Dr David Aldridge, Department of Zoology, University of Cambridge
Part of the show Invasive Species, Conservation and the Last Giant Tortoise
Chris - Tell us a little bit about the problem of invasive species, because rabbits are very much in your face in Australia but what about other more subtle examples.
David - Invasive species are one of the biggest threats to the world's ecosystems and wildlife. Many species go pretty much unnoticed until we start looking in a bit more detail. I work in rivers and lakes, and there some of the biggest threats to natural biodiversity are from non-native species getting in. A good example that we're working on at the moment is something called the zebra mussel, which has actually got into the Great Lakes of North America and is spreading through Britain and threatening our native organisms.
Chris - What does it look like?
David - It's very much like the marine mussels that you like to eat but it's a freshwater version. It as a beard or byssus thread which it uses to attach to solid surfaces, and it has a stripy shell, which is where the name zebra comes from.
Chris - How big are they?
David - They're about three centimetres in length.
Chris - So they're quite small actually compared with the big things you see on the dinner table in the South of France.
David - Yes, they're small but they live at really high densities. When they get into places they can live in densities of over 100 000 in a square metre so they engulf absolutely everything.
Chris - So where do they tend to make a bee line for?
David - They do very well in rivers and lakes and they're not very choosy. This is one of the characteristics of invasive species, that they can often live in a broad range of habitats, are generalists and can get into disturbed habitats very quickly. In the case of zebra mussels there's nothing else in freshwaters that attaches to solid surfaces, so they can exploit unexploited niches.
Chris - So why are they a problem?
David - They've got a lot of publicity, especially in North America, because they affect industry by blocking pipes in power stations. They've closed down water treatment processors and that's because they can attach to surfaces. In North America they cost five billion dollars a year. From a conservation viewpoint they're very worrying because they change the entire nature of the ecosystem them invade. They'll sit on top of anything solid including our native mussels which they choke and smother. We have a rare mussel in Britain called the depressed river mussel, which gets completely covered in zebra mussels and is completely prevented from breathing and feeding.
Chris - How did they get here?
David - The zebra mussels have a really interesting method of dispersal in that they have planktonic larvae that can remain in the water column for up to a month. A lot of the overseas dispersal has been in the ballast water of ships. Ships will go from one freshwater port to another, cross the sea and dump the water when it gets to a new port. So actually this is one of the major vectors of invasive species. There's been some work done on ships moving from Japan to North America, and they've found over 300 non-native species living in the ballast water. So now there's a big move towards treating ballast water to control all the little invasive pests inside.
Chris - I guess no-one thought about the potential for this problem when ships began to do this.
David - No, and still it's very difficult to regulate. A lot of people turn a blind eye and don't actually recognise the value and importance of it because changing ballast water out at sea is costly and time consuming. People don't want to do it.
Chris - You made headlines fairly recently with a novel approach for dealing with this problem.
David - Yes. I'm a conservation biologist but on this occasion I've actually been out to kill the pests so that's a slight challenge to my nerve! We've developed something called the Biobullet, which is an environmentally friendly solution for controlling zebra mussels. This is particularly something that you can use in pipelines and in industry. Traditionally, if you pour bleach down a pipe line, which is what they do, the zebra mussels are very clever, sense the toxin and close up their shells. What we've done is encapsulated the toxins, in this case a salt, in a little edible coating which tricks the zebra mussels into eating it. They think they're getting tasty feast and they concentrate it out of the water as they busily filter out the food. They then swallow this toxic pill without realising it.
Chris - And why doesn't this take down native wildlife?
David - What we do is put it down a pipe line and engineer the coating to break down within two or three hours of going in the water. So all the water that goes out into the wider environment has pills that have already degraded and the salt has dissolved so there's no toxic build up in the environment.
Chris - Why isn't this a problem in parts of the world where these mussels are native?
David - That's a very good questions and very often when people are trying to find ways of controlling invasive pests, they'll go to their native range and ask why is it that these things are being kept under control. Sometimes it can be a biological agent such as a predator or a virus or a parasite which has evolved in association with that organism and keeps it in check. But often when species are taken out of their range, there's nothing present to regulate it and this is why invasive species often do very well in island habitats where there is a low diversity of organisms. There's a lower chance that there's going to be something there to actually eat it or kill it through disease.
Chris - What's the chance of something evolving to be able to prey on it and therefore make the problem go away?
David - There are possibilities over time. Often invasive species will establish, become very abundant and then the ecosystem will adapt in that those organisms that can deal with the invasive species are selected for and therefore start to persist. But in the meantime, we've got problems with these invasives just establishing and causing us ecological and economic problems.