Alien Nation

April 9, 2007  •  Nature  •  0 Comments

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• Here in Andalucía there has been an explosion in the number of red eared terrapins

THEY came by night. They came by day. Silently slipping over borders, stealthily avoiding detection. Some came by boat, others by plane. Only a few people knew about them. Still less cared. “It’s just not right, they don’t fit in here!” cried a few sane voices in the wilderness. But nobody took any notice until it was too late. Some of the invaders even had government agents to help them settle. And then they waited, their numbers swelling year by year until…
No, it is not an Op Ed from the Daily Mail. The aliens here do not want their children to attend faith schools and they are not overly worried about compulsory cultural immersion courses. In fact, they are more likely to have exoskeletons, pincers, scales or flippers. They might even be pond life. What’s more, they are all, overwhelmingly, happy in their new home.

We are talking ecology here, of course. Spain, like many other countries, is currently suffering from the rampant overpopulation of various species that have found their way here and got out of hand. Some of them, such as the Gambusia holbrooki, a small river fish, were brought in to fight pests. In this case, it was introduced to the Ebro river delta in Northeast Spain to eat mosquito larvae. For the last few millions of years this small fish had been happily living in the rivers of North America, where it was part of a finely balanced ecosystem. It ate; it got eaten – in roughly proportional amounts.

But in the 1960s, at around the time Aretha Franklin was singing Chain of Fools, this small fish decided to unshackle itself from its restrictive food chain. Taking advantage of a Spanish government grant, this American ecotourist introduced itself to the river Ebro. It was just swell. There was a continuous all you can eat mosquito larvae buffet and, what’s more, those elitist European fish just turned up their snouts and refused to eat it. It had children. Several billion of them. And now there are so many of them that the whole delta ecosystem is clogged with little brown fish with bulging bellies.

Further up the same river, in 1974, another type of fish was let loose. A type of catfish (Siluras glanis) was put into the water to improve sport fishing in the area. But these were no tiddlers. Growing to over two metres in length the wels catfish turned into monsters. Carl Smith, a Briton, hauled one out of the water in 2006 weighing 102.7 kilograms – the largest freshwater fish ever caught in Spanish waters. It is not known whether Carl Smith ate the wels catfish, but it is well known that the wels catfish eats a lot of other fish and has been known to destabilise ecosystems around the world away from its native eastern Europe. Females lay 30,000 eggs for every kilo they weigh.

But not all introduced species are so spectacular, although the effects they can cause are often dramatic. Take the aptly named assassin algae (Caulerpa taxifolia). This was first noticed in the sea at Monaco in 1984, covering about one square metre. Now it has spread around the Mediterranean, coating the fronds of the protected sea grass (Posidonia oceanic). This sea grass covers large areas of the seabed and is akin to a rainforest in terms of the great biodiversity harboured within. Scientists are so worried about it they warn of irreversible changes to the Mediterranean coastal ecosystems and large scale efforts are afoot to contain its spread.

Other invaders take the form of beetles, weasels, mussels, crayfish, ducks and turtles. These species are often termed “aggressive invaders” but there is nothing particularly nasty about them. They are so called because of the way they exploit a particular niche in the ecosystem that had previously been off limits to them. Given the slow nature of evolution every species we see today is a survivor in its own particular patch. Not many species are able to travel long distance without human help – and even in the case of lone specimens travelling across oceans on coconuts they usually die out when they reach land due to a lack of mates (in the reproductive sense of the word, although it is not inconceivable that they might die for lack of friends).

This is why the world of plants and animals is so diverse. Up until now (i.e. in the last few hundred years) the world was truly a world of islands. Human engineered travel has mixed things up so much the entire biological world is currently fighting pitched battles as one wave of invaders sweeps in after another. Some invaders have proved very successful and can thank us humans (think of rats and rhododendrons). Other less so (think dodos, Puerto Rican flower bats and the West Indian porcupine).

If you want to see an unwelcome invader in Spain look no further than your local river – the Rio Guadalfeo will do fine. Here in Andalucía there has been an explosion in the number of red eared terrapins (Trachemys scripta elegans). These fresh water turtles eat almost anything and are another invader from America – in this case the Mississippi Valley. Cute and toy-like when young, the terrapins are often sold to children or given away as prizes at fairs. When they reach adulthood they become snappy and are not as much fun. That is when they find themselves being booted out of the family home and chucked into the local pond or river (sometimes via the sewage works). It is a method of dispersal that works so well for the terrapin there are now huge populations of them all over the world from Australia to Israel. The popularity of the Teenage Mutant Ninja Turtles did for the red eared terrapin what Jaws did not for the great white shark.

And what can we do about it? The answer is: not much. We must do our best to try and contain the worst of the outbreaks and try to prevent any further unwelcome organisms entering areas where the local population has no defence against them. Things will take a few million years to settle down again – and there will doubtless be a lot less species around. In the meantime we can sit back, throw another giant red crayfish on the barbie and watch that huge flock of Jamaican ruddy ducks fly off into the sunset.

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