Invasive Species in the Pacific NorthwestThe U. S. government defines invasive species as "an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health." Invasive Species in the Pacific Northwest describes these species, how they got here, and the effects of their invasions on the region's environment. Each of 108 invasive species of fish, plants, invertebrates, mammals, and birds including earthworms, domestic cats and pigs,