An invasive plant is one that "doesn't belong" in the area, and somehow damages the (native, local) plants around it.

A corn stalk in a field of soy beans is an invasive, and farmers remove them carefully.

For Colonial Park, an invasive is usually from out of North America.  Buckthorn, honeysuckle and phrgmities all came from Europe.  Emerald ash borer -- the insect -- came from Asia.

Buckthorn shades out and poisons seedling trees, preventing forest regrowth.  Honeysuckle shades out the ground when big also.  Even oaks can't grow up through them.  Just to keep this discussion local, these plants do not support insects well, so fish in the river have less to eat.

All invasives reduce the amount and number of insects that feed the birds, frogs, fish, etc.  All invasives reduce the _diversity_ of plants and animals -- diversity that sustains the local ecology as well as supports our life on this planet.