The debate: Is hostile architecture designing people – and nature – out of cities?

It looks like something designed to stop inmates from escaping a prison complex. But this shocking photograph of metal spikes covering the branches of a tree was actually taken in a wealthy residential suburb of Bristol, England. Their purpose? To prevent birds from perching on the branches, and defecating on the cars below.

Despite outrage surrounding the spikes exploding on social media this week, they will remain. “There’s nothing the council can do because it’s on private property and there’s no tree protection order in place,” a spokesperson for Bristol City Council told CNN.

London, United Kingdom —

Hostile architecture is a form of urban design that aims to prevent people from lingering in public spaces. The anti-homeless spikes here, for example, were installed to deter beggars and those sleeping rough.

Washington DC, United States —

The armrests on this bench also prevent people from sleeping on it.

London, United Kingdom —

A rusty garbage bin at the end of this bench makes it an unpleasant environment for those hoping to rest on it.

Philadelphia, United States —

Design firm Veyko created these benches for Southeastern Pennsylvania Transportation Authority's train stations. The wire-based seats are said to be impervious to vandalism and other types of damage -- though they also look hard to sit on.

Dusseldorf, Germany —

This men's room is illuminated with ultraviolet light, which makes it impossible for heroin addicts to find a vein. However, the uncomfortable lighting doesn't only deter drug addicts and dealers -- it also creates an unpleasant atmosphere for other visitors.

Washington DC, United States —

Sometimes hostile architecture is subtle. Instead of unwelcoming armrests, this wooden bench is designed with a curved base, to prevent users from lying down on it.

London, United Kingdom —

Photo of a sloped bench design captured by Cara Chellew, a Toronto-based researcher who focuses on the design, regulation and politics of public spaces. She compiles photographs of defensive architecture on her website #defensiveTO.

Paris, France —

Besides benches, hostile architecture can come in other forms -- like this window sill with anti-loitering spikes, which stop people perching on the concrete ledge.

Paris, France —

Metal spikes on the bench are designed to prevent skateboarders.

Paris, France —

They also stop people from using it for long periods of rest.

London, United Kingdom —

Crisis UK, a charity for homeless people, took photos of this hostile architecture outside a casino on Wardour Street, in the capital's busy Soho district.

Colorado, United States —

The Federal Reserve Bank of Denver, surrounded by fences and spikes.

Houston, United States —

Countless stones have been placed outside this car park -- the sharp rocks make it impossible for homeless people to camp here.

Bromley, United Kingdom —

Produced by UK firm Factory Furniture, the FLO granite was designed to prevent criminal and antisocial activities. The undulating top makes it awkward to lie on, while the smooth surface means there are no slots or crevices in which to hide drugs.

United Kingdom —

Also a Factory Furniture design, the Scroll Seat has been installed across the UK. Its armrests make it impossible to lie down.

London, United Kingdom —

The Serpentine bench's curves deter skaters and rough sleepers alike.

Boston, United States —

Perhaps the antithesis of hostile architecture, this solar-powered phone charging bench was created by US firm Soofa. Launched in Boston in 2014, Soofa benches are now found in more than 100 cities. This new technology changes how people spend time in public areas and encourage dwelling.

The rise of hostile architecture

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This installation is a prime example of hostile architecture – a controversial type of urban design usually aimed at preventing people from using public spaces in undesirable ways. It can come in the form of spiked or sloped benches, bolts installed on shop doorsteps and windowsills, and even water features that operate at surprising intervals on flat surfaces.

While use of design to block homeless people from finding a place to sleep or young people from skating in public angers many people, others feel it’s a necessary measure to secure public safety.

Dean Harvey (L) and James Furzer (R).

CNN invited architect James Furzer, whose designs try to combat hostile architecture, to debate this issue with Dean Harvey, co-founder of the Factory Furniture: a company that produces many of the offending benches.

Read: Is “female architect” an offensive title?

Here’s how their Skype conversation went.

You’ve both seen the shocking tree spikes in Bristol. What is your reaction?

James Furzer: This is totally hostile architecture. Hostile architecture doesn’t affect humans only – the landscapes we design can also be manipulated to deter all sorts of acts. It’s such a shame that it now applies to nature.

Not only does it look unattractive and aggressive, it shows the selfish nature of society and its thoughts on the environment. Has anyone considered who takes the blame if children wish to playfully climb the trees, unaware of the barbaric devices situated on top of the branches? (Or) if a branch falls, with an unsuspecting person relaxing beneath?

Dean Harvey: My initial thought is that this cannot be considered an extension of hostile architecture: spikes have been used to protect damage to stonework from bird droppings for many years.

This is an extension of pest control. I think the issue here is the damage to the ecosystem of the tree and the aesthetic problem it poses. It’s totally unnecessary. A tree is a natural object, and this (the addition) is a complete eyesore. The time and effort they spent putting those spikes on trees is probably totally outrageous in comparison to any actual problem.

How do you define hostile architecture?

Dean Harvey: