We've seen so many instances in which humans interfere in natural processes without carefully thinking things through. For instance, all of the species of plants and animals that have been artificially introduced into systems in which they have no natural enemies, to the detriment of native species.

Kudzu in the southern US, rabbits and predatory mammals in Australia, purple loosestrife in the northeastern part of the US, the Japanese beetle in the US, the Norway rat in the US... the list is vast. And the cost to economies to try to control these species is also vast.

A pest is any species that occurs where it should not occur, and that causes problems as a result. GMOs are essentially new species. There is no place in nature where they belong. They are no different than any other introduced species. Introduced species tend to escape cultivation. Sometimes they don't cause problems. But when they do cause problems, these problems are often spectacular.

This is a reality with which we have been dealing, to our detriment and great frustration, for many years. It's a bit beyond me to understand why we have not yet learned our lesson in this regard.

The Norway rat...

Came into...United States in 1775... Wild Norway rats destroy about 300 million to 2 billion dollars' worth of farm products yearly in United States and 20 million dollars' worth in Canada. Our worst pest.

--Fieldbook of Natural History (second edition), by E. Lawrence Palmer and H. Seymour Fowler

This book was printed in 1975, so the numbers are rather dated. My guess is that they would be much higher in today's dollars. We don't know what the result will be of GMO species escaping cultivation. However, considering our prior experiences, I'm at a loss to understand why we wouldn't want to exercise the utmost caution.