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Land‐use transformation is one of the most important and pervasive ecological changes occurring across the Earth, but its long‐term effects are poorly understood. Here, we analyze the effects of urban and agriculture development on bird biodiversity and community structure over a 16‐yr study period. We found that long‐term effects of land‐use change are dependent on spatial scale and land‐use type. At the regional scale, we found that gamma diversity (total number of species observed) declined by ~10% over time. At the landscape spatial scale, we found that beta diversity (uniqueness of bird communities) increased by ~16% over time. Additionally, the average contributions of urban riparian bird communities to beta diversity were generally the highest but declined by ~26% over the study period. Contributions of urban communities to beta diversity were generally the lowest but increased by ~10% over time. At the local scale, we observed different responses for different measures of alpha diversity. For bird species richness, temporal changes varied by land use. Species richness declined 16% at sites in desert riparian areas but increased by 21% and 12% at sites in urban and agricultural areas, respectively. Species evenness declined across all land uses, with some land uses experiencing more rapid declines than others. Our analysis of species groups that shared certain traits suggests that these community‐level changes were driven by species that are small, breed onsite, and feed on insects, grains, and nectar. Collectively, our results suggest that biodiversity declines associated with land‐use change predominate at the regional and local spatial scale, and that these effects can strengthen or weaken over time. However, these changes counterintuitively led to increases in biodiversity at the landscape scale, as bird communities became more unique. This has implications for conservation and management as it shows that the effects of land‐use modification on biodiversity may be positive or negative depending on the spatial scale considered.

Cultivation and spread of non‐native plant species may result in either phylogenetic homogenization (increasing similarity) or differentiation (decreasing similarity) of urban floras. However, it is unknown how non‐native species influence homogenization of cultivated versus spontaneously occurring species in cities, and which traits are associated with species that promote homogenization versus differentiation. In this study, we compared homogenization effects of cultivated and spontaneous non‐native species in yard floras across and within seven widely distributed U.S. cities. Additionally, we explored which traits explained their particular contribution to homogenization. We recorded plant presence/absence in 178 private yards distributed among seven metropolitan statistical areas in the United States. We compared phylogenetic homogenization effects of non‐native species within both the cultivated and spontaneous species pools using phylogenetic dissimilarities and the homogenization index. Then, we expressed contributions of non‐native species to the homogenization of each pool as a function of two different sets of plant functional traits using phylogenetic generalized least square (PGLS) models across and within cities. Across cities, spontaneous non‐native species homogenized, and cultivated non‐native species differentiated, yard floras. Within the spontaneous pool, short, small‐seeded non‐native plants and non‐native grasses significantly homogenized yard floras. Within the cultivated pool, species contribution to homogenization was best predicted by plant height, presence of showy flowers, and growth form, with non‐native grasses significantly homogenizing cultivated yard floras. Within cities, non‐native species—whether they were cultivated or spontaneous—consistently homogenized yard floras of the three northern cities and differentiated yard floras of three of the four southern cities, suggesting that homogenization processes are context‐ and scale‐dependent. Likewise, traits explaining homogenization differed substantially among cities. The inconsistent patterns among cities in the plant traits that promoted homogenization of both cultivated and spontaneous species suggest that local environmental and anthropogenic conditions of individual cities imposed strong constraints on trait selection. Linking plant functional traits that promote homogenization with residents’ preferences for vegetation may further enhance understanding of how yard plant communities assemble at regional and local scales.