Invasive species are organisms that have expanded their geographic ranges and become established in new habitats. Although species invasions have occurred throughout evolutionary history, human activities have greatly increased their frequency and