Image caption Although far fewer species were found in cities, the authors said it was wrong to view them as urban deserts

A dataset, described as the largest of it kind to date, has assessed the impact of urbanisation on biodiversity levels around the globe.

It found that cities supported far fewer species of birds and plants compared with similar areas of undeveloped land.

However, it showed the vast majority of flora and fauna in a city reflected an area's "unique biotic heritage".

The findings appear in the journal Proceedings of the Royal Society B.

Image caption Green spaces provide an "important refuge" for native and migratory species, the team observed

"While we show that urbanisation has caused cities to lose large numbers of plants and animals, the good news is that cities still retain endemic native species, which opens the door for new policies on regional and global biodiversity conservation," said co-author Myla Aronson, a research scientist from Rutgers University, in the US state of New Jersey.

"In particular, the study highlights the value of green spaces in cities, which have become important refuges for native species and migrating wildlife."

The international team of researchers collected lists of plants in 110 cities and lists of birds found in 54 cities around the world.

"We were interested in plants and birds that were found in all parts of the city - from buildings, roads, vacant lots, brownfields, managed greenspaces and natural greenspaces - to understand how cities support biodiversity as a whole," Dr Aronson told BBC News.

Unique biotic heritage

The team analysed the data and found that cities retained about just 8% of bird species and 25% of plant species of comparable undeveloped land.

But Dr Aronson added: "Contrary to popular belief, we show that the plants and birds of cities are not all the same across the world.

"Owing to the fact that cities around the world share similar structural characteristics - buildings, roads etc - it is thought that cities share a similar biota, no matter where they are in the world.

"Few species are shared across cities, such as pigeons and annual meadow grass, but overall, the composition of cities reflects the unique biotic heritage of their geographic location."

She said the data revealed that, overall, cities supported close to 20% of the world's bird species and 5% of known plant species.

"Conserving green spaces, restoring natural plant species and adding biodiversity-friendly habitats within urban landscapes could, in turn, support more bird and plant species," Dr Aronson suggested.

Commenting on the study's findings, Prof Philip James, of the University of Salford - who was not involved in the research - said that many cities grew in areas that were diverse and rich in natural resources, plants and animals.

"So the challenge is to use our knowledge of urban ecology to enrich the lives of the ever-increasing number of people living and working in cities," he told BBC News.

"Seeing birds from our windows, hearing their songs and having pleasant, natural places to walk are all beneficial to our health and well-being."

Dr Aronson said that by improving the understanding of the ecology found in urban areas, city planners and managers would be better placed to preserve and promote biodiversity, which provides important ecosystem services, such as water quality and flood protection.

She added: "Of course this is a coarse-scale study but we still come up with patterns that are similar in cities around the world."