…the feral olive, a ‘horticultural time bomb’
By Graeme O’Neill
THEY’RE tough, prolific breeders and not keen on making friends with the natives.
Now two leading environmental scientists have called for stricter controls on olive orchards, describing them as a horticultural time bomb. They say orchards need to be netted to prevent birds spreading olive seeds into native woodlands.
Large areas of eucalypt woodland could be invaded by tough, long-lived wild olive trees that would be enormously expensive to eradicate without stricter controls.
In the latest edition of their book Practical Conservation Biology, Australian National University ecologist David Lindenmayer and Mark Burgman, an environmental risk analyst at the University of Melbourne, warn against the industry’s unregulated growth.
They say feral olives spread from abandoned orchards after South Australia’s original olive industry collapsed in the late 1800s, creating an “olive zone” in the foothills of the Adelaide Hills. After being introduced in 1836, naturalised olive seedlings were initially kept in check by sheep, but the trees spread unchecked as grazing retreated from Adelaide’s rural fringe.
Olives are highly tolerant of drought and poor soils and are very long-lived. Many olive trees in the Mediterranean region are at least 1500 years old.
Dr Peter Martin, of the Cooperative Research Centre for Australian Weed Management, said: “Olives are the biggest tree weed species we have. They’re very tough and accumulate a huge seed bank, so they keep coming back after you clear them.”
The industry had so far invested very little money in research into its potential environmental impact.
“A levy on olives and olive products would fund containment measures, but the local industry insists modern varieties don’t spread and become weeds like the old ones did,” he said.
Organic olive growers Chris and Julie Green, from Gisborne, said Victoria’s soil types made it hard for pest olives to take root here, but they were mindful of the problems they could cause.
“South Australia has alkaline sandy soils but our soils in Victoria tend to be on the acid side,” Mr Green said. “The sandy soils tend to make it easier for any seedlings or pollen to take hold — it’s a little bit different here.”
But he agreed olives were a naturally hardy species that could survive without great quantities of water — even if they need it to produce bountifully.
Dr Lindenmayer and Dr Burgman say olives not only threaten the integrity of woodlands in parts of South Australia, but have endangered populations of several significant plant species. “To limit the further spread of wild olives from existing orchards, the Animal and Plant Control Commission has recommended that all orchards be netted, at a cost that is far less” than the $10,000 to $15,000 a hectare cost of weed control, they write.
Paul Miller, president of the Australian Olive Association, said his association expected to implement a code of practice within 12 months. However, he believed problem areas were confined to Adelaide.
But Dr Lindenmayer and Dr Burgman say many parts of Australia offer climatic and soil conditions favourable to olive trees, and warn that the weed problem could be repeated in many other Australian woodland communities.
THE GOOD OIL Australia’s olive industry
■Number of hectares in 1990: 4615. In 2006: 24,615*
■Australian olive oil imports 2004: 32,629 tonnes
■Main sources: Spain, 17,140 tonnes; Italy 12,826 tonnes.
■Australian olive oil exports 2004: 500.7 tonnes. Main customer: Italy, 178.4 tonnes
■Australian exports of table olives 2004: 250 tonnes. Main customer: New Zealand 96.9 tonnes.
Practical Conservation Biology is published by CSIRO.
* Based on an estimate of 1.5 million trees in Australia in 1990, 8 million trees in 2006.