Most commonly, invasive species are defined as aggres sive alien species from other continents. A common example of this type of alien species is the butternut
(Juglans cinerea) canker, a disease which likely came to North America from Asia. The butternut canker has devastated populations of butternut trees since the 1960s, a similar situation to that of Dutch elm disease and chestnut blight, affecting American elm and chestnut, respectively. These three diseases have nearly eliminated important tree species and undoubtedly altered the func tion of the Eastern deciduous forests of North America.
Surprisingly, scientists do not always know on which continent an invasive species originated, but the source continent and population of the invader can be deter mined by genetic analysis. In the case of the North American populations of Hydrilla verticillata, the dioecious form of this aquatic invasive is similar to types found in Bangalore, India, while the monoecious form resembles plants from Seoul, Korea. Hydrilla verticillata in New Zealand may have been introduced from Australia, where the species invaded centuries ago. The determina tion of the geographic origin of an invasive species can be a useful first step toward determining factors that can control the invasive species. The place of geographical origin can be checked for parasites that control the spread of the invasive species there, and subsequently this knowledge could be used in the design of biological con trol methods.
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