The Pestilential Power of Potamopyrgus

Forest and Bird, Issue 314, 1 November 2004, Page 6

The Pestilential Power of Potamopyrgus

is an expert in

freshwater ecology with the National Institute of Water and Atmosphere.

—R.M. McDOWALL

tiny freshwater snail New Zealand has invaded Britain, Europe and North America, becoming something of a ‘pest in reverse’ from a country commonly invaded by northern hemisphere plants and animals. The snail has the tonguetwisting name of Potamopyrgus. It is widespread in New Zealand, a little, pitch-black snail seldom more than about 5 mm high, that can be found in most New Zealand fresh waters, especially at low elevations. Its full scientific name is Potamopyrgus antipodarum, the species name meaning simply ‘from’ or ‘of the antipodes;, from which it has now spread. About 10 years ago, an American report listed Potamopyrgus as one of three recently established species described as being ‘of most

concern’; some American biologists describe it as "highly invasive’, finding that it has become more abundant than native snails in several different habitat types. It is not only in North America that Potamopyrgus has become invasive. This little mollusc has also been found in Australia, the United Kingdom and parts of Scandinavia. It actually arrived in the United Kingdom as early as 1859 — so long ago that British biologists thought it was a native species and described it as Potamopyrgus jenkinsi. It was not until the 1970s that a New Zealand biologist, Mike Winterbourn, realised that the British populations were actually invasive from New Zealand. In the nearly 150 years since this invasion, it has spread throughout Europe — I found

records from Switzerland, Poland, Germany, France, Finland, Denmark, the Czech Republic, and even in Iraq. It seems to be still spreading, the first records for Finland being in the year 2000. A study in France found that Potamopyrgus made up no less than 99.5 percent of the snails in some streams. Some American studies are suggesting that Potamopyrgus ‘competes strongly’ with native snails. Australian studies likewise suggest that it is a ‘very successful invader’. Thus, where introduced, Potamopyrgus may have some distinctly negative impacts on native aquatic animals. One of the issues that has gained some attention, internationally, is that Potamopyrgus acts as an intermediate host for an array of parasites, raising the possibility that these may have been introduced with the shellfish, and may then have been transferred to native fish species in other countries. It seems that this could have happened already. Moreover, it seems that the snails may be acting as secondary hosts for some parasites in the countries where they have become established. Thus there could be some unexpected impacts. The Americans have become so seriously concerned about it that Joe Holumovski, an aquatic ecologist from Ohio State University, has been able to obtain funding to study the snail’s ecology. He has been to New Zealand several times to undertake experimental studies on environmental factors that might influence its abundance. There has been a whole series of scientific papers written about this animal as an invasive species, to the extent that I suspect it is better known in some of the recipient countries than it is in New Zealand.

The global ecological theorists would probably not have predicted that a species from a small, isolated group of islands in far off seas, like New Zealand, would be able to successfully invade the supposedly more complex, diverse, and aggressive ecosystems of northern hemisphere lands. So it is against all the predictions, that Potamopyrgus has made the reverse relocation/invasion, and seems to be doing ‘just fine, thank yow in many northern cool and cold temperate lands. What has enabled it to do so well, against prediction, is quite unknown, but this success does show that widely accepted ‘ecological rules’ do have their exceptions. Overall, the global ecosystem effects of Potamopyrgus are not serious, but act as a continual reminder of the global issue of invasive species.