Not all invasive species are bad, and in the case of plants it seems that they usually increase biodiversity rather than making native plants extinct. A recent article in PNAS by Dov Sax, an ecologist at Brown University and Steven Gaines, a marine biologist at the University of California, investigates how species invasions and extinction influence the future of biodiversity on islands. They point out that despite the introduction of 22,000 non-native plants to New Zealand and the naturalisation of 2,436 (NZPCN) of these, only 3 of the 2,357 native plants are now extinct. Vascular plants, as well as mammals and freshwater fish have seen dramatic increases in richness across both continental and oceanic islands, with a doubling in plant richness on oceanic islands over the last 2 centuries – many exotics have become naturalised, whereas few native species have gone extinct. However, the story is very different for birds which usually fare worst during species invasions, e.g. 38 of 91 native land bird species in New Zealand have become extinct. For native birds, the number of extinctions is high worldwide and is largely matched by the number of exotic birds that have become naturalised.

Invasive species receive a lot of bad press for driving native species extinction but is this view too simplistic? Sax and Gaines argue that competition from exotic species shows little sign of causing extinctions as ecosystems show no sign of being saturated. Similarly, James Brown of the University of New Mexico argues that the 5 native species of freshwater fish in Hawaii are not becoming extinct because they compete better than the 40 invasive species in certain refuges. Invasives can also contribute to increased biodiversity by hybridising with natives e.g. the American saltmarsh cordgrass (Spartina alterniflora) interbred with the native English small cordgrass (Spartina maritima), resulting in a new species, common cordgrass (Spartina anglica).

Invertebrate richness may also have increase with >2,500 species naturalised on Hawaii alone, but records of extinction are less certain. Geerat Vermeij and Peter Roopnarine’s recent article in Science considered the invasion of the North Atlantic by molluscs of the North Pacific 3.5 million years ago as a result of global warming making the Arctic Ocean less forbidding. As a result the Atlantic’s diversity rose. As the Arctic Ocean is warming again now in another wave of climate change, Vermeij and Roopnarine predict that the same will happen again causing another increase in diversity. Critics, however, argue that today’s biological invasions are different from those of the past. “Invasions and extinctions have always been around, but under human influence species are being transported faster than ever before and to remote areas they could never reach,” says Anthony Ricciardi of McGill University. “Invasions will interact with climate change and habitat loss… We’re going to see some unanticipated synergies.”

It is difficult to predict whether the trends in increasing plant richness on islands will continue in future. Some argue that the extinction debt is yet to be paid. Sax and Gaines point out that plant invasion patterns may suggest that islands are becoming saturated with species, in which case will further invasions cause increased extinctions or will the rate of invasion decrease?

Asian_carp_attackThe impact of invasive species is more than just biodiversity though. For example the Asian long-horned beetle (Anoplophora glabripennis) is infesting forests across the United States and is expected to harm millions of acres of hardwood trees (look out for the new distribution map coming in December); zebra mussels (Dreissena polymorpha) have clogged water supply systems in the Midwestern United States; and dead stems of Japanese knotweed (Fallopia japonica) contribute to flooding risk in the UK, Europe and USA. In Illinois the Army Corps of Engineers has even built an underwater electrical river barrier ominously named “the Chicago Sanitary and Ship Canal Aquatic Nuisance Species Dispersal Barrier” to dissuade further spread of the Asian carp  which, as well as threatening native fish, launch an assault on passing boats, rocketing like slimy torpedoes out of the water when a boat passes, hitting the hull of the boat or injuring the passengers on the boat. On the other hand there are many beneficial naturalised species, including the honeybees that pollinate many crops in the USA and were originally introduced from Europe. There’s clearly a lot more to it than “exotics are evil” and each new introduction will need a lot of careful thought and consideration.

Coming soon from CABI:
Invasive Species Compendium – instant access to vital scientific information about invasive species – to help with that careful thought and consideration!
Environmental Impact – the complete online information service that covers climate change and other influences of humans on the biosphere.

To find out more about CABIs research on invasive species click here.

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