## 1 Introduction
Therefore, in this study, drawing upon the assessment of economic resilience, where the change rate of GDP (also known as the sensitivity index) is used to measure resilience of the economic system [28], a resilience evaluation index is built to measure urban ecological resilience. Specifically, we use the change of net primary productivity (NPP) as the resilience surrogate. A concrete calculation method will be presented in the following method section. The surrogate selection of NPP is referenced from research that uses normalized difference vegetation index (NDVI) and GPP to quantify ecosystem resilience [29; 30]. In terms of the impact factors of ecological resilience, as mentioned before, some studies use the impact factor of resilience to quantify system resilience and identified factors such as climate (e.g., precipitation, sunshine hour, temperature), hydrology (e.g., surface and groundwater resources), and land cover factors [31]. Among them, land cover is an important and widely studied impact factor of ecosystem services and resilience [5; 32; 33]. Research shows urbanization increases land cover fragmentation, thus decreasing urban ecological resilience. In addition to biophysical factors, social-economic factors, such as GDP, population, land use, industrial structure, infrastructure, institutional arrangements, etc. also influence the resilience in urban systems [3; 34; 35; 36]. An urban system, as a complex adaptive system, has a socio-economic subsystem that directly contributes to system resilience with its own urban functions, such as engaging in social production, providing employment, increasing productivity, and improving the livelihood of urban residents. Meanwhile, it influences system resilience indirectly through the interactions with the ecological-environment subsystem, which also directly impacts urban ecological resilience. Therefore, this study sets out to measure urban ecological resilience with the empirical case of the Beijing-Tianjin-Hebei Urban Agglomeration and explore the impact factors including physical geographical factors (temperature, precipitation, elevation, slope, land cover type) and socio-economic factors (GDP, population, industrial structure, urbanization rate and carbon emission). Rationales for impact factor selection will be presented in the empirical results section. In doing this, we aim to empirically assess the ecological impacts of urbanization through the quantification of urban ecological resilience and explore the influencing mechanism of urban change on ecosystems. The following sections of this paper are structured as follows. The next section presents the overall methodology of this study with elaborations on the case study, research data and method. The empirical results are illustrated in Sections 3 and 4, with Section 3 presenting the resilience assessment result and analyzing the spatial difference of ecological resilience in the study region. Section 4 demonstrates the impact factor analysis and the correlation between ecological resilience and various impact factors. Section 5 concludes this paper by discussing the empirical, methodological, and theoretical implications and contributions of this study.