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Recent estimates suggest that there are over 600 aquatic and terrestrial non-native species present in the Great Lakes region. Many terrestrial non-native plants were brought over with early settlers because they reminded them of their home countries, while others arrived as contaminants in seed for agriculture. More recently, a number of plants imported by the horicultural trade have demonstrated serious invasive tendencies once released into the wild by birds which eat their fruit and disperse them widely. Ironically, many of these were planted for their wildlife value, before their invasive tendencies were noted.
There are a variety of introduced shrub species originating in Europe, Asia and Africa that have invaded native woodlands with disastrous results. Typically, they are dispersed by birds which eat their abundant berries and are most common in disturbed forest, edge and forest openings, although they can invade healthy forest interiors as well. They leaf out before native species and shade out tree seedlings and herbaceous groundcover, inhibiting forest regeneration. They include a number of honeysuckle species, glossy and common buckthorn and autumn olive.
Garlic mustard is an herbaceous biennial that invades forested communities and edge habitats. The plant has no known natural enemies in North America and is difficult to eradicate once established. It is one of the few herbaceous species that invade and dominate the forest understory. In its first season it develops a distinctive basal rosette and in its second year sends up one or more flowering stalks. A single plant averages 136 - 297 seeds but can produce over 7,000 seeds, effectively dominating the seedbank. Garlic mustard dominated woodlands are characterized by low native herbaceous diversity.
For more information, see: Habitat Assessment, Section V (PDF)