MANILA, Philippines?A story published nearly four decades ago tells of a witch who entered a peaceful, happy land and sowed doubt and discontent among neighbors over their not-exactly pets known as Fuzzies.

In a way, invasive species behave like that witch, entering an ecosystem and destroying the natural balance by competing with native species over access to resources such as food and water.

One of the most well-known examples of such a species is the cane toad, which was introduced to Australia to protect the sugarcane fields. Without natural predators in its new home, the toad population swelled and remains an uncontrolled and poisonous menace to this day.

In the January 2008 issue of the Journal of Ecology, Australian researchers identified three factors that make it easier for a foreign plant species to establish itself in such regions.

Better chance of survival

One consideration is that an introduced plant has a better chance of surviving in an environment that?s undergone recent environmental change. An example of this is cogon grass, which has spread out over several million hectares of land over the Philippines, notably in areas which had once been forested.

If the new species fills a previously unknown ecological gap, it also has a better chance of surviving. The golden apple snail introduced and cultivated as a protein supplement for farmers arguably fell into this category, but it turned out to inflict heavy damages to the ricefields such as those around the Ifugao rice terraces.

Finally, if the introduced species has no natural enemies in its new home, it can spread uncontrollably. The crown-of-thorns starfish can multiply and prey on coral reefs, for example, because its natural enemies have all been overfished.

Reducing the impact of a potential invasive species then, would seem to require a stable ecosystem with diverse species. In the supplement to last month?s issue of The American Naturalist, ecologists John Maron and Marilyn Marler from the University of Montana-Missoula described how they tested this theory through a four-year study on how native plant communities can lessen the impact of invasive plant species.

Checking introduced species

In 2003, Maron and Marler prepared 26 plots of land containing various occurring native plants that typically coexist. Each of the plots was then further subdivided into four subplots, and received equivalent plantings. To check the notion that plant diversity was essential to keeping introduced species in check, a dozen of the plots were planted with different combinations of plant species, while each of the remaining plots received only one type of plant.

The following year the researchers began introducing designated ?invasive? species in three of the four subplots on each area of planting. Between caring for the plots between 2006 and 2007, they calculated how many of the invasive plants had taken root in the subplots, and also calculated the ratio of invasive to native plants per subplot as well as per plot. If the native plants outnumbered the invasive plants, then the invasive species was determined to have zero impact per mini-ecosystem they?d created.

Maron and Marler found that the variety of the species growing in the plot as well as what they called ?functional richness,? referring to plants such as grasses, shallow-rooted plants that flowered early and deep-rooted, late-blooming plants, was necessary to reduce the negative effects of an invasive species. Their results suggest that when maintaining (or restoring) an ecosystem, its ability to control and minimize the impact of an invasive species is dependent on maintaining plant diversity.

* * *

E-mail the author at massie@massie.com.