FIGURE 4  Contribution value of environmental variables to predicting result of the MaxEnt model

3.4 Potential geographical distribution of species

HSI is an important or key factor that affects the survival and development of species. It refers to the potential ability of a habitat to support the survival of a particular species. The results of the study revealed that potential distribution areas of species appears to be decreasing with the improvement of habitat suitability (Figure 5), which indicated that the remaining natural habitat suitable for species was becoming less and less. In addition, the research results also indicated that the species with a larger potential distribution areas were Echinocodon lobophyllus , Gymnadenia crassinervis ,Rhodiola atuntsuensis and Aristolochia delavayi . Their potential distribution areas were 44,963.53 km2, 30,830.51 km2, 22,481.77 km2 and 22,396.28 km2, respectively, accounting for 56.36%, 38.64%, 28.18% and 28.07% of the total area of Northwest Yunnan (Table 3; Figure 6). The species with smaller potential distribution areas were Diphylax uniformis , Paris dulongensis andTerminalia myriocarpa . Their potential distribution areas were 2,137.05 km2, 1,054.28 km2 and 826.33 km2, respectively, occupying only 2.68%, 1.32% and 1.04% of the total area of Northwest
Yunnan (Table 3; Figure 6). Generally, the potential distribution areas of each species were relatively concentrated. The HSI values of the mid-western, mid-eastern and northern regions of the study area were between 0.40 and 1.00, which were deemed the main concentration area of the potential distribution of species. Furthermore, the area of high suitability plaques (i.e., Sum area of high suitability level habitats, where 0.40 ≤ HIS < 0.60) was larger than the area of optimal suitability plaques (i.e., Sum area of optimal suitability level habitats, where 0.60 ≤ HIS < 1.00) (Table 3; Figure 6).