Aliens on the shores.

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In the early 1980s, a biologist at the tropical aquarium in Stuttgart, Germany, noticed that the beautiful green alga Caulerpa taxifolia (Fig. 1) would make an ideal aquarium decoration. It does not wither, it is fast growing and it withstands cool temperatures. In the following years, many aquariums acquired Caulerpa plants from Stuttgart, among them the Oceanographic Museum of Monaco, from which it escaped into the Mediterranean Sea. In 1984, the alga covered an area of around 1 m2 on the coast of Monaco. Since then, it has invaded large areas of the French and Italian coasts and has spread as far as the coasts of Spain, Croatia and North Africa. The alga grows everywhere, from the surface to the lower limits of underwater vegetation, forming huge monocultures that displace other plant species and, being highly toxic to many fish species, also threatens aquatic fauna wherever it roots. > Invasive species are the second biggest cause of loss of biodiversity after habitat destruction Figure 1. The green alga Caulerpa taxifolia In the summer of 1999, New York City witnessed an unusual spate of bird deaths, particularly of crows. During the same summer, on August 23, a physician from a hospital in northern Queens reported two encephalitis patients to the New York City Department of Health, and further surveillance identified other cases. A month later, scientists from the Centers for Disease Control and Prevention (CDC) in Atlanta, GA, USA, identified the link between these seemingly unrelated events. They discovered that the causative agent was the West Nile virus, which had never before been seen in North America. To contain the outbreak and prevent transmission of the virus by mosquitoes, New York's mayor, Rudy Giuliani, ordered the spraying of whole neighbourhoods in New York with pesticides. It was in vain. During 2001, 149 cases …