Urbanisation's varying impacts on ecosystem services

Loss of green spaces can exacerbate extreme flooding events, the study shows

Different urbanisation policies have varying impacts on a region's ecosystem services, researchers report.

Dense housing leads to an increase in concrete and asphalt, reducing areas' flood mitigation services, they say.

And low density housing does not affect flood mitigation services but does reduce land availability for food and carbon storage, the UK team adds.

The study was presented at the British Ecological Society's (BES) annual meeting at the University of Sheffield.

"Predicting exactly how cities are going to grow was extremely difficult because every city does it a little bit differently," said co-presenter Felix Eigenbrod from the University of Southampton, who was part of a University of Sheffield research team during the study.

"So what we did was to look at what we saw as the two extreme, yet realistic scenarios."

The two scenarios - "densification" and "sprawl" - were based on classifications used in the UK Land Cover Map, in which urban areas were divided into either "dense urban" or "suburban" housing.

"We assumed under the 'sprawl' scenario that in order to accommodate the growing population, the majority of people would want to live within an area of suburban housing densities rather than dense urban conditions," Dr Eigenbrod told BBC News before his presentation to the BES meeting.

"Then it was possible to calculate how much extra land was required in order to accommodate the increase in people."

High, rising population

Under the "densification" scenario, this process was reversed.

The team figured that densification policies would firstly try to increase the density of existing cities, meaning that suburban areas would become locations with dense urban housing.

"This would mean that there would be less or no green space, but the cities would not expand in order to accommodate the population growth," Dr Eigenbrod suggested.

Within the UK, National Statistics projections estimate that the population will grow by 16% by 2031 from 2006 levels, reaching about 70 million people.

The team looked at what impact the two scenarios would have on three ecosystem services:

• Agricultural production

• Carbon storage in soil

• Flood mitigation.

Dr Eigenbrod explained the basic process behind the team's modelling: "If a city expands, then that land is no longer useful for agriculture, so production would go down.

"We did the same thing for carbon storage - by constructing new houses, you are disturbing the soil, which will basically remove the stored carbon."

However, assessing urbanisation's impact on flood mitigation services presented more of a challenge, he added.

"Two of the team members were hydrological modellers from the Centre of Ecology and Hydrology (CEH), which has produced a Britain-wide state-of-the-art large scale hydrological model.

"One of the things that goes into that model is the percentage of land that is either suburban land cover or high density land cover."

Each classification has its own dynamics in terms of flood peaks and flow rates as a result of the amount of the area covered by impermeable surfaces, such as concrete and asphalt.

It also has to take into account the type of drainage and sewerage systems.

"If you have these high density areas where you have much less green space," Dr Eigenbrod said, "the water will run off much more quickly and contribute to flooding - much more so than in suburban areas where you have a lot of gardens and parks where the water can go into the soil for longer periods, so you do not get these flood peaks."

City trade-offs

Dr Eigenbrod said the scenarios did seem to reflect current policies.

The researchers identified that each of the urbanisation scenarios had a number of trade-offs

In recent years there has been media coverage and campaigns against "garden grabbing", because these sites were deemed to be "brownfield sites" and not subject to such tight planning controls as greenfield sites.

However, he did say it was too early to judge whether the current government's proposed changes to the planning process would lead to an increase in "sprawl"-type urban development patterns.

But he did add that the issue of urban ecosystem services was an under-researched topic.

"The thing about ecosystem services is that you really need to look where people are because, by definition, in order to have an ecosystem service you need to have people benefiting from the natural environment.

"So things like parks often have enormous value in terms of ecosystem services, just because they are used by so many people. And it has been shown that there are really strong health benefits from even walking around parks.

"So in these areas, there are these places that may not be overly impressive in terms of biodiversity but have these huge human benefits."

But the study showed, he added, that there were trade-offs. "If you have these bigger cities with a lot of green space, then they are bigger. So then, of course, land that could be used by wilder natural habitats or for agricultural production is lost.

"The challenge is to have smart cities that are quite dense so they do not take up too much space and do not have the disadvantages of sprawls (longer commuting distances etc), but still have enough green spaces that give all these benefits for people living there, including flood mitigation services.

"There is an opportunity here, with clever planning, to try and maximise the benefits identified from both scenarios."