Science Interviews


Sun, 10th May 2009

Invasive Alien Species

Dr David Aldridge, Cambridge University

Listen Now    Download as mp3 from the show Clean Water and Alien Invasions

Helen - Well, in the studio now we have Dr David Aldridge from Cambridge University and he works on the understanding how the ecology of invasive species affects our rivers and fresh waters and finds out ways of trying to stop them.  Hi David, thanks for coming in to the studio.

David -   Hello Helen.  It is a great pleasure to be here.

Helen -   Fantastic.  Well first of all, what do we really mean by invasive species and why are they a problem?

David -   Most people understand the term Ďinvasive speciesí to relate to species which are not native in a new area but which also have a negative impact on the ecosystem.

Helen -   So we can have invaders that we donít notice or they generally stand out, we know that they are there and thatís why they are a problem?

David -   We sometimes talked about this ĎTens Ruleí that about ten per cent of the species which invade a new system donít establish and then only ten per cent of those that do establish actually have a measurable impact so thereís lots of things around in our rivers and elsewhere and in our environment which we donít really notice and donít really have a measurable impact.

Helen -   And the things that get to the British waterways and are a problem, what sort of things, are we are seeing arriving in our shores?

David -   Well we have just finished an inventory of all the non-native species in Britainís waterways and we haveÖ

Helen -   How many are all there?

David -   Thereís about 120.

Helen -   Well, okay.

David -   But thereís lots waiting to come and we are trying to help predict which is going to be the next big problem but some of the really big problems throughout the U.K. at present in the Cambridge areaÖ

Retrieval of zebra mussel-encrusted Vector Averaging Current Meter near Michigan City, IN. Lake Michigan, June 1999. Photo by M. McCormick.Helen -   Youíve got something in front of you there I see.  Youíve brought some dead critters I am pleased to say, into the studios.  Well what have you got there?

David -   I have got a little menagerie of goodies here and I have got some zebra mussels, I have got a signal crayfish of North America and I have got a Chinese mitten crab.

Helen -   Excellent, we heard a little bit about the zebra mussels last time you were on the show a couple of years ago and I take it they are still a problem very much within the U.K. waters.

David -   They have become an increasingly larger problem and actually they have got a very close relative, something which belongs to the same genus, something called the Quagga mussel which is building up in the river Rhine and almost displacing the zebra mussels and so thatís going to arrive with us very soon.

Helen -   So another invasion of something thatís getting rid of another invasion, and why are they a problem?  Whatís wrong with having these zebra mussels?  They look rather beautiful I have to say.  The collection youíve got right there, the stripey and you know, they donít look problematic.  They look quite small and sort of size of thumbnail, I mean why should that be a problem in the wild?

David -   You are right they are beautiful but unlike our native fresh water mussels which just sit in the bottom of rivers with their foot digging into the mud zebra mussels have a beard, a byssal thread which is like the marine mussels that you eat.

Helen -  Moules Mariniere, tasty, lovely...

David -   Indeed.  So zebra mussels are able to sit on solid surfaces and they can attach to each other and sit in dense layers so they can foul pipelines and drinking water supplies, cooling systems to power plants, irrigation systems but also they sit on our native wildlife and one of the things they really threaten such as this specimen I have got in front of me, are our native mussels which provide a really good substrate and they choke them and cause them to die.

Helen -   Because thatís a huge mussel youíve there and I can hardly see it, itís kind of covered in smaller zebra mussels, thatís incredible.  And then the crayfish youíve got there, that looks quite tasty.  Can we eat those?

David -   We can and thatís the reason they were brought over here.  The American Signal crayfish was brought over in the 1970s as a commercial aquaculture food and the problem with these crayfish is that they can walk over land so they escaped out of these little ponds they were put in and they can move into the wider environment.

