Keystone WILD! Notes

WINTER 2007-2008

dvd details alien invasion creeping across pa landscape


The giant hogweed should be approached with caution as contact with its sap can cause blisters and scarring for those who are sensitive to it. Photo /Pennsylvania Department of Agriculture
(click on photo to view full-size)

As images of seemingly harmless plants flash on the screen, an ominous score in the background suggests something is very wrong. Far from benign, these plants are often conquering the state’s forests, rivers and yards.

It’s an invasion that started nearly with the first settlers. Upon his arrival, William Penn observed that “the woods are adorned with lovely flowers with color and variety. I have seen the gardens of London with that sort of beauty, but think they might be improved by our woods.”

It turned out, however, that Pennsylvania’s woods—as well as its meadows, rivers, wetlands and other habitats—were about to be overrun with plants and other exotic species.

The story of that transformation is told in “Dangerous Invasions,” the Wild Resource Conservation Program’s first educational film since 2003 and its first to be released on DVD.

The makeup of the woods began to change with the first settlers, the 25-minute film notes, as arriving ships usually carried dirt as ballast in their holds. Upon arrival, the Old World dirt was commonly shoveled onto the New World landscape. No thought was given to the insects, seeds, snails or other organisms it might contain. Others hitched rides in cargo or were deliberately imported for medicinal purposes or to adorn gardens.

Today, more than 7,000 nonnative plants have taken root in the United States. Not all become “invasives” with the ability to rapidly expand and crowd out other species. But those that do, because of a lack of predators or other controls, become a major problem. Invasive species are second only to loss of habitat as the greatest threat to rare species.
Pennsylvania has about 100 highly invasive plants. The film presents something of a rogues gallery of some of the worst.

Among them:
• Tree of Heaven was introduced from China in 1784 to adorn gardens, but each tree can produce 325,000 seeds a year that thrive in disturbed soil.
• Purple Loosestrife was brought from Europe in the late 1800s for ornamental and medicinal purposes, but can quickly take over wetlands, reducing their habitat value.
• The aptly named Mile-a-Minute vine quickly covers and smothers plants and trees in meadows and forest edges.
• Giant Hogweed, originally imported as an ornamental plant, is a 10-foot giant that outcompetes native plants and also produces a toxic sap that causes painful blisters on humans when coated skin is exposed to sunlight.

Invasive species can fundamentally alter ecosystems. Leaves that fall into streams form the base of the aquatic food web, but insects and other aquatic life that evolved to eat native vegetation can’t stomach exotic plants such as multiflora rose which, ironically, was imported to improve habitat.

But the film also offers hope.

It goes into the field to show examples of where habitats have been restored, largely because of the work of committed volunteers. Members of the Tinicum Conservancy, for instance, successfully worked with landowners to control Japanese knotweed, which dominates streambanks throughout the Tinicum watershed, a tributary to the Delaware River. The plant grows 10 feet high and shades out native plants, sharply reducing habitat value in the critical riparian zone.

purple loosestrife
Purple loosestrife can quickly take over wetlands, reducing their habitat value. Photo / U.S. Fish and Wildlife Service
(click on photo to view full-size)

With these types of efforts, coupled with public education aimed at helping today’s gardeners ensure they put the right plant in the right place, Pennsylvanians may yet stem the wave of invasives.


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