Tropical cities lose more bird species to urbanization

A global analysis of more than 10,000 bird species reveals that urbanization weakens a fundamental pattern of biodiversity. Researchers found the latitudinal diversity gradient, which describes how species richness increases toward the tropics, becomes three times weaker in cities compared to natural areas. This indicates urban development significantly alters global patterns of bird diversity.

The study compared bird diversity between urban and natural areas across different latitudes. Near the equator, cities contained 42 percent fewer bird species on average, while cities at 65 degrees north lost only 12 percent. This pattern held true when comparing equal-sized areas, showing tropical ecosystems face greater losses from urban expansion.

Tropical bird communities tend to include more specialized species with narrow habitat requirements. Researchers suggest these specialists are particularly vulnerable to urbanization, which favors generalist species like crows and geese. The disproportionate loss of specialists in tropical cities helps explain why urbanization reduces diversity more severely in these regions.

The research relied on community science data from eBird, a platform where birdwatchers worldwide submit millions of observations. This dataset provided unprecedented spatial and temporal detail, allowing researchers to distinguish bird communities in cities, suburbs, and natural areas. The high-resolution data made it possible to track species movements and compare diversity across different urbanization levels.

To measure urbanization's impact, researchers divided the world into one-square-kilometer grid cells. Each cell was categorized as urban, suburban, or natural based on population density and land-use changes. Only cells with sufficient bird observation data were included, ensuring reliable diversity estimates even in highly diverse tropical regions.

The study's findings remained consistent even when applying strict data quality thresholds, confirming that the results reflect real patterns rather than sampling biases. Researchers note that urbanization's effects on bird diversity are not limited to where cities were built, but represent a global phenomenon affecting biodiversity patterns worldwide.

One encouraging discovery is that greener cities support higher bird diversity. At any given latitude, urban areas with more green space hosted more bird species than less vegetated cities. This suggests that incorporating natural habitats into urban planning could help mitigate biodiversity losses from development.

The research team is now investigating how changing urban bird communities affect ecological interactions like pollination and predation. Their work highlights the importance of biodiversity-friendly urban design as governments consider strategies to balance development with wildlife conservation.