Any substance that enters an ecological system, spreads throughout that system, and kills enough members of the ecosystem to disrupt its structure and function. For example, on July 14, 1991, a freight train carrying the pesticide metam sodium fell off a bridge near Dunsmuir, California, spilling its contents into the Sacramento River. When mixed with water this pesticide becomes highly poisonous, and all animal life for some distance downstream of the spill site was killed.