Invaders come in all shapes and sizes! The last ice age wiped out endemic species of earthworms in much of North America. When the ice melted, forest trees evolved to depend on large piles of leaf litter for nutrients and protection of seeds and seedlings. European human colonizers changed that forest ecosystem when they introduced alien earthworms. The European earthworms most likely hitched a ride across the Atlantic in the root balls of plants that the early settlers brought with them. For example, settlers in the Jamestown colony brought West Indian tobacco, European varieties of grapes, and fruit and nut trees.