Why the tropics lose the most birds when cities expand

Tropical ecosystems are renowned for their incredible biodiversity, which follows a striking latitudinal gradient from the poles to the equator. This pattern of increasing diversity is consistent across various branches of the tree of life, including birds. However, human activities, particularly urbanization, pose a significant threat to this natural diversity.

Cities replace natural ecosystems with paved surfaces and buildings, leading to the loss of wetlands, desertification, and the planting of non-native grasses. Notably, urbanized landscapes are rapidly becoming the fastest-growing biome on Earth, now extending from the Arctic to the equator. Research indicates that urbanization significantly reduces local bird diversity, with the latitudinal diversity gradient becoming three times weaker in cities compared to natural areas.

A study analyzing data from over 10,000 bird species worldwide found that tropical cities lost three times more bird diversity than high-latitude cities. For instance, a 1-by-1-kilometer square in a city near the equator would have 42% fewer bird species than a similar area in a natural habitat, while the same size square at 65 degrees latitude would have only 12% fewer.

Tropical bird communities are known to contain more habitat specialists, which are more likely to be absent from urban areas. This loss of specialists explains the greater decline in bird diversity in tropical cities. The study highlights the importance of green spaces in urban planning, as cities with more vegetation tend to have higher bird diversity.

Policymakers attending the COP17 biodiversity summit in Yerevan, Armenia, were urged to consider greener urban designs that incorporate more urban green space to preserve both human and wildlife habitats.