Cities can be deeply unwelcoming places for wildlife. They are noisy, difficult to get around, full of people and heavily reliant on artificial lighting. Yet some species do better in urban areas than in rural ones.
Research is showing that animals of the same species that live in cities and the countryside are behaving differently. These disparities will probably grow since
over half of people worldwide now live in urban areas, and cities and towns are getting bigger.
A recent study from Tel Aviv University found that Egyptian fruit bats living in urban parts of Israel gave birth two and a half weeks earlier than rural populations. This gives them an advantage as they are more likely to reproduce twice per year.
In the urban areas in the study there was a higher abundance and diversity of fruit trees. In Tel Aviv, for instance, the trees are watered. This means there is fruit for a longer period across the year, meaning more reliable food supplies for the bats.
They may also be benefiting from the urban heat island effect, with warmer temperatures reducing the harshness of the winters felt by their rural neighbours.
Most species perceive humans as predators, so our presence disturbs and distracts them from feeding and breeding. To survive in human-dominated cities, animals must therefore be bold.
This is something researchers have studied for a while in wildlife like foxes. Urban foxes are often more confident in their response to new food when it is presented in a novel object like a puzzle box.
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Urban birds, from robins to feral pigeons, are also bolder. In a 2008 study scientists found that urban birds are more tolerant of human disturbance than rural ones), allowing humans to approach them closely.
The birds that reacted less to approaching humans were descended from a large number of generations since urbanisation, showing a long history of adaptation. This behavioural change helps these animals to adjust their stress responses when they are exposed to new situations. If they did not do this, they would suffer with chronic stress.
To test whether this boldness in birds is due to evolutionary adaptations, one 2006 experimental study in Germany hand-raised blackbird chicks taken from both an urban centre and a nearby forest.
They kept all the birds in the same environment until they were adults and then tested their acute stress responses when the birds were caught and handled. The birds from the city had a lower stress response, suggesting that this difference was genetically determined.
However, urban birds tend to be less successful in raising chicks than those in more natural areas. Although birds can take advantage of food provided by people in many cities and towns across the world – whether directly in bird feeders, or by scavenging on our discarded food – urban areas do not provide enough of the invertebrate prey that many nestlings need.
One study published in 2020 found that the biggest challenge for urban great tits was the low abundance of nearby insects.
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Same species, different city
Many of these changes in urban species are difficult for people to detect, but one in particular becomes clear when you spend time in cities across the world. Have you noticed that whichever city you visit there seem to be many animals of the same species?
Scientists call this biotic homogenisation. It happens when places start to become increasingly similar over time with the species that you can find there.
This process begins with the exodus of species that cannot tolerate living alongside humans. Large mammals, often predators, are the first to go as an area becomes increasingly urbanised.
Then the non-native species begin to move in. Feral pigeons, rats, starlings and many other species are introduced by people over time, whether accidentally or deliberately, until a point is reached when the biodiversity found in one city, say in the US, starts to resemble another in Europe.
These species often have broader dietary and habitat niches, which makes them good at exploiting urban areas.
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Urbanisation is continually changing our relationship with animals and how we perceive nature. Although scientists debate whether we have entered the Anthropocene (a new geological age based on significant planetary changes caused by humans) it is undeniable that humans have and still are moulding landscapes to suit our needs.
The growth of cities and other urban areas is set to continue, with future urban expansion predicted to swallow 11-33 million hectares of natural habitat by 2100, an area the size of Norway. Indeed, humans are becoming the largest driving force in the evolution of wildlife.
Becky Thomas does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
