Objective  Rural areas serve as the base and hinterland space for urban development, the important foundation for homeland ecological security, and the key region for ecological civilization construction. However, in urban and rural construction, many linear projects pass through the city and split the countryside, bringing severe challenges to rural ecological security, and resulting in the rapid decline of biological habitat connectivity and the decline of biodiversity in rural areas. It is of great significance to pay attention to the influence of the construction of suburban railway in typical linear projects on bird habitat connectivity in rural areas.

Methods  Firstly, this research adopts the species distribution model to simulate the habitats potentially suitable for birds in rural areas of Beijing and the suitability index thereof, and accordingly obtains the suitable habitat patches. Secondly, the research adopts the Conefor 2.6 software to analyze habitat connectivity through such indicators as probability of connectivity (PC), integral index of connectivity (IIC) and number of links (NL). Then, the research conducts a scenario simulation in combination with the planning scheme for Beijing Suburban Railway, based on which the following four scenarios are set up: ideal scenario (S0), preliminary construction (S1), partial completion (S2), and full completion (S3). Finally, the research analyzes the influence of linear projects on bird habitat connectivity with the help of the SDMtoolbox, quantifies the connectivity of ecological sources and ecological corridors as well as the complexity and ecological efficiency of the ecological network by virtue of relevant ecological network structure indices, and accordingly puts forward suggestions for optimization.

Results  Research results are as follows. 1) The distribution of suitable habitats for birds in the research area obviously presents the following characteristic: The habitat distribution suitability index is the highest for such habitats as reservoirs, watercourses and their adjacent wetlands, marshes and cultivated lands. The total area of suitable habitats in rural areas is 1, 052.2 km². Habitat patches are small and unevenly distributed, mainly distributed near Guanting Reservoir in northwest Beijing and Miyun Reservoir in northeast Beijing, and in waters and woodlands around the main urban area, as well as parks, reservoirs and scenic spots along rivers and lakes. 2) The research extracts 26 patches with a dI index greater than 0.5 as ecological sources. It is found that the overall connectivity level in rural areas of Beijing is low and uneven, and the IIC, PC and dI indices are higher in the northeast and lower in the southwest. The dI and IIC indices are much higher than NL in Miyun Reservoir and Guanting Reservoir, indicating that it is costly for birds to migrate to both habitats and there is no "stepping stone" between batches in the aforesaid habitats. 3) Under the scenario simulation, the migration path of birds in the south is severely blocked by Beijing Suburban Railway, which produces an avoidance effect among migratory birds, making the original migration path change to the main urban greenway and the northern mountainous area, accordingly leading to the decline of habitat connectivity upon corridor fracture. 4) The ecological network structure index of S3 is significantly different from that of S0 and S2, indicating that the construction of a suburban railway on a certain scale may affect habitat connectivity. The cost for migration path is quite high in all four scenarios, indicating that the quantity and quality of bird habitats are both low in rural areas, and none of the ecological network structure indices shows any obvious advantage. Based on the analysis results, the research proposes a few environmentally friendly and resource-saving optimization methods for linear project construction. 1) Assign different widths to the 25 important ecological corridors, 15 relatively important ecological corridors and 26 general ecological corridors extracted from the 325 potential ecological corridors identified based on a gravity model, and take different measures for them in combination with their respective importance. Specifically, for important ecological corridors, mainly take natural measures to ensure the stable connectivity thereof; for relatively important ecological corridors, take both natural and artificial measures to protect the connectivity thereof; for general ecological corridors, attract birds through artificial nesting and artificial feeding equipment to activate the ecological functions thereof. 2) Divide ecological sites into three levels by the natural breakpoint method according to the dI index, distinguish the functions of each level to improve the quality of habitat. 3) Add ecological nodes at the 17 intersections between important corridors within the area of high-frequency paths that are identified in this research, to supplement the blank of the ecological source area. As shown by the habitat simulation results, after the addition of ecological nodes, the level of ecological network index is improved compared with S3 scenario, indicating that the connectivity efficiency is improved, the material flow and information flow between birds become smoother, and the path cost is effectively reduced. 4) Optimize the 25 ecological fracture points identified at the intersections between ecological corridors and Beijing Suburban Railway, and take measures such as forest phase transformation, shelterbelt construction, planting type and quantity optimization, and farmland conversion into wetland to achieve zoning-based optimization of the habitat quality of buffer zones.

Conclusion  Beijing Suburban Railway reduces the bird habitat connectivity in rural areas of Beijing. The comprehensive optimization measures proposed to alleviate the negative ecological effects of Beijing Suburban Railway are helpful to the ecological civilization construction of Beijing and the implementation of the strategy of rural revitalization, and can provide a certain reference for the conservation of biodiversity in rural areas and the ecological planning of rural landscape.