. Kahl 2021, pers. comm.). Note that the given density of nymphs / 100 m2 should be interpreted as the annual number of I. ricinus nymphs, which one would collect by monthly flagging an area of 100 m2. The abbreviations in the map at the top left indicate the names of the federal states of Germany (BW: Baden-Württemberg, BY: Bavaria, BE: Berlin, BB: Brandenburg, HB: Bremen, HH: Hamburg, HE: Hesse, NI: Lower Saxony, MV: Mecklenburg-Vorpommern, NW: North Rhine-Westphalia, RP: Rhineland-Palatinate, SL: Saarland, SN: Saxony, ST: Saxony-Anhalt, SH: Schleswig-Holstein, TH: Thuringia). State and federal state boundaries were taken from the GADM, the database of global administrative areas (https://gadm.org).
Table 2.
Overview of the climatic parameters and the resulting nymphal I. ricinus densities for the three future scenarios.
Fig 9.
Germany map of nymphal I. ricinus density differences depicting the change between the reference period (1971–2000) and the long-term scenario (2070–2099).
If the density difference exceeds the threshold of 2,000 nymphs / 100 m2, the grid cells are indicated in grey. State and federal state boundaries were taken from the GADM, the database of global administrative areas (https://gadm.org).