Most of these plants have been introduced from other places and are dubbed invasive exotics, nonnative plants or simply noxious weeds.

It is only the aspect of being harmful to native species and ecosystems that earns a nonnative plant or animal the "invasive species" designation.

However, the habitat on the ridge where 8 of the 9 living Newcomb's snail were found is badly degraded by the invasion of nonnative invasive plants.

In a comparison of two experimental gardens, each with four native plant species, the plot with Chinese Privet contained almost entirely nonnative plants after 64 weeks.

A recent careful estimate of the economic damage done by nonnative plants puts the cost at $35 billion a year.

A nonnative plant can aggressively take over a field, for example, crowding out native ones and affecting the habitat of insects and animals.

But in truth there are plenty of nonnative plants around.

But the researchers say it may be possible to use that factor to gauge whether a recently introduced nonnative plant will become an aggressive invader.

Pollution, road salting and competition from nonnative plants like the Oriental bittersweet have also contributed to the decline or disappearance of species.

Damage to the habitat has occurred via development, the introduction of nonnative plants and animals, agriculture, recreational activity, and fire.