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Pigeons in the city
Pigeons in the city

What can the evolution of urban animals teach us about our cities?

From detoxifying pigeons to urban blackbirds, evolutionary biologist Menno Schilthuizen speaks about urban animals and how we can support biodiversity through design

 

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Isat down with Menno Schilthuizen, evolutionary biologist and author of When Darwin Comes to Town before his lecture for the UK Green Building Council on the rapid evolution of urban animals and what it means for our cities.

 

In this interview, Schilthuizen describes how species such as the blackbird have adapted to city life, how we can support greater biodiversity by design, and why he’s not sure humans will stick with city life – after all, we’ve only been an urban species for 1,000 years...

 

 

In your book, Darwin Comes to Town, you describe a number of species that are changing in urban habitats. Is it surprising that species are adapting to our cities?
I’m an evolutionary biologist, and in evolutionary biology you are often forced to just infer evolution based on patterns you find in nature or fossils. You cannot watch evolution taking place, see it in action, but since the 1960s we have realised there are situations when species can evolve so quickly that we can see it happening in a human lifetime. Those are usually situations when the environment changes drastically – and that’s of course the situation in cities, where all of these human-induced environmental changes come together. You have an entire ecosystem that is in the process of evolving rapidly and changing rapidly. I wasn’t surprised by this rapid evolution, but I still find it very exciting when, as an evolutionary biologist, I can actually observe a species changing and evolving, especially in our own streets and backyards.

 

I was struck by the evolution of pigeons described in the book – they’re considered a pest in the city, and in a way we’ve forgotten that they once had a wild habitat.

Pigeons are so common that we almost don’t notice them any more. We don’t even consider them wildlife – we consider them furniture, or worse. But pigeons are originally a wild bird and they still live in the wild – their ancestors live in rocky environments in the south of Europe, Greece and Italy. So it’s not actually surprising that they do so well in the city, which is basically a collection of artificial rocky cliffs. Of course, they have also been domesticated, having evolved to live close to humans.

 

They are one of the most widespread urban birds, and live in every city all over the world, from the tropics to the poles, and they’re ideal subjects for studying urban evolution. There is one set of studies being carried out by a researcher in Paris who is looking at the colour of the plumage in the city pigeon, which can range from pale grey to a really dark, sooty grey. She’s found that the darker forms of pigeons are better at detoxifying themselves because heavy metals like zinc and lead bind to the melanin, the pigment in those dark feathers, so the pigeons that are genetically predisposed to make darker feathers are also the ones that are better at getting rid of the heavy metals in their system. In big cities where there’s a lot of pollution, metals like zinc are just constantly flaking off street lanterns and electricity pylons, and these bigger cities have a high proportion of dark-feathered pigeons. So pigeons, which are already adapted to being a city bird, are getting even better at it by making themselves darker.

 

“The darker forms of pigeons are better at detoxifying themselves from heavy metals like zinc and lead”

 

The species you described as having evolved the most was the blackbird – can you tell me about what you found?

Pigeons are just changing one aspect of their appearance. We’re not really talking about the evolution of a completely new species. However with the urban blackbird, which was one of the first birds to colonise cities more than 200 years ago, all kinds of characteristics have changed.

Urban blackbirds have shorter beaks than the forest blackbird, they have shorter digestive systems, they sing at a higher pitch, they sing earlier in the day, they don’t migrate any more, they breed earlier in the year, they have a more relaxed personality – and that is genetic because it has to do with serotonin receptors. Because there’s not much interbreeding between the urban and the forest blackbirds, the urban blackbird might be on the way to becoming a separate species. It’s not there yet – it’s very early stages – but in a few hundred years it could be.

 

What other characteristics help animals adapt to urban life?

It can be all kinds of things. For example, cities are heat islands, so being tolerant of higher temperatures is something we find in some urban animals. There are many organisms like lizards, for example, that in the natural world would scramble up trees and branches, and in cities adapt to be able to cling to smooth, human-produced artificial surfaces such as rain pipes, windows and iron. You see changing limb shapes.

