Land-use change to eucalypt plantations decreases taxonomic and functional diversity of bird communities

Global expansion of eucalypt plantations raises concerns over their ecological impacts, particularly on bird communities. We evaluated how land-use change to eucalypt plantations affects taxonomic and functional diversity of birds, comparing plantations with shrublands (directly replaced), agricultural lands (alternative land use), and natural forests (potential native vegetation) in central Portugal. We also assessed how habitat heterogeneity within plantations, defined by vegetation structure, composition, and landscape context, shapes bird community assembly. Taxonomic diversity was measured using species richness and Shannon’s equitability, while functional diversity was quantified via functional richness, evenness, divergence, and dispersion. Eucalypt plantations had the lowest species richness and functional richness, shrublands were intermediate, and agricultural lands and natural forests had the highest diversity. Functional divergence was higher in plantations than in agricultural lands but similar to shrublands and natural forests. Plantations shared a functional composition with shrublands, suggesting that simplified vertical structure limits tree-associated species. Within plantations, bird assemblages were shaped by cross-scale interactions between local vegetation structure and landscape context. Tree height and understory development determined local habitat suitability for multiple functional groups, while landscape-scale plantation cover mediated broader patterns of functional group abundance. These results indicate that conversion to eucalypt monocultures reduces both taxonomic and functional diversity, representing a net loss of ecological roles relative to directly and indirectly replaced land uses. Our study underscores that enhancing structural complexity and integrating stand-level management with landscape planning are essential to reconcile production goals with bird conservation in plantation-dominated regions. • Eucalypt plantations reduce bird taxonomic and functional diversity. • Plantations share functional diversity with shrublands (directly replaced land use). • Highest diversity in agricultural lands and natural forests (indirectly replaced). • Despite their forest-like structure, plantations do not favour tree-specialist birds. • Vegetation structure and landscape context drive bird communities in plantations.