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Page last updated at 09:18 GMT, Monday, 17 August 2009 10:18 UK
Invasion of the 'island snatchers'
Matt Walker
Editor, Earth News

Common gorse (Ulex europaeus) in Hawaii
Common gorse (Ulex europaeus) a yellow-flowered shrub native to Europe, is taking over a slope on Mauna Kea, Hawaii

Almost 400 invasive plant species have set up home as weeds on some of the world's most distant oceanic islands.

About half now dominate their new habitat, and hundreds more species are expected to invade these once pristine islands in the coming years.

So says the most comprehensive survey to date of invasive plants on island archipelagos.

Worse, people are mainly to blame, having repeatedly introduced these weeds into their farms and gardens.

Non-native plants and animals can be extremely destructive.

But while it is undisputed that many invasive animals such as rats and cats pose a major threat to biodiversity, it is less clear what role invasive plants play in changing native habitats.

A vast majority have been deliberately introduced and planted
Botanist Dr Christoph Kueffer

So botanist Dr Christoph Kueffer of the University of Hawaii in Honolulu and colleagues across Europe analysed how many species of invasive plants have become established on island archipelagos.

They collected data from 30 island groups across four oceanic regions, including the Atlantic, Caribbean, Pacific and Western Indian Ocean.

In total, they found 383 species of spermatophyte, or seed-bearing plant, had invaded at least one of the 30 island groups studied, with between three and 74 species invading each archipelago.

Of those, 181 species or about 50% had become the dominant species of a habitat on at least one island.

If this rate of invasion continues, the researchers calculate that between 500 and 800 spermatophyte plants will become weeds on islands lying at latitudes between 35 degrees North and 35 degrees South.

Of those 250 to 350 will become dominant on at least one island.

Lost islands

Hawaii has been particularly inundated by invasive weeds.

"For Hawaii alone, it is said that 10,000 non-native plant taxa have been introduced to the islands," says Kueffer.

While most have remained as ornamentals in people's gardens, 47 have become a significant weed, dominating parts of the Hawaiian landscape.

Other badly affected islands include Reunion in the Indian Ocean, where 35 weeds dominate certain habitats, and the Canary Islands in the Atlantic Ocean, which is now home to 22 dominant weeds.

One particular surprise to come out of the survey was how difficult it is for a plant to invade many different islands, and to do so naturally without help from people.

Kahili ginger (Hedychium gardnerianum) growing in Hawaii lowland forest
Kahili ginger (Hedychium gardnerianum), a common understory herb in the Himalayas, is now rampant in lowland forest in Hawaii

Conversely, the study revealed the extent to which people are to blame for the problem.

"A particular challenge is to predict which non-native plant species may become a problem, in order to prevent [their] introduction to a new place," says Kueffer.

So as part of the analysis, the research team examined which characteristics best predicted whether a weed would take hold, including aspects of the plant's biology, the isolation, geography and ecology of the islands or the extent of human development.

The Gross Domestic Product per capita of an island best predicted whether it would be overrun by invasive weeds, the researchers report in the journal Perspectives in Plant Ecology, Evolution and Systematics.

An island's GDP is strongly linked to the amount of development that has taken place, and that in turn is linked to how often people bring non-native plants onto the island either for agricultural or horticultural reasons.

"Human action is a predominant factor in driving invasive species patterns on islands," says Kueffer.

"A vast majority have been deliberately introduced and planted."

In the past this happened most often when people tried to restore deforested areas by planting new seeds.

But today the horticultural trade is mainly to blame.

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