Long‐Term Legacy of Exotic Pioneers: Over Two Decades of Biodiversity Monitoring Reveals Constrained Succession in a Restored Sanitary Landfill

Long‐term ecological studies are critical for evaluating restoration strategies on challenging sites, such as closed landfills. This study presents a 23‐year biodiversity monitoring program (2000–2023) at the restored Southeast New Territories sanitary landfill in Hong Kong. We tracked the succession of woody trees and shrubs and of soil and litter invertebrates to assess the impact of initial revegetation with exotic pioneer species, particularly nitrogen‐fixing legumes. Our results reveal a constrained ecological trajectory dominated by a few exotic/pioneer species, with the leguminous trees Acacia confusa and Leucaena leucocephala becoming overwhelmingly dominant, correlating with a significant decline in overall plant diversity over time. This low‐diversity plant community supported a depauperate invertebrate assemblage, notably favouring detritivores and harbouring invasive ants such as Anoplolepis gracilipes (yellow crazy ant). While bird surveys indicated the presence of seed‐dispersing species, the ecosystem failed to develop high species richness. The findings demonstrate that while exotic pioneer trees effectively establish initial cover, their long‐term dominance, potentially exacerbated by allelopathy and site management practices like mowing, creates a biotic filter that hinders the development of biodiverse, self‐sustaining ecosystems. We conclude that landfill restoration requires frameworks that go beyond mere vegetation establishment to actively manage the legacy of pioneer species and promote heterogeneous habitats, such as constructed wetlands in restored landfills, to support diverse ecological communities and enhance functional biodiversity.