3.2 Important environmental variables
The choice of environmental variables is the key to determine the accuracy of modeling. Therefore, it is necessary to evaluate which environmental variables contribute the most to the model. Temperature was the main environmental variable affecting S. ciliatum, S. nepalense, and S. yunnanense, according to the MaxEnt model’s variable contribution rate (Fig. 2). Bio3(52%) and Bio4(12.7) for S. ciliatum reached 64.7%. Furthermore, Bio3 (61.8%) and Bio4 (14%), both for S. nepalense, achieved 75.6%, much greater than the other contribution rates. Bio3(28.6%) and Bio6(54.3%) for S. yunnanense were 72.9%. This reveals that Bio3, Bio4, and Bio6 were the most important influencing variables in S. ciliatum, S. nepalense, and S. yunnanense distribution.
The Jackknife test was used to determine the impact of environmental variables on S. ciliatum, S. nepalense, and S. yunnanensepotential distribution (Fig. 4). When a single variable was applied in relation to S. ciliatum , the jackknife test showed that the most influential environmental variable is Bio4 followed by Bio3 and Bio11. When a single variable was applied in relation to S. nepalense , the most influential variable was Bio3 followed by Bio4 and Bio19. When a single environmental variable was applied in relation to S. yunnanense , the most influential variable was Bio4 followed by Bio6 and Bio3. The most important variables influencing S. ciliatumdistribution were Bio3, Bio4, and Bio11. S. nepalensedistribution was influenced by the variables were Bio4, Bio3, and Bio19. Bio4, Bio3, and Bio6 are the most important variables influencing the distribution of S. yunnanense.
The response curve was used to calculate the thresholds (existence probability > 0.5) for the main bioclimatic parameters (Fig. 5). In relation to S. ciliatum , Bio3 ranged from 44 to 51.5, Bio4 ranged from 420 to 580, and Bio11 ranged from 1℃ to 13℃.
In relation to S. nepalense. Bio3 ranged from 44 to 53, Bio4 ranged from 400 to 580, and Bio19 ranged from 25 to 80 mm.
With regard to S. yunnanense , Bio3 ranged from 41 to 53, Bio4 ranged from 300 to 600, and Bio6 ranged from -6 to 6℃.