It is hard to tell the difference at times which hypothesis is true for which invasive species. .
II. Japanese Barberry, an invasive species. .
Japanese barberry (Berberis thunbergii) is an invasive species in the United States. It belongs to the family Berberidaceae and is native to Japan. The Japanese barberry is a deciduous shrub three to six feet tall with alternating deciduous leaves that are ovate and have a smooth edge. The leaves are wider at the base then they are at the tip. At each node there is a single spine. The leaves are green in the summer and can be orange to purplish color in the fall. The Japanese barberry has small yellow flowers in April and red berries around October. Many cultivars of the Japanese barberry have been bred for use in landscaping. They are easily propagated by seed and by cuttings (Silander and Klepeis 1999). Japanese barberries are distributed from Maine as far south as Georgia, and as far west as Missouri (USDA). It is currently used in landscaping as decorative hedges and edging (Coleman, Withey, et.al.2002). .
In traditional herbal medicine barberry was used to treat ailments such as infections and psoriasis. In fact barberry bushes produce an antimicrobial alkaloid called berberine (C20H19NO5). It has been studied for its medicinal effects. It was found that bacteria can easily eliminate berberine from their cells by using a membrane protein to pump out antimicrobials before they can act, which is why berberine is considered to be a weak antimicrobrial. Through further studies of the Berberidaceae family three species of the genus Berberis has been found to contain a second compound, 5" methodxyhydnocarpin, that inhibits the membrane pump in the bacteria (Ben-Ari 2000). I have been unable to find specific documentation that the Japanese barberry contains both compounds or only the berberine, but if both compounds are found in Japanese barberry then it could be an invasive species due to allelopathy and not just resource competition.