Extreme Ecology On Christmas Island

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Australia's remote Christmas Island is home to 1700 people and one hundred and fifty million red crabs, peacefully coexisting. Then came exotic yellow crazy ants, which scientifically preyed on the crabs. (How did they learn to do that?) It was a minor problem until the further arrival of scale insects. Feeding on plants, these produce a nutritious liquid called honeydew. The ants learned to farm the scale insects, boosting their strength so much they formed gigantic colonies. They could spray so much formic acid into the air that it became a chemical weapon. Clouds of it wiped out crabs en masse, whereupon the ants occupied their abandoned burrows to raise more ants. The colonies grew so large they threatened to wipe out all other animal species.

The people didn't like that. They tried ant poison, no luck. Next they introduced a tiny wasp that preys exclusively on scale insects. That turned the tide.

 
Earth sciences news on Phys.org
Many island countries face these issues. In New Zealand, biosecurity is strict: travelers declare risky goods, including food, plants, seeds, wood, flowers, animal products, outdoor gear, herbal meds, and natural products. Bags are x-rayed, searches are conducted, and sniffer dogs check travelers.

Some declared items may be allowed, but others, like fresh fruit, meat and dairy products, seeds and untreated plant material, will be confiscated to protect NZ from pests and diseases.

They would check your boots and ask where you've been in the past few weeks, like on a farm or some related ground where pests and diseases might follow you to NZ.