FIGURE 3  The AUC value calculated and output by the Maxent model

3.3 Environmental variable contribution

The percentage contribution values of environmental variables to the predicted results of the MaxEnt model were defined through heuristics, and depended on the specific path that MaxEnt codes used to provide best solutions (Li et al., 2020). In this research, the percentage contribution values were the average contribution values established over cross-validation and repeated runs for each species (Li et al., 2020). The results showed that among the 11 independent environmental variables, annual precipitation (Bio12), precipitation of coldest quarter (Bio19) and population density (Pop) had higher average contribution rates to the species distribution model than other environmental variables. Their average contribution values to the model were 25.92%, 15.86% and 17.95%, respectively, and the cumulative contribution value accounted for 59.73% of the total contribution value of all environmental factors to the model (Figure 4). However, compared with other environmental factors, mean diurnal range (Bio2) and altitude (Alt) had the lowest average contribution values to the species distribution model. Their average contribution values to the model were 1.07% and 1.64%, respectively, and the accumulated contribution value accounted for 2.71% of the total contribution value of all environmental variables to the model (Figure 4).