Introduce an exotic species and ecological chaos usually results. In Pennsylvania, examples such as house sparrows, multiflora rose, emerald ash borers and house mice come to mind. Two more exotic species have made headlines recently.

A Japanese ship sank off the coast of an uninhabited, 10-square-mile island in Alaska's Aleutian Island chain in the early 1780s. A few Norway rats (think New York City sewer rats) made their way to shore, and eventually they decimated the island's populations of ground-nesting birds.

Last year, the U.S. Fish and Wildlife Service joined forces with The Nature Conservancy and Island Conservation in a $2.5 million project to eradicate the rats on Rat Island, part of the Alaska Maritime National Wildlife Refuge. For 10 days helicopters dropped rat poison around the island. Today Fish and Wildlife reports there are no signs of living rats, and some birds have already returned to nest. If Rat Island is truly rat-free -- that has yet to be determined -- waterfowl, gulls, murrelets, auklets, storm petrels and song sparrows are expected to resume nesting on the island.

Thousands of miles to the south in Florida, wildlife and residents are dealing with an exploding population of Burmese pythons. Pet owners release these huge constrictors when they realize these snakes can grow from 20-inch babies to 8-foot juveniles in just one year. Adults that reach lengths of 20 feet and weights of nearly 200 pounds simply don't make good pets. The problem began more than 20 years ago when snake owners began releasing pythons in the Everglades.

Biologists estimate that 20,000 to 30,000 pythons currently inhabit the Everglades. Their most disturbing trait is their diet. Pythons eat everything from endangered wood storks to adult white tailed deer and adult alligators.

Recently, nine pythons have been confirmed on Key Largo, which is separated from the Everglades by 6 miles of water. Florida biologists admit it's no longer possible to eliminate pythons; containment is the current management objective.

The ecological lesson of these examples is clear. Intentionally introducing exotic species invariably upsets ecosystem function. It's never a good idea.