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).