So they are very good at sort of changing the ecosystem through feeding on the bottom rooting plants and macrophytes and they dig burrows which can cause destabilisation of the banks but perhaps of greatest or immediate concern is that they carry a fungus, something called crayfish plague which kills our native crayfish species but these are pretty resistant and the American ones are pretty resistant too.

So we have had for instance in the Cam in 2000 there was an outbreak of plague which wiped out native crayfish from about 20 kms of river within weeks.

Helen -   Wow, and could we kind of combat that fungus?  Is that one way of looking at sort of the problem or do we really just need to get rid of the invasive crayfish themselves? 

David -   The Signal crayfish and the other non-native crayfish we have in Britain are a real problem.  Nobody has really found a way of controlling those.  The environment agency have tried sort of heavy duty trapping, people tried pheromone traps to try and skew the sex ratio in the rivers. 

Eriocheir sinensis, Chinese mitten crab.One of my sort of observations is that certainly in sort of some of the chalk streams the river Shep, for instance in Cambridgeshire where you have got cooler headwaters the fungus seems to be less virulent there and so they could be refuges for the native crayfish in those areas but generally speaking thereís no known way to eradicate this.

Helen -   So far we donít know what to do about them and you have got another crab in front of you and you said that was a Chinese Mitten crab, did they come from China?

David -   Yes, and quite a few of our non-native species have come from China.  They are very popular food items in Asia and the Chinese mitten crab is quite tasty but unfortunately in China it also carries a fluke which is harmful to humans.

Helen -   So thatís like burrow into a liver or something.

David -   Thatís right, yeah.  The things that flukes usually do.

Helen -   Nasty things, okay.  But you also work in China.  You are looking at similar sort of issues and invasive species out there?

David -   Yeah, I have been working in China for the last five years with the World Bank and there the focus is on sort of trying to rehabilitate some of the worldís largest but most polluted freshwater ecosystems.  So I am working on the border with Tibet in some of the Chinaís largest lakes that were created during the Himalayan uplift andÖ

Helen -   How big is the lake?

David -   Lake Dianchi, which is the largest one we work in, is about 300 square kilometres.  The second largest one we have lake Fuxian, which is 160 metres deep.  They are pretty big things.

Helen -   Pretty huge then, yeah.  So thereís a lot of space there for lots of creatures to get into?

David -   Yeah, I mean historically these lakes we have discovered had massive endemism.  We have found in the areas sort of 50 species of fish which you find in these lakes and nowhere else but with habitat degradation we are discovering these ecosystems have really crashed. Over 90% of the biomass in these lakes are non-native organisms, things like introduced Carp, the water hyacinth which totally smothers the lake surfaces.

Zebra mussel cluster. Photo taken by D. Jude, Univ. of Michigan.Helen -   So only ten per cent of the living stuff in those lakes is stuff that should be there really.

David -   Yeah, and actually a lot of the stuff that hasnít been described before, the fish have been well studied but when I went out the first time five years ago I put a dry suit on and jumped in to the water which a lot of Chinese people tend not to do.  I rolled over some rocks and I found a new species of leech, a huge thing with green and purple stripesÖ

Helen -   Oh gosh, that sounds horrible.

David -   All sorts of things, yeah. 

Helen -   So amazing creatures there.

David -   Yeah, absolutely.

Helen -   And finally really what can we do to try and protect ecosystems from these invaders?  It sounds like thereís an enormous problem in China, how on Earth can you start to actually do anything about that?

David -   A lot of it is education but the problem is that until we understand and we can conduct risk analyses and identify what invaders are going to be the most likely next big problems.  It is very difficult to develop policy and decide what we should be doing to stop things moving around.  We have to understand the vectors, there is implication for climate change on this and whether a new species might be better suited to invading things.  So all sorts of complicated issues to consider. 

Helen -   It sounds like a very thorny issue indeed but thank you very much for coming and introducing us to the world of invasive species.  That was David Aldridge.  He is from Cambridge University and works on understanding the aliens that are invading our waterways.


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