Researchers analyzing data from more than 10,000 bird species worldwide found that urbanization weakens one of nature's most fundamental patterns: the latitudinal diversity gradient, which describes how biodiversity increases from the poles to the tropics.
The study, led by Anna Hargreaves, Jennifer Sunday, and Jory Griffith, examined bird observations submitted to eBird, a citizen science platform. The team divided the world into one-by-one-kilometer grid cells and categorized each as urban, suburban, or natural based on human population density and land-use data.
In natural areas, bird diversity decreases as latitude increases. Cities disrupted this pattern dramatically. The latitudinal diversity gradient proved three times weaker in cities than in natural areas. Overall, cities had a third fewer bird species than natural areas at the same latitude. But the effect was unequal: tropical cities lost three times more bird diversity than high-latitude cities.
In a tropical city near the equator, a one-by-one-kilometer area would contain 42 percent fewer bird species than a natural area at the same latitude. By contrast, a city at 65 degrees latitude, such as Reykjavik or Fairbanks, would lose only 12 percent of species.
Hargreaves, Sunday, and Griffith identified why tropical regions suffered greater losses. Tropical bird communities contain a larger proportion of habitat specialists, whereas cities favor generalist species like crows and Canada geese. Specialists were disproportionately absent from urban areas, the researchers found.
The work appears in Proceedings of the National Academy of Sciences. The researchers noted that cities with more green space showed higher bird diversity than less green cities, suggesting that urban planning incorporating more vegetation could help preserve bird species.