Impact of urbanization on native plant pollination networks in riparian corridors

Thesis Abstract

Urbanization alters habitat structure, resource availability, and disturbance regimes in riparian ecosystems, with potential cascading effects on plant–pollinator interactions and the integrity of native pollination networks. This study addresses the gap in empirical evidence linking urban-driven environmental change to the structure and function of pollination networks within riparian corridors, aiming to quantify shifts in plant-pollinator assemblages and assess the resilience of native networks amid increasing urban pressure. The central aim is to determine how varying intensities of urbanization influence (i) network structure metrics (connectance, modularity, nestedness, and species degree distribution) of native plant pollination networks along riverine corridors, (ii) pollinator assemblage composition and visitation rates to native plant species, and (iii) the potential mediating role of habitat quality, floral resources, and disturbance regimes in shaping these networks. Specific objectives include (1) characterizing riparian plant communities and their native pollination partners across a gradient of urbanization (low, medium, high) within three metropolitan river systems; (2) quantifying visitation rates, pollinator diversity (Apis mellifera excluded as a managed species, Bombus spp., hoverflies, bees, and non-bee taxa), and pollen transfer effectiveness using standardized observation blocks and fluorescent dye tracking over two flowering seasons; (3) constructing and comparing quantitative pollination networks to evaluate changes in network metrics with urban intensity; (4) testing the influence of habitat variables (fragmentation, corridor width, vegetation cover, and floral resource density) on network robustness through generalized linear mixed models and Bayesian hierarchical models; (5) identifying keystone plant species and critical pollinator groups vital for network stability under urban pressure. The study adopts a mixed-methods, quasi-experimental field design. The population comprises native plant species and their pollinators in riparian corridors along three mid-latitude urban rivers. Sampling involves 30 transects per river, stratified by urban intensity, yielding approximately 90 transects with standardized plot sizes of 20 m × 5 m, across two blooming seasons. Data collection employs direct observational pollinator counts (20-minute sessions per plot, repeated monthly), pollinator identity to species or functional group, floral trait measurements (corolla depth, nectar volume, bloom phenology), floral resource mapping, and pollen deposition assessments using recipient stigmas collected from tagged flowers. Instrument validity is established through pilot sampling and expert validation, while observer calibration ensures consistent species identification. Plant-pollinator interaction matrices will be compiled per site, integrating visitation data with pollen deposition and pollen tube growth assays on a subset of documented interactions to estimate actual pollination effectiveness. Analytical techniques include network ecology methods (quantifying connectance, modularity via modularity-maximization algorithms, nestedness with NODF, and interaction turnover), generalized linear mixed models to assess relationships between urbanization indices (impervious surface, human disturbance, and road density) and network metrics, and structural equation modeling to evaluate direct and indirect pathways linking habitat variables to network stability. Temporal replication over two flowering seasons allows assessment of inter-annual variability and resilience indicators. Anticipated findings suggest that higher urbanization will reduce native plant diversity and disrupt specialized pollination linkages, leading to sparser networks with lower connectance and increased modularity, while functional pollination may be maintained for generalist taxa through urban-adapted pollinators, potentially revealing a threshold urbanization level beyond which network robustness declines. The study contributes to knowledge by integrating plant–pollinator interaction theory, metacommunity concepts, and network resilience frameworks to a real-world urbanizing landscape, offering quantitative benchmarks for riparian corridor management. Implications include evidence-based recommendations for urban planning and restoration—such as maintaining corridor width, enhancing native flowering plant richness during peak pollination periods, and creating pollinator-friendly habitats to sustain network integrity. The main conclusion is expected to be that maintaining and restoring native floral resources and habitat connectivity within riparian corridors is critical to preserving functional pollination networks under urban growth. Recommendations emphasize implementing green infrastructure that prioritizes native plant diversity, continuous habitat corridors, and targeted pollinator habitat enhancements, with further research focusing on long-term monitoring across multiple cities and river systems to generalize findings.