Researchers at McGill University have found that urbanization is weakening one of Earth's most enduring ecological patterns: the tendency for biodiversity to increase toward the tropics, a pattern known as the latitudinal diversity gradient.
Analyzing bird populations worldwide, they found that on average, urban areas near the equator contained 42 per cent fewer species than nearby natural areas, while the comparable global figure was 32 per cent fewer, and 12 per cent fewer at 65 degrees of latitude.
The researchers said this disproportionate loss is driven partly by more intense urbanization in tropical cities and partly because many tropical bird species are less able to cope with changing environments.
"Travelling from northern latitudes like Canada to the tropics, you notice how many more species of plants and birds there are," said Anna Hargreaves, senior author and Professor in the Department of Biology. "That pattern has existed for tens of thousands of years. It's strong, it's striking, it's old and it's super consistent. There are not many ecological patterns like that. And now, one of Earth's most prominent, obvious biological patterns is almost being erased due to urbanization."
The researchers suggest such urban planning strategies as preserving green space are "hopeful measures" that might allow birds and other species to share space with humans.
Leveraging citizen science
Previous research on the latitudinal diversity gradient has used smaller, more localized datasets. This study looked at more than 10,000 bird species across five continents using eBird, a citizen-science database created at Cornell University that allows users to record bird sightings.
"We downloaded the data and divided the world into square-kilometre grid cells, only including cells where we had enough eBird data to be confident we were capturing at least 95 per cent of the diversity in each cell," said Jory Griffith, the graduate student who led the analysis.
The researchers then classified each cell as urban, suburban or natural, based on human population density and the degree of landscape modification, and compared diversity patterns across the three categories.
Hargreaves said that the intensity of urbanization had a marked effect, noting that suburban areas and cities with considerable greenspace saw less of a decline.
The team is now working to understand not just which species are in cities, but how changing patterns of diversity affect interactions among species in cities, like predation and pollination.
About this study
"Urbanization weakens the global latitudinal diversity gradient in birds," by Jory Griffith, Jennifer M. Sunday and Anna L. Hargreaves, was published in PNAS.
The study was funded by the National Science and Engineering Research Council of Canada and a Canada Graduate Scholarship-MSc level.
