Shrub encroachment on fishpond transition zones alters avian assemblages and insect biomass in a Central European biosphere reserve

Wetlands provide essential ecosystem services but are increasingly threatened by agricultural intensification, land-use change, and the abandonment of traditional management, leading to habitat fragmentation and declines in wetland-dependent species. In the Třeboňsko Biosphere Reserve, historically diverse mosaics of woodlands, marshes, and peat bogs have been extensively modified for fish and timber production, while open wetlands are increasingly overgrown by willows. This study investigated how willow encroachment on wetland transition zones exerts an effect on bird diversity and invertebrate biomass across three littoral biotopes: reed beds, low-vegetation marshlands (e.g., sedge stands), and willow stands. We hypothesized that primary habitats, particularly reed beds, would support higher bird diversity and abundance than secondary willow growth. Bird surveys and invertebrate biomass sampling were conducted at 27 evenly distributed sites. Reed beds supported the highest bird diversity, followed by willow stands, whereas low-vegetation marshlands showed significantly lower species richness and densities. Willow stands contained the most homogeneous bird assemblages and lacked most threatened or wetland-specialist species, indicating biotic homogenization associated with woody encroachment. Species dissimilarity was greatest among low-vegetation sites but lowest in willow stands, indicating that willow stands supported the most homogeneous bird assemblages. Species–area relationships were positive overall but differed among habitats. Invertebrate biomass was negatively correlated with bird density, suggesting complex trophic interactions. These findings demonstrate that wetland overgrowth can reduce habitat heterogeneity and conservation value, highlighting the broader importance of structurally diverse wetlands for maintaining biodiversity and ecological resilience. Conservation strategies in Třeboňsko and similar temperate wetlands should prioritize preserving and restoring open littoral habitats to support specialist species and reduce habitat homogenization.