Alien invasions in the ocean realm

Helen - We often hear about invasive freshwater species wreaking ecological havoc in places they shouldn't be - things like Chinese mitten crabs and signal crayfish here in the UK, and zebra mussels all around the world. And invasive species are also causing problems in the marine realm. Later on we'll hear the story of lionfish causing problems in the Caribbean and how science could actually be contributing to species movement, but first, when we're talking about 'invasive' species, how do we define what they actually are?

Boze - An invasive species can go by a number of different definitions. A fairly broad definition would be any species that's living and persisting outside native range. For it to be invasive some people would impose on that definition another layer that it causes a problems, usually an economic problems. But at the most basic level it's a species that's been moved and is able to persist outside its native range.

Helen - That was Boze Hancock from the Nature Conservancy.

One of the key questions asked when it comes to invasive is about the scale of the problem. How many invasive species are there in the world and where are they localised? Back in 2008, Mark Spalding, also from the Nature Conservancy, was involved in the first quantitative study of marine invasive species around the globe ...

Mark - What we wanted to do was try to tease apart the challenges from marine invasions. Now, some species are moved - there are always species moving into new environments - some of those are far more threatening, far more problematic than others. So what we were trying to do was both to look at that geographic spread, where were these things happening, were there hotspots on invasion. And also trying to tease out how many of those invasions are actually posing problems either for people or for nature more widely.

And we showed that they were found in pretty much every kind of biogeographic region in the world. They're everywhere. And where we had gaps, chances are that no-one has data rather than they're not there. But there were huge hotspots, the Mediterranean being one, the Californian coast being another, Southern Australia. A lot of those hotspots are around ports or areas of dense, technologically advanced human populations.

Helen - And how are these species moving around?

Mark - You can have natural invasions. You can get species moving into a place naturally through some unusual weather event and so or. But invasions generally are humans bringing species into places. So we're introducing species all the time. The numbers are phenomenal. One estimate which is probably reasonable is that at any point in time at the moment there's 10,000 different species being carried around in boats. A boat will pick up ballast water in one port and carry it to another, and then lets that water out with a host of species, most of which won't invade, they'll die, the conditions are too cold, too hot, too fresh, too salty, and so the won't survive. But every now and then some will.

Helen - As well as shipping, aquaculture or fish farming can also be a major way for species to move about the globe...

Boze - There's a lot of movement of either the species that is being aquacultured itself, fish or shellfish, around the globe and also food. Some of the problems that have occurred have been with say fish pens and moving bait fish from one area to another to actually feed those farmed fish. Or alternatively in the NW of the US is a particularly high level of invasive species and a good many of those came from the days when there were large volumes of small Pacific oysters shipped in from China predominantly, and they were brought to the NW and grown out there for sale. So with boxes of oyster shells there are numerous hitchhikers that come along with them and a good percentage of those have actually stayed and found a new home. And cause a number of problems as a result.

Helen - Certainly aquaculture and shipping ballast water are two of the main routes of invasive transmission, but as we'll hear later, aquarium trade can also play a role, and a new study shows that scientific research could be helping to move species around too. Climate change may also have a role to play...

Mark - I think in the changing world, particularly with climate change, there's going to be natural movements that are going to look like invasions. It's already happening. We're watching the spread of species up coasts, as coasts get warmer, the warm water species are moving along and into new areas. So we're seeing that as a form of natural - semi natural - invasion.

It's not a nice neat movement that every species shifts its range at the same time at the same rate, so you're going to get species appearing that will then predate other species that they've never met before so you will get imbalances and changes and they will look like invasions in a more scary sense, they will be causing changes to ecosystems.

Helen - Yet another example of humans providing a route for species to move to new areas is the building of the Suez Canal. Built in the 1860s, this forms a link between the Eastern Mediterranean and the Red Sea, allowing goods to be transported from Europe to Asia without circumnavigating Africa. But, it also acted as a passage between two bodies of water that had been separated for tens of millions of years, allowing species to migrate between them. Especially from the Red Sea into the Med, where around 1000 species have been recorded. So what is the ecological reason for the success of this invasion? Mark Spalding again...

Mark - If you look at what's in an ocean evolution works its magic and species appear so on the eastern Mediterranean as always been a little bit depauperate, it's been lacking the number of species you'd expect in a body of water that warm in that location. And the reason for that if you look at it is the origins of those species, they've all had to come through colder water to get to the eastern Mediterranean. It's actually a kind of subtropical basin, the eastern Med, but the only route in for the last tens of millions of years has been through the straights of Gibraltar where waters are much colder.

And what happened when the Suez canal opened up and species started to come in a lot  of fish, but a lot of other stuff too, and doing very well Interestingly they don't seem to have replaced or pushed out anything. So you've got suddenly a much more diverse array of species but nothing so far as been documented as disappearing.

Helen - So in this case, the invasions don't seem to have threatened the local ecosystem. But there are many more examples of species that have become a menace. Macro algae have been smothering reefs in Hawaii, and comb jellies caused a crash in the anchovy fishery in the Black Sea in the 1980s after being introduced there. And it's not just animals and plants that can be invasive, but diseases as well, that hitch a ride with transported species.

But is it all doom and gloom? Can anything be done to remove invasives? An eradication program to control the algae taking over Hawaiian reefs has had some success because of its whole ecosystem approach. Invasives are generally not just a threat on their own, but when taken together with other threats like pollution, nutrient runoff and overfishing of species that might eat the invasive, they present a problem.

In Hawaii, measures are being taken to improve the health of the ecosystem to help the fight against the algae - increasing the numbers of local herbivorous sea urchins, working with local government to reduce nutrient runoff, and physical removal of the algae using what are essentially giant ocean vacuum cleaners.

The idea of introducing additional species to control an invasive problem is one that has been used on land, but often with disastrous consequences. Could we see this kind of intervention in the sea? Boze Hancock...

Boze - We are newer at the impressive art of manipulating the marine environment. It's been done on land for a long time but we are still working to understand the ecosystems and the interactions in the ocean. So I don't think we are anywhere near as advanced and I would not advocate or encourage anybody to do that. The incidents of introductions going wrong outweigh the incidents of introductions doing what was intended by such an enormous amount that I think you'd be crazy to try.

Helen - So the key is really to prevent rather than treat. Equipment for testing ballast water, and cleaning it, is being developed, but it's really the increased awareness of the problem that is working in our favour, particularly when it comes to aquaculture...

Boze - It doesn't mean accidental introductions are not still possible. Wherever animals are moved around in our oceans there is a potential for invasive species but people are just far more aware of it now and aquaculture operations actually rely on a clean environment, that's what their business relies on, so the understanding of the impact of what has happened I think is one of the things we've got working in our favour at the moment.

Find out more:

The Nature Conservancy
Invasive species in the oceans