Discussion
What happens when alien species are introduced unintentionally ?
Answer: D
They turn invasive and cause decline or extinction of indigenous species.
An alien species introduced by accident, for example in ships' ballast water or with imported grain, often arrives without the predators, parasites and diseases that kept it in check at home. It can then spread rapidly and out-compete, prey on or infect native species. Lantana, water hyacinth and Parthenium in India are examples; invasive species are one of the main causes of biodiversity loss.
A decrease of the alien species is the opposite of what usually happens to a successful invader. 'Increase in habital' and 'cause increase species' do not describe any recognised effect of invasion.
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