Monitoring Ecological Changes on a Rapidly Urbanizing Island Using a Remote Sensing-Based Ecological Index Produced Time Series

Abstract

Island ecosystems are susceptible to the considerable impacts of increasing human activities, landscape reconstruction, and urban expansion, resulting in changes in the ecological environment and urban ecological security issues. Remote sensing techniques can achieve the near-real-time ecological environment monitoring of these rapidly changing areas. The remote sensing-based ecological index (RSEI), as a comprehensive remote sensing ecological environment index, was adopted to dynamically monitor urban ecological quality (EQ) over time in this study, combined with the Landsat-based detection of trends in disturbance and recovery (LandTrendr) algorithm. Annual composite images were generated using Landsat 5, Landsat 7, and Landsat 8 imagery to extract four metrics (Greenness, Moisture, Heat, and Dryness) to calculate RSEI from 1991 to 2021. The ecological quality in the study area was evaluated using a five-level classification (poor, inferior, medium, good, and excellent), and the changes in EQ on a pixel basis were identified by the LandTrendr algorithm. The results showed that (1) the average value of the RSEI ranged from 0.47 to 0.57 over 31 years, indicating that EQ was maintained at the medium level; (2) the distribution of different EQ levels had visible patterns, and an area of 47.87 km2 was affected by a poor EQ at least once in 31 years; (3) 38.22 km2 of this area experienced EQ poor disturbance once, and 3.05 km2 of the area had poor disturbance twice. Urban expansion, forest degradation, and policy are the main factors causing the reduction of the RSEI. The results demonstrate that combining time series of RSEI and LandTrendr can effectively monitor the changes of EQ, which is helpful to identify the spatial–temporal variation patterns of EQ and provide valuable information for policymakers and protection.