Surprisingly, botanical gardens have often contributed to the spread of invasive aliens, because they've long prided themselves on the number of requests they get from around the world to distribute their plants or seeds. Other exotic invaders make the trip as hitchhikers seeds or insects can attach to cars, trains, ships, or planes. Purple loosestrife, whose seeds are able to survive submersed in water, was partly spread by water used as ship ballasts temporary loads used to increase stability until ships pick up cargo. And diversity is a good thing. Randall points out that insects and other small animals need a variety of native plants to feed on.
Exotic pests can cause damage too.
The balsam wooly adelgid has killed Fraiser firs in the North Carolina mountains. This sap-sucking insect can kill a tree in five to seven years, White says. Trees are even more susceptible to pests when stressed by air pollution. Hugh Morton, a Carolina alum and owner of Grandfather Mountain, has often spoken out about the deadly mix of air pollution and adelgids. The balsam wooly adelgid, originally from Asia, traveled to North Carolina in the 1930s attached to some balsam fir trees from New England.