Concrete jungle, silent canopy: Why tropical birds vanish in cities

Tropical ecosystems are famous for their teeming, exuberant diversity. In fact, the dramatic increase in biodiversity from the poles to the tropics — known as the latitudinal diversity gradient — is pervasive across branches of the tree of life, and fossil evidence suggests it has existed for millennia.

It might seem that if anything could be immune to human disruption, it would be the latitudinal diversity gradient. But human impacts on nature are also global, pervasive and dramatic. One of the most dramatic is urbanization.

Urbanization replaces natural ecosystems with environments dominated by pavement and buildings. Wetlands are drained, deserts are irrigated and grass lawns are planted.

Urbanized landscapes are much more similar to each other than the ecosystems they replace, so much so that urbanized areas have been called the fastest-growing biome on the planet. Unlike any natural biome, urbanized landscapes now exist from the Arctic to the equator.

Ecologists have known for decades that urbanization generally lowers local diversity. We wanted to know if urbanization was altering global diversity patterns as well.

How urbanization weakens biodiversity

By analyzing data from more than 10,000 bird species worldwide, we found that the latitudinal diversity gradient is three times weaker in cites than in natural areas — an enormous reduction in one of the world’s most important ecological patterns.

Cities had a third fewer bird species than natural areas overall, confirming that around the world, urbanization lowers bird diversity. But tropical cities lost three times more bird diversity than high-latitude cities.

For example, imagine if we compare a one-by-one square kilometre in a city to one the same size in a natural area at the same latitude. On average, the city square would have 42 per cent fewer bird species if it was close to the equator (think Singapore or Quito), but only 12 per cent fewer if it was at 65 degrees latitude (think Reykjavík or Fairbanks).

One of the reasons tropical cities lost more species lies in the traits of tropical birds. Tropical communities have long been thought to contain more specialized species, and previous work has suggested that cities tend to favour generalists — jack-of-all-trades species like crows and Canada geese.

Our work showed exactly that: tropical bird communities contained a much larger proportion of habitat specialists, and these specialists were disproportionately missing from cities, at least partly explaining why urbanization excludes so many more species in the tropics.

Leveraging community science

Our analyses show the power of community-driven science and would not have been possible without the millions of birdwatchers around the world who submit their sightings to the platform eBird.

By doing so, birders have created a dataset of unprecedented spatial and temporal detail. Watching a time-sequence of eBird observations, you can see a species migrate north in spring and south in winter. And the high spatial resolution enabled us to tell which checklists came from cities, suburbs or natural areas.

To test how urbanization affects diversity patterns, we divided the world into one-by-one kilometre grid cells and categorized each as urban, suburban or natural, using data on human population density and land-use change.

We retained only cells that had sufficient eBird data to capture most of the diversity in even hyper-diverse cells. Sensitivity tests suggest that our results only get stronger if we apply even stricter thresholds for data quality, and were not an artefact of bias in where cities were built.

A more liveable future

One hopeful finding of our study is that urban diversity losses may be stemmed by greener urban planning. On average at a given latitude, cities with more green space had higher bird diversity than less green cities.

Delegates from around the world recently met in Yerevan, Armenia, for the COP17 summit of global biodiversity. Governments around the world should consider how building greener cities that incorporate more urban green space would create more liveable landscapes for both humans and wildlife alike.

We are now working to understand not just which species are in cities, but how changing patterns of diversity affect interactions among species, like pollination and predation, in the urban jungle.