"They took all the trees, put 'em in a tree museum" went Joni Mitchell's 1970 song Big Yellow Taxi, ruminating about the loss of nature.

Forty-five years later, scientists have warned that urban nature might not be replaced with museums, but digital equivalents such as images and sound recordings.

This virtual replacement of nature has been listed as a potential future threat to city ecosystems, along with some other possibilities as surprising as cremation ashes, drones - and even energy-efficient homes.

"We don't want to throw the baby out with the bathwater - some of these new technologies bring a range of environmental benefits," Dr Margaret Stanley said.

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"But clever solutions are going to be needed to mitigate threats to urban biodiversity if we are to maintain our connection with nature as we become increasingly urbanised."

With her colleagues at Auckland University's School of Biological Sciences, Dr Stanley brought together experts from Australia, the UK and New Zealand to identify current trends and new technologies that pose the biggest threat to urban ecosystems.

Their study, published in the journal Frontiers in Ecology and the Environment, pointed to growing evidence the natural world was a benefit to human health and well-being, particularly if more of us were going to be living in cities in the future.

Dr Stanley told the Herald that most of the threats identified in a Top 10 list applied to Auckland - including digitally replacing nature, as has already been exemplified in static displays of forests in Britomart.

In another example, greenery was being removed to make parks and other places because of safety and security issues.

The research follows another major study, authored by Dr Stanley and published earlier this year, which showed central Auckland was losing what urban tree cover it had.

Across the isthmus, the amount of tree cover was just 6 per cent, and of that, 63 per cent of the trees were on private land and 15 per cent - few of them native - were protected.

The loss ran against international studies finding how plenty of green spaces in cities could improve the health of pregnant women, speed up recovery times among hospital patients and cut the use of anti-depressants.

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They also sucked hundreds of tonnes of pollution from the environment each year, reduced stormwater run-off, and served as crucial "wild-links" that helped birds and pollinators move between food sources and habitats.

In August, the Herald investigated potential solutions to the wider threats that faced New Zealand's largest city, as part of its World Class Auckland series.

The Top 10 potential threats facing nature in cities today

Health demands on green space:

As more people are encouraged to use green urban spaces for exercise, these spaces could become highly maintained for people rather than wildlife; with more tracks, artificial lighting and fewer plants.

Digital replacement of nature: There is a risk that nature in cities could be replaced with digital equivalents of nature, such as images and sound recordings.

This gives people some of the benefits of nature, but without the maintenance and messy side of nature, however, it could lead to city dwellers undervaluing nature in their immediate environment.

Scattered cremains: There has been a growing trend for cremation as space for burial of human remains is at a premium.

However, in some cities land for interring "cremains" has become very expensive and scattering cremains has become more culturally acceptable.

Because of high levels of phosphate and calcium in cremains, there is a risk of polluting urban ecosystems and waterways.

Spread of disease by urban cats: Globally, there are more than 600 million pet cats, and the increase in pet cat ownership is resulting in the disease toxoplasma spilling over into wildlife populations, in urban areas as well as to species in more remote locations, such as the endangered Hector's dolphin.

Switch to LED lights: Cities across the globe are switching their lighting technology to LED lights. However, the whiter spectrum of LED lights overlaps with the visual systems of wildlife and can disrupt their physiology and behaviour.

Solar cities: Many cities are implementing city-wide solar panel installation programmes. However, solar panels can disrupt the behaviour and reproduction of animals that are attracted to the polarised light they produce.

Nanotechnology: Nanoparticles, such as graphene, are now an increasing but invisible part of cities, found in everything from smartphones to clothing. However, there has been almost no research on the effects of these particles on animals, plants and entire ecosystems.

Self-healing concrete: This is a new concrete product infused with specialised bacteria that is about to be commercialised. If use of this product becomes widespread, it could spell the end for the often unique biodiversity that manages to thrive in cracked concrete all around cities.

Energy-efficient homes: Many countries are implementing large-scale retrofitting of buildings to make them more energy efficient. However, this effectively seals the building off from the outside, resulting in loss of breeding sites for wildlife such as bats and nesting birds.

Drones: The recent popularity of using drones (unmanned aerial vehicles) in cities is likely to result in issues for wildlife, such as nesting birds, which are particularly sensitive to stress and repeated aerial disturbance.