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Bat Colonies Overwhelming Australian Gardens

Elizabeth M. Tasker
for National Geographic News
October 29, 2002
 
The demand for real estate in Sydney and Melbourne is at an all-time
high, and not just among people. The grey-headed flying fox, a large
Australian bat that feeds on fruit and nectar, has established permanent
"camps" in the botanical gardens of both cities.

The thousands of flying foxes living year-round in the oldest groves in both gardens are stripping leaves and breaking branches, damaging—and even killing—some of the trees.

Both bat camps are increasing in size, escalating the need to find ways to minimize damage to the Royal Botanic Gardens in Melbourne and Sydney. At the same time, the Australian government has classified the grey-headed flying fox as a vulnerable species, as its numbers decline throughout much of the country.


This dilemma—how to save the bats and also protect the gardens—has conservationists and arborists searching for solutions.

Real-Estate Connoisseurs

Grey-headed flying foxes are normally a nomadic species, moving through the landscape as the seasons change and food becomes available.

Flying foxes have always been drawn to Sydney's center. There are reports dating as far back as the 1850s of the bats trying to colonize the botanic gardens. But it wasn't until the 1980s that they were allowed to stay.

The bats camp in the trees; during the day up to 9,000 of them can be seen hanging upside down on branches like some sort of bizarre hairy screeching fruit. At dusk they stream out of the gardens by the thousands to feed on fruit trees and eucalyptus flowers throughout the city's suburbs.

The grey-headed flying foxes also moved into the botanical gardens in Melbourne during the 1980s. Previously, they had shown little interest—flying foxes are essentially tropical mammals, and the temperatures were too cool for them.

However, the city has grown warmer. The growing lack of vegetation and extensive paved development—roads, sidewalks, parking lots, and buildings—have created a phenomenon known as the "urban heat island effect."

A recent study by ecologist Mark McDonnell, director of the Australian Research Centre for Urban Environments, suggests that this situation has contributed to the flying foxes taking up permanent residence in the city.

The Melbourne colony now has 5,000 permanent year-round residents, and swells to 20,000 bats in summer. It is the only breeding colony of flying foxes in the state of Victoria, and the southern-most colony in the world.

"The bats have a good eye for real estate," jokes Kerryn Parry-Jones, a bat researcher at the University of Sydney.

Changing Environment

Human changes to the environment are forcing the once-nomadic animals to settle down.

"From the 1970s onwards, there was a big movement to plant native plants [in urban areas] to attract birds. An unexpected outcome was the bats," said Peggy Eby, a flying fox researcher and independent consultant.

By planting large numbers of trees, many of which are winter-flowering, local councils and the public in effect created the bat equivalent of a year-round smorgasbord.

At the same time, the coastal forests that provided the bats with food in the winter are being lost to development at a rapid rate.

"The flying foxes use the botanic gardens to roost in because the planted rain forest groves provide humid and sheltered areas with lots of branches to hang from," said Parry-Jones. "It's ideal habitat for them."

Balancing Act

Ideal for the bats, but not good for the trees.

"Several trees have been lost, and some others are in sharp decline," said Patrick Houlcroft, environmental coordinator at the Sydney gardens.

Sydney garden officials have tried a variety of innovative methods to protect both the bats and the trees. They have installed "wailers"—suspended speakers playing loud noises—hung smelly substances such as rotten meat, prawns, and chili peppers on the trees; smeared the branches with python dung; and used water sprinklers to deter the bats.

So far none of the techniques has shown much success; the bats seem to be more curious than scared. Researchers studying the effectiveness of wailers in Melbourne observed some bats actually hanging from the speakers to investigate the strange sounds more closely.

Melbourne officials are not enthused by their bat colony, and last year authorized a cull of the flying foxes. This led to a public outcry, night-time vigils at the gardens by protesters, and extensive media coverage. The killing was subsequently halted, and non-lethal bat control methods are now being tried.

An attempt is also being made in Melbourne to relocate part of the colony. Bats are being trapped and moved to temporary cages at a new site. Officials hope their calls will attract other bats and encourage them to settle in the new environment. It is too early to tell whether the plan will work.

Battle for Survival

The increase in grey-headed flying foxes in urban areas camouflages an overall decline throughout the rest of their range, resulting from forest clearing and persecution by orchardists.

"The numbers of grey-headed flying foxes have declined by about 30 percent over the last ten years," said Eby.

This decline threatens not just flying foxes, but also native forests. Flying foxes are important pollinators and seed-dispersers. Because they can fly long distances, they are crucial to maintaining the health of isolated forest fragments.

But the bats' future remains uncertain. Australia has the highest rate of native vegetation clearing in the developed world, and urban areas will continue to increase in importance as wildlife refuges.

Flying fox experts meet later this month in Sydney to plot a course that they hope will protect both the bats and the gardens.

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