Native vs immigrant?

Ecologists have long believed that extinction of native species, besides being caused by destruction of habitat, is also linked to the introduction of new species into their habitat. Ted Case, ecologist at the University of California, San Diego, says that competition between native and introduced species is not responsible for the extinction of the former. After collecting information from 70 islands in various parts of the world, Case noticed that the number of introduced species roughly equalled the number of species that had become extinct. But what he also noted was that the species that became extinct disappeared before the introduction of the new species. With regard to introduced species of birds, he says they do not inhabit the same natural territory as the native species but prefer 'unnatural' habi tat -- areas transformed by humans. Case says the loss of natural habitat and the creation of unnatural habitat are responsible for the disappearance of native species ( The Economist , Vol 343, No 8022).