Many Dutch people are becoming more and more wary of the numbers of immigrants streaming into the Netherlands. But it's not just people that they're worried about; hundreds of species - sometimes invasive - cross the Dutch border each year.

The names sound like something from a science fiction novel: whangamata sea squirts, western tubenose gobies, and floating pennywort. These species have, in various ways, hitched human rides into the Netherlands.

Because of global trade and modern wanderlust, more and more exotic plants and animals are becoming world travelers - often without the knowledge of their carrier. They come along with banana shipments, in the ballast water of ships and in birdseed. And what better holiday souvenir than an exotic new pet or potted plant?

Population explosion
"Every year hundreds of new species come to the Netherlands," says Ella de Hulle, co-founder of the new Centre of Exotic Plant & Animal Expertise in Nijmegen.

"Some are here only briefly; they can't survive in a colder climate. But, for some other species, the Netherlands is a great place to multiply. Sometimes, when a species is successful, the population growth explodes. Such an invasive species may even displace native plants and animals."

Exotic species have many different effects on an ecosystem. They enrich biodiversity but also introduce new risks. So the burrowing muskrat undermines Dutch dykes, the Asian tiger mosquito threatens public health and the Asian ladybug puts pressure on indigenous insects.

But not all exotics are a threat; the nile goose, for example, fits in perfectly and does no harm (an example that many Dutch politicians wish human immigrants would follow).

Exhibition of newcomers
Visitors to "Newcomers to the Netherlands", an exhibit at Leiden's Naturalis museum, can learn more about exotic species and their impact on the Netherlands.