 

But more important is the fact that cities are where animals and plants live together which have not evolved to be together. An urban ecosystem is built up from species that come from all over the world, it’s a melting pot. The human trait has brought them together. And these species have to evolve ways to build a food web together. Wherever those species came from is irrelevant to the place where they now live together. One important species in that whole system is us, so this tolerance for humans is something that is evolving in many urban animals.

 

“The urban blackbird might be on the way to becoming a separate species. In a few hundred years it could be”

 

Are there negative characteristics that mean a species will fail to adapt?

It’s a bit hard to predict which species will thrive in a city and which will not. Some species which you wouldn’t expect to be common in cities somehow manage to adapt and live with us.

 

I think you would find that urban species often come from naturally dynamic habitats, like floodplains, for example, where there is lots of change. Forest fringes also – a lot of crows come from those dynamic edges of forests. They are adapted to live in an environment that is constantly changing, so that helps them adapt to the city. In larger animals like birds and mammals, living in that sort of environment makes them better at dealing with new situations.

 

In cities, there is constant change. There are new ways that we package food, for example. So problem-solving and being interested and curious about new things are personality traits that you see in urban animals. It’s certainly true that there are many species that will probably never be able to become an urban species. It’s sort of an elite group of species that make up this urban ecosystem that you find in almost every city in the world. Biologically that’s very interesting, because there’s never been a situation on Earth where you have a global new habitat that is distributed across the world and shares the same species. Usually there are barriers: mountains or oceans that prevent the species from crossing. That’s why you find a completely different set of species in a forest in Madagascar compared to a very similar looking forest in Borneo... But cities are a new environment that seems to be homogenised all over the world, and of course we transport species as well. In addition, species that are well adapted to city life can easily then colonise another city.

 

“Don’t only think of parks and green spaces, but also of creating very small pockets of vegetation”

 

It also means that a lot of biodiversity gets lost because you find more or less the same kinds of species all over the world. Urban biodiversity is not the best way to conserve global biodiversity. You’re still going to need the conservation of completely natural environments as well.

 

I was thinking that’s the danger of your book – if it supports thinking that animals will adapt to the city, does that mean we don’t need to preserve natural habitats?

That is only true to a very limited extent. There will be, as time goes by, more and more species that will be able to colonise cities. But that’s never going to replace or compensate for the loss of species on the global scale – we will still have a relatively limited set of species living in cities.

And many species will still need reserves of pristine environments. You need to do both things: maximise ecosystems in anthropogenic environments, which are very much dominated by people, and at the same time, preserve pristine environments as much as we can to preserve species in there.

 

Do you have advice for city-makers and designers on how to preserve biodiversity?

One of the most important things is not only to think on the large scale, We still tend to think of large animals, like mammals, birds, trees and shrubs, but those are just a relatively small part, albeit a very visible part, of the urban ecosystem. The majority of species that will make a healthy and rich urban ecosystem are much smaller and usually not noticed by people – they’re insects, snails, and spiders, mosses, lichens and small plant-like organisms that you wouldn’t notice. But these make up the bulk of the species and the most important part of the food web.

 

For that, you need to manage vegetation and provide places where they could settle on a very small scale, don’t only think of parks and green spaces, but also of creating very small pockets of vegetation. I think creating new building materials that would allow these species to settle is something that is going to be very helpful. You may not want insects in your house, but you could have them on your house. Roofs and buildings can be habitats for very rich communities of small animals and plants.

 

When you look at old buildings you see wildlife in them; you see moss growing on the roof, or bees nesting in the mortar, and usually that interferes with the function of the building – the roof in the end collapses or gets too heavy from all of the moss, the mortar gets soft and water gets in.

 

But nowadays we are able to design building materials that allow plants and animals to settle without impairing the function of the material itself. If we can work with those sorts of solutions, then you can get a very rich ecosystem which includes the small organisms I was talking about. This would allow people to live sustainably and durably in those buildings, and enjoy the presence of those urban ecosystems.

 

“I think managing the species that live in the cities in general is a bad idea; let these communities develop naturally”

 

What else do you think people who design cities could do better?

There’s a movement to increase vegetated surfaces on and around buildings – things like green walls and green roofs, but usually those plants are selected from the catalogue of a garden centre. If, instead, you would simply create the spaces for trees and shrubs to take root naturally, then you would automatically select species that are already present in cities and let them colonise those spaces. I

 

f they grow then they are the right species, so let the vegetation appear naturally rather than pre-designing them. I think managing the species that live in the cities in general is a bad idea; let these communities develop naturally.

 

We place all kinds of values on species that don’t have any ecological basis. We make choices that are to do with size, with being furry or feathery; we value native species higher than exotic species, even though they may serve the same ecological role. I think it’s important to try and minimise those feelings as much as possible, and to value every species for its role in the ecosystem. Try not to let things like size or the taxonomic group, or the region from which they come, play a role in how we value them.

 

Why is it important to bring nature into the city?

In general, it’s known that people feel happier if they have day-to-day contact with wild animals and plants. You can also train people to also notice the small ones, so that people can become happy if they see a bumble bee – it doesn’t necessarily have to be a fox.

 

And even if you’re only interested in the large ones, they usually need a food web composed of small ones to survive. Many of the birds eat insects, so you need a food web made up of many species of insects that can maintain those birds. Even if you don’t notice them, they’re still there, and they need to be there to maintain other parts of urban nature.

 

We’ve heard in recent years how important nature corridors are in the city – how railway lines or canals can provide pathways for nature. But you question whether this is always the right approach. Why is that?

I think that’s also an example of how policies are shaped by what we know about large animals, and not so much about small animals, or even medium-sized animals.

 

In my book, I mention that connecting every green space in a city is not necessarily a good thing. For example, there is a detailed study on white-footed mice in city parks in New York, where every park harbours an isolated population of these wild native mice.

 

Genetic studies have shown that each of these populations is adapted to the specific conditions of the park where it lives. So the Central Park mice are perfectly adapted to Central Park and Prospect Park mice as well – and they are different. Prospect Park has less junk food, so the diet of the mice has also diverged.

 

If you connect those two parks, and create a corridor between Central Park and vice versa, some of those optimal adaptations would be genetically polluted by the non-adapted mice. So for smaller organisms which can have a healthy population even in fragments of green, you may prevent local adaptation or undo local adaptation.

 

“I often get the question, ‘Will we also evolve over time into urban species?’ I think we probably will”

 

For many larger species, they have so few individuals in a fragment of forest in the city that you can get problems with inbreeding, and what they call a loss of genetic variability, which makes them vulnerable to diseases, so a corridor is useful. But if you are a small organism and can have thousands of populations in a small fragment, those problems don’t play a large role.

 

You mention that whether humans continue to live in cities is “a big if”. Why is that?

Well, our urban environment, the way we live, is just as extreme and new for us, as it is for the animals and plants that live in cities. We have also not evolved to live in such dense concentrations of people. We’ve only been doing that for a few thousand years – that’s nothing in evolutionary terms. Our bodies and our minds are not adapted optimally to living in these conditions, which means that probably many of the problems we are facing now in city life - in our psychology and sociology – have to do with the fact that we live in an environment that we are not meant to live in.

 

Maybe this will change. I often get the question, “Will we also evolve over time into urban species?” I think we probably will, although we don’t have such a high potential for evolution as the wild species. Firstly, because our generation time is quite long, an urban insect will have two or three generations in a year, but for us that takes much longer. We need 25 years for a new generation. Plus, fortunately, we’re able to keep most people alive, while wild animals have a lot more death and death creates selection, and selection creates evolution, so we may not be able to evolve and adapt ourselves to the city we have created, as far as wild animals and plants can.

 

What is your best hope for your book?

I hope that people will realise that evolution is not a very abstract process, that it happens all the time, everywhere around you, in your own backyard, in the street where you live. There are all kinds of citizen participation projects you can get involved with to watch urban species and study how they adapt and how they evolve. I hope my book is contributing to that, and that you start noticing these things, and that just walking down the street is going to be so much more fun when you see those pigeons and realise they have dark feathers because they have adapted to heavy metals, and when you see those mosquitoes in the Underground that are almost a new species. Realising that you are walking in a real-time, very exciting biological experiment is something that people could definitely benefit from.

 

Transcript by Lewis Duncan

 

Listen to the full interview in the podcast below by clicking on the link and sign up to The Developer Weekly to be updated when new episodes go online.

 

 

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