## 3 Results
### Habitat Quality Change
#### 3.2.2 Spatiotemporal Change in Habitat Quality  
Next, the average habitat quality indices in China from 1980 to 2020 (Figure 6) reveal a decreasing tendency in habitat quality. This decline was initially slight over the first two decades, followed by a sharp decrease between 2000 and 2010. A slight improvement was observed after 2010, but overall, the habitat quality in China has declined during the last four decades. Figure 5: Spatial pattern of habitat degradation degree (1980–2020). Figure 6: Average habitat quality indices (1980–2020). The natural break approach was utilized for classifying the habitat quality index into five levels (Table 6 and Figure 7). The spatial pattern of habitat quality has changed significantly over the past 40 years, with the largest values in the eastern and southern regions and the smallest in the northwest. The good-quality habitats were initially concentrated in Inner Mongolia and the Qinghai-Tibet Plateau. However, in the last decade, a small part of the Qinghai-Tibet Plateau has dropped from good to medium quality. The Heilongjiang Basin, the Greater Khingan Range, the Yunnan-Guizhou Plateau, and the southern Yangtze River region, characterized by lush vegetation and developed water systems, had excellent habitat quality. Moderate-quality habitats were often adjacent to excellent habitats, such as the Northeast Plain, North China Plain, and Pearl River Delta, which were dominated by cropland. Conversely, habitats with very poor quality were concentrated in the Tarim and Junggar Basins, with the remaining habitats dispersed within urban agglomerations. Poor-quality habitats were primarily located in the northwest, often adjacent to those with very poor quality. Overall, China's habitat quality was closely linked to land use, terrain, and resource availability. Figure 7: Spatial pattern of habitat quality levels (1980–2020). Over the 40-year period, the size of excellent-, good-, and medium-quality habitats all decreased. This simultaneous decrease in medium-quality habitats and the expansion of very poor habitats suggests a concerning polarization trend. Despite the overall declining trend, certain regions like the Greater Khingan Range maintained their excellent quality, suggesting the effectiveness of ecological protection in these areas. These trends, along with the patterns observed in Figures 6 and 7, indicate an overall decline in China's average habitat quality from good to medium. Next, a transformation matrix analysis was performed for quantifying the spatial changes in habitat quality levels (Table 7). The transformation outcomes were classified into three categories: increased, decreased, and unchanged habitat quality (Figure 8). Overall, most habitats exhibited no discernible change in quality, but the habitats displaying a decline in quality exceeded those demonstrating an improvement. Unchanged habitats accounted for the largest proportion, concentrated in the Tarim Basin, Sichuan Basin, and Inner Mongolia Plateau. Increased quality habitats (blue value in Table 7) comprised the study area, primarily located around northwestern and southwestern zones, with the remainder dispersed throughout China. In contrast, decreased quality habitats (red value in Table 7) comprised the study area, mostly distributed in small clusters in alpine regions such as the Junggar Basin, the Qinghai-Tibet Plateau, and coastal urbanized zones. Furthermore, substantial changes were observed across habitats with varying quality levels. The excellent habitats were the most stable, remaining unchanged, degrading to good quality, and a minor proportion degrading to medium, poor, and very poor quality. The transformation showed a \"jumping\" pattern of degradation, with habitats often degrading beyond the adjacent quality level. Good-quality habitats were particularly vulnerable, with a significant proportion experiencing severe degradation. Notably, medium-quality habitats showed the highest instability, suggesting that they may serve as a critical threshold in the degradation process.