Invasive species: they came, they conquered, and native varieties suffer

In fact, if we do any of those things we may be committing a colossal error and being less green than we think. Why? Because we are helping species, which are often exotic (not native) and invasive, to implant themselves in a habitat which is not theirs.

According to the International Union for Conservation of Nature, accidental introductions - those which are unintended, or are brought in for commercial or other reasons - are the ones that produce "the most successful invasions".

"Not all exotic species are invasive," Joan Pino, the director of CREAF (the Spanish ecological and forestry applications research centre) told SUR.

"To be invasive, they must successfully establish themselves in an area - in other words reproduce in it - and then spread and increase in number."

But that is often just a matter of time and that is why experts are warning that a rapid response and prevention are necessary and are almost the only way to prevent these species expanding.

Why do we need to fight them? Because when they "invade" they impact the biodiversity (they take over by eliminating many rivals, for example), the economy (they can affect crops or infrastructure) and even human health (some transmit diseases).

"The generalist species, which adapt easily, win and the native ones, which are more demanding, lose," said Pino.

In other words, being very fussy about food, mating and climate change reduces the chances of survival (something that also applies to humans). However, playing devil's advocate, he also said the disappearance or decline of a species is not only influenced by "outsiders" but also by climate change and human activity.

Here, we list a selection of invasive species which came to Spain (maybe as pets, or by air or other form of transport without anyone realising, or for commercial reasons...) and have become established.

American crab: This was brought to Spain in 1930 as a food and ended up "eating" nearly all the native crabs, which are in danger. They were bred for human consumption on a fish farm in the Guadalquivir marshes, but, as they say in Jurassic Park, life finds a way and, after escaping or being released, they spread. The American crab is endangering native species, damaging agriculture, degrading riverbanks and clouding the water.

Tiger mosquito: It is bigger than the native ones, comes from south-east Asia and was detected for the first time in August 2004 in Sant Cugat del Vallés (Catalonia). It is believed to have arrived in Spain inside cargo. Now, 18 years later, it is very well-known.

Florida or American turtle: Oh, what a mess this little pet has made of things! In the 1980s these became fashionable pets all over the world. They first arrived in Spain in the 1990s and in 1997 the EU banned their importation. By then, however, more than one million of them had been sold and, as many people got bored of their turtles, some decided to release them into the wild. That led to their invasion. The consequences? They exclude other native turtles and eat masses of invertebrates, fish, amphibians and aquatic vegetation. And beware - they can even transmit salmonella!

Asian hornet: "This species arrived less than 20 years ago and it is still extending," said Joan Pino. The Asian hornets feed on many insects in Spain's ecosystem, especially honey bees, and on occasion they attack fruit trees and vines, as has already happened in some parts of the Catalonia region.

American mink: Scientists also know when these arrived in Spain for the first time. It was back in 1958 in Segovia, when American minks were brought to breeding farms for the fur industry. But they continually escaped or were intentionally released, so they spread over a large part of the country. Their main victim has been the European mink, which is smaller, less aggressive and easily succumbs to viruses spread by its American "brother".

Zebra mussel: This species drastically changes the ecosystem. By increasing the level of organic material in the waters, it covers all the ground in its path. It is native to the Black and Caspian seas and was detected for the first time in 2001 in Catalonia, although it spread rapidly to other regions. It causes considerable damage to hydraulic infrastructure (amounting to about two million a year in the Ebro river basin).

Argentine parokeet: "This green bird is one of the most successful invasive birds in the world. It originated in South America and a lot were sold as pets, so we believe some were bought by people here in Spain and they either escaped or were deliberately released. This country has the second-highest number of Argentine parakeets in the world, around 20,000. And they are a problem. They compete for food with our native species and they feed on the eggs and chicks of other birds. In Barcelona alone there are six species of parrots in different phases of expansion," said Joan Pino. Large, built-up areas are also popular with "invaders" such as magpies and gulls, he explained.

Pampas grass. Plant species also invade, not just animals. One example is Pampas grass, which is widespread in Spain. It comes from South America and displaces native species. Another example from the world of flora? The water hyacinth: it reduces the oxygen level in the rivers and affects both flora and fauna.