## 4 Discussion
### Reasons for Selecting Indicators and Testing for Collinearity and Correlation  
#### 4.2.1 Principles and Basis for Selecting Indicators  
Ecological quality is significantly influenced by various changes in the natural physical environment [102], encompassing climate variations, alterations in urbanized landscape patterns resulting from human activities, and the resilience of ecosystems in resisting disturbances and maintaining stability [103]. Drawing upon the established \"FIPS\" framework, we discuss the reasons for selecting indicators and their impact on ecological quality based on prior research and indicator concepts.  
Figure 9: Principal component analysis results of ten variables in three urban agglomerations. The larger the absolute value of box is, the greater the contribution of the variable to the principal component.  
Ecological function (EF) characterizes the functional pattern and ecological status of the ecosystem, determined by parameters such as NDVI, LAI, GPP, and LC. Both LAI and NDVI reflect the current state of vegetation cover and biological resources [67]. Though NDVI showed certain correlation with LAI in some research, recent research continues to suggest that both indicators should be used when exploring the effects of climate change or urbanization on vegetation in large areas [68]. NDVI saturation in high vegetation areas, soil factor interference in low vegetation areas, and seasonal differences caused by various ecological vegetation types make it impossible for a single index to comprehensively evaluate the ecological quality of large scale [104]. Changes in Land Cover (LC) exhibit a strong correlation with alterations in regional ecological patterns and ecological quality, especially the conversion of ecological land to urban construction land [105]. Gross Primary Productivity (GPP) represents the productive capacity of plants and other individuals in the ecosystem, affecting the energy flow and equilibrium state of the ecosystem [69].  
Ecological interaction (EI) explores the relationship between ecosystems and climate change interactions, utilizing temperature, precipitation, and PM2.5 concentration as indicators. Rapid urbanization intensifies the urban heat island, making monitoring temperature changes crucial for maintaining a suitable climate environment and establishing sustainable urban ecosystems. Abnormal precipitation disrupts human production, daily life, and critical components of urban ecosystems such as agriculture and green spaces [106]. PM2.5 levels reflect the air quality of urban agglomerations, playing a vital role in assessing livability and ecosystem health, especially given its significant impact on human health and environmental quality. In fact, PM2.5 is the main source of air pollution that troubles the three major urban agglomerations, with a greater impact on human health and environmental quality [107]  
Ecological pressure (EP) quantifies the pressure of human activities on the ecological environment. Human density and GDP are closely associated with the level of urbanization development, quantitatively describing the pressure and perturbation of economic development on energy flow and material cycles under human activities' intervention.  
Ecological stability (ES) reveals the stability and resilience of ecosystems [76]. GPP plays a crucial role in maintaining the stability of ecosystems and the balance of energy and material processes [69]. The structural complexity of ecosystems and soil properties profoundly influence stability and resilience when ecosystems face disturbances. Ecosystem stability improves with higher soil nutrient availability and higher reproductive capacity[77],