Immigrant species aren't all bad

Mark Davis, New Scientist 25 Sep 09;

HUMANS have been transporting species around the world, intentionally and unintentionally, for centuries. Many of the species we think of as a natural part of our landscape are, in fact, non-native. For example, the honeybee, which nearly a third of US states have named as their state insect, was introduced into North America from Europe in the 1600s.

Naturalists have long been aware of this bio-globalisation, but widespread research on introduced species did not begin until the early 1980s. In those days, the message from invasion biologists was clear and simple: introduced species were bad news. They were referred to as invaders, aliens, exotics or even "biological pollution". A common refrain was that invasive species were one of the greatest extinction threats for native species, second only to habitat destruction.

Make no mistake, some introduced species have caused great harm. For example, the brown tree snake, introduced into Guam in the mid-20th century, caused the extinction of most of the island's native birds. Many other island and lake species have been driven extinct by introduced predators. The global cost of damage by non-native species to farming, timber, fisheries and waterways is estimated at well over $100 billion annually. Many of the human diseases of greatest concern are viruses that have been transported to new regions, such as SARS, West Nile virus, Ebola, H1N1 flu and HIV.

However, you may be surprised to learn that only a few per cent of introduced species are harmful. Most are relatively benign; some, such as the honeybee, can even have beneficial effects.

Despite this, many people cling to the idea that non-native species are uniformly undesirable. In my discussions with people involved in conservation and restoration programmes, for example, I find that many view native species as inherently more desirable than non-native ones. Many land managers and ecologists also hold this view.

If most non-native species are not harmful, why have so many people adopted this "nativism paradigm"? I believe there are two explanations. The first is the message sent by invasion biologists to the public over the past three decades. The second involves some general predispositions that we seem to share as humans.

Some anthropologists argue that the desire to classify is inherent in humans, as we crave a sense of order. Similarly, recent evolutionary research suggests that humans are predisposed to impose group boundaries and to see outsiders as a threat. We seek every opportunity to identify with a home land, a home tribe, a home religion, a home team, and to declare everybody else the enemy. This declaration of identity extends to the natural world. Most countries, states and provinces have designated a species of animal or plant as "theirs", declaring it the national bird or the state flower. Given this powerful predisposition, it is not surprising that people, including ecologists, show favouritism toward native organisms.

We need to move on. Scientific disciplines are often guided at their outset by a few simple ideas. However, as the field matures, participants typically recognise the complexity of their subject and the need for a more nuanced approach. This is what is happening in invasion biology.

Philosophers, social scientists and some invasion biologists have challenged the choice of language used to describe non-native species and have argued that conclusions about them sometimes rest more on prejudice than science. Others have criticised the preference for native species as scientifically unsound, arguing that invasive species do not represent a separate category, evolutionarily, biogeographically or ecologically. Others have pointed out flaws in the claim that non-native species are the second-greatest extinction threat after habitiat destruction. In fact, with the exception of insular environments such as islands and lakes, there are very few examples of extinctions being caused by non-native species.

Despite this more nuanced approach, many of my invasion biologist colleagues are reluctant to discard the nativism paradigm. Some have told me that "message enhancement" is a necessary strategy when dealing with the public and policy-makers, in order to get their attention.

I believe this strategy carries serious risks. If messages are enhanced to the point of exaggeration, they can end up eliciting well-intentioned but misguided responses, such as spending scarce resources on projects that should not be a priority. The harm being caused by introduced species is well documented. We do not need to enhance the message.

Whether we like it or not, the world's biodiversity is becoming globalised. We can call them aliens, exotics or biological pollution, but the fact is that introduced species are our new residents. It is common for humans to resist change, particularly when change is rapid and profound, as is the case with globalisation of cultures and biota. In some cases, real economic, social or ecological harm is caused. When the harm is great, society needs to mobilise its human and financial resources and respond quickly and effectively.

But resources are finite. It is crucial that we distinguish harm from mere change so that we can spend scarce human and economic capital wisely. We scientists can best assist by making sure that our messages are accessible, interesting and meaningful without compromising the science. As long as the harm is real, it should not be necessary for us to overgeneralise, exaggerate, use incendiary language or misrepresent data in order to attract attention.