Humans often move species around, introducing species that are not native to an ecosystem and disrupting the delicate balance that evolved among species in that ecosystem. Species can be moved both accidentally and intentionally. The introduced species may compete with native species for food or nest sites, or they may prey on native species.
As humans penetrate into more remote places, we allow other species to do the same by using the roads we build. In addition, we transport species by sea. Ships take on water in one location for ballast, travel across the ocean, and then dump the ballast water, carrying new aquatic life forms to habitats already occupied by other species.
We travel from island to island for trade or recreation, taking foreign species with us. Islands are particularly vulnerable because they are isolated and native species have nowhere to go when other species move in.
Note: Emphasized words can be found in the glossary.