In Sapzurro Bay on the Colombia-Panama border, the blue land crab can be found surrounding human infrastructure, burrowing into the nooks and crannies of the coastal town. The species, which can grow up to 15 cm in size and ranges in color from violet to bright cerulean blue, is considered critically endangered or vulnerable in this region, although it may be classified as invasive elsewhere. Historically, it lived in the region’s rich mangrove forests, many of which are now urbanized – habitat loss that scientists blame for the crab’s decline.
But when scientists studied the species’ distribution around Sapzurro Bay, they were surprised to find that the species still thrived in areas where vegetation had been eliminated: creeping into meadows, banana and coconut plantations, and scurrying under concrete structures. Although the dens were fewer and smaller in urban areas, it had successfully built houses along sewer channels and between houses.
A growing number of studies are collecting data on species like this crustacean – endangered wildlife that is learning to thrive alongside humans in urban areas.
“We often forget that we are dealing with living animals,” says José Marin Riascos, a marine ecologist at the Corporation Center of Excellence in Marine Sciences of Colombia, who published the blue land crab study in April 2024. passive, they are active. If you change something, they respond with a new change.”
These findings also complicate the long-standing idea that cities cannot be hotspots for animals and plants, and that conservation is something that must be done far away, in pristine places.
“We assume that when humans modify an ecosystem, habitat for biodiversity is lost,” says Riascos. That is not always the case, he says. In some contexts “it’s just changing.”
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In general, cities have an overwhelmingly negative impact on nature. If a region contains 100 species, on average only 25 would occur in the city, and populations could be up to 92% smaller than outside the urban area. But among the portion of wildlife that remains, there are also some species that actually do better in cities than outside of them. This group can provide useful insights into how animals can adapt – or not – to human spaces, but especially how humans can adapt their cities to become more nature-friendly.
Research has shown that 66 of the 529 bird species that live in cities occur only in urban areas. In Santo Domingo, Dominican Republic, endangered Spanish-speaking parakeets hold their own in urban green spaces and old buildings. Across North America, round, downy burrowing owls have found new dens in cities everywhere. Three endangered species of cockatoos in Australia – Baudin’s black cockatoo, Carnaby’s black cockatoo and the forest red-tailed cockatoo – have adapted to munching on urban pine plantations. In London, peregrine falcons have found imitations of tall trees in tall buildings.
“This is definitely something we’ve been ignoring,” said Erica Spotswood, an urban ecologist at Second Nature Ecology + Design in California. In a 2021 study published in the Journal of Bioscience, she argues that cities can provide various services to the surrounding nature.
Building a city and having places for people to live does not mean losing nature and wildlife; we can have both
Kylie Soanes, urban ecologist
“We’ve created cities in places we like, along rivers, along the coast, in alluvial plains, at the bottom of valleys,” says Spotswood, and human preferences overlap with those of many species. This means that cities ultimately also have a wide variety of habitat types, and a lot of diversity.
Urban spaces can provide a refuge during periods of stress and wildlife scarcity, allowing easier access to an abundant diversity of resources year-round. And cities can help species escape the threats that animals face in the surrounding landscape – for example, by offering pollinators a refuge from pesticides that are systematically spread across farmland.
Research has found that several species of native bees are more abundant and diverse in cities than in the surrounding landscape, and that cities can be hotspots for some pollinators. “Bees are a good example of this,” says Robert Francis, professor of urban ecology and society at King’s College London. “The growing season for plants is extended in cities, so there are more plants over a longer period of time and the resources are really good.”
The smalltooth sawfish, once abundant in North American waters, now thrives only in the urbanized coastal waters off the coast of South Florida, according to a 2020 article. And research shows that endangered western ringtail possums have found refuge in residential yards across Australia, even if they have access to more rural areas.
When builders went to fill in an abandoned industrial estate in Sydney to build a new stadium for the 2000 Olympics, they discovered the dirty water was filled with green and gold bell frogs: plump, cartoonish-looking amphibians that are being hunted to extinction across Australia threatened.
“Everyone thought it was a horrible, degraded urban area, but it turned out to be a crucial habitat for these frogs that were there,” says Kylie Soanes, an urban ecologist at the University of Melbourne.
Like this frog, the city is home to “the vast majority” of endangered species found in cities, and humans are built on top of it. But this also means that the city is the last place for the species to gain a foothold – a category of animals that Soanes calls the ‘last chance species’ – so urban spaces are a crucial opportunity to protect and conserve them.
Doing this requires overturning the assumption that cities cannot be places for conservation, she says, and recognizing that they offer opportunities for intensive human stewardship.
Related: The world’s cities go head-to-head in the race to spot the most urban wildlife
“Building a city and having places for people to live doesn’t mean we have to lose nature and wildlife – we can have both in the same place,” says Soanes. She points to growing more nature-friendly plants in private gardens and spreading more nature-supporting infrastructure, such as bird nest boxes, bee hotels and frog ponds outside homes and around the city, blurring the boundaries between urban and natural.
In Brazil, the Programa Macacos Urbanos has built sky bridges of wood and rope over roads to prevent monkeys from electrocuting themselves by swinging from power lines. In Britain, building products manufacturers have started making ‘swift bricks’ – plastic bricks designed for swifts to nest in, and bus stop roofs have been converted into small patches of grass called ‘bee stops’.
“It just saturates the city with biodiversity-friendly things: green space and green infrastructure,” says Francis, who lives in a residential area where some buildings have bat nest boxes built into them. However, he notes that it is too early to know whether these small changes will make significant differences in urban animal communities, at a scale large enough to support population growth over generations and also reshape all surrounding landscapes. populate, “or that it is only a very small difference”.
But, says Francis, “Recent research on cities has really changed our understanding of urban ecosystems.”
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