There are also hundreds of native species of plants and animals that we consider undesirable. For centuries we have referred to them as weeds, pests, vermin and disease. There are also many exotic species that fall into this category. And, of course, there are many native species that are considered extremely beneficial, especially those that provide food for a growing population. The point is, both exotic and native species can be desirable or undesirable from a human perspective, depending on how they effect our lives. Our almost innate dislike of rats and spiders has nothing to do with whether or not they are native or exotic, it is due to the possibility of deadly disease or a fatal bite. And even though dandelions in the lawn are hardly a life-and-death issue, millions are spent each year to rid lawns of these “weeds”.
Certain exotic species have resulted in severe negative impacts. The most notorious case involved the introduction of European species of animals to Australia, New Zealand, and the Pacific Islands when Europeans colonized these regions beginning about 225 years ago. Many native species, flightless birds and ground-dwelling marsupials in particular, were not able to survive the introduction of predators such as rats, cats and foxes. As a result, hundreds of native species were eliminated. Another well known exotic is Dutch elm disease, a fungus that actually originated in Asia, came through Europe and on to North America where it has resulted in the death of many native elms in the US and Canada.
There can be no doubt that we should always be careful when considering the introduction of a new species, and that regulations are needed to prevent undesirable accidental introductions. At the same time we must not lose sight of the fact that introduced species play a vital, indeed essential, role in modern society. Each species must be evaluated on its own merits. The introduction of some species may be desirable in one region and yet undesirable in others. Islands are particularly susceptible to introductions because they are isolated and their native species are not subjected to as wide a variety of predators and diseases. When rats are introduced to islands that support large bird rookeries there is often a precipitous decline in bird populations due to predation on eggs and nestlings.
There is really no difference when considering the use of an exotic species of tree for managed forests. The main reason we tend to use native species of trees for forestry in North America is because they are the best available in terms of productivity and wood quality. In other regions this is not the case. Radiata pine from California has been very successful in New Zealand, Australia, and Chile. Eucalyptus from Australia is the forestry species of choice in many parts of Brazil, Portugal and South Africa. Douglas-fir from Oregon has become the number two species of softwood produced in France. And Chinese larch is a favorite for reforestation in Scotland where forest cover was lost centuries ago to sheep farming.