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Page last updated at 09:06 GMT, Thursday, 8 October 2009 10:06 UK
How cities drive plants extinct
by Matt Walker
Editor, Earth News

Tree in New York City
Green shoots of recovery in New York City?

How towns and cities cause the extinction of local plants has been revealed for the first time.

An international team of botanists has compared extinction rates of plants within 22 cities around the world.

Both Singapore and New York City in the US now contain less than one-tenth of their original vegetation, reveals the analysis published in Ecology Letters.

However, San Diego, US and Durban, South Africa still retain over two-thirds of their original flora.

Both the pace of urban change and how many plants remain in a city are good predictors of whether plant species will survive there in the future, says the report.

"The rapid and ongoing growth of cities and towns significantly threatens global biodiversity," says Dr Amy Hahs, a scientist working at the Australian Research Centre for Urban Ecology at the Royal Botanic Gardens in Melbourne, Australia.

So Hahs and colleagues from universities in Australia, New Zealand, Singapore, the UK and US came together to try to understand how this process occurs, in order to find ways to prevent it happening.

We need to consider vegetation as a long-term investment rather than as a disposable asset
Dr Amy Hahs

For the first time, the team compiled raw data on plant extinctions within 22 urban areas, organised into three categories.

First, they examined cities in which the native flora started to be transformed more than 400 years ago.

Almost all are European cities, including Glasgow in the UK, Vienna in Austria, and Zurich in Switzerland. Hong Kong is also included as it experienced extensive landscape transformation prior to 1600AD, before urbanisation.

They also categorised cities in which the native flora started to be transformed after 1600AD, but before any floral surveys could be completed. Such cities include New York City and Chicago in the US and Auckland, New Zealand and Singapore.

Grassland surrouding Melbourne
Urban expansion threatens Melbourne's native grasses

The third category included cities that had large areas of native flora at the time they were surveyed, but subsequently were transformed by urban development. Los Angeles and San Diego in the US, Melbourne and Adelaide in Australia and Capetown and Durban in South Africa were included here.

By doing this, the researchers were able to unpick how the history of a town or city influenced how many plants became extinct as a consequence.

Future threats

Cities belonging to the first two categories had by far the highest extinction rates.

"For example, Singapore and New York City have both lost 600 species of plants from an initial species richness of 2179 and 1361 species respectively," says Dr Hahs.

However, in general, cities with more than 30% vegetation cover had much lower extinction rates.

For example, San Diego and Durban both retain over 60% of their native vegetation cover, and have both only lost only 11 species each from an initial floral diversity of 926 and 2218 species respectively.

"Under current planning and design practices, it is very hard to maintain 30% native vegetation within an urban area, but finding ways around this problem either through innovative design or restoration will help preserve local biodiversity," says Dr Hahs.

Singapore Parliament building
Singapore, a pleasant but less green land

The analysis also revealed that more recently built cities can expect to lose a significant proportion of their native plants over the coming decades.

"The cities we expect to lose the most plant species over the next 100 to 150 years are Melbourne and Adelaide," says Dr Hahs.

Indeed, unless more is done to intervene, the researchers expect Melbourne to lose more than half its 1200 original plant species over the next 100 years.

"If we want to keep plant diversity in our cities, we need to protect and restore areas of native vegetation," says Dr Hahs.

"Plants and people can coexist in urban areas. We just need to consider vegetation as a long-term investment rather than as a disposable asset."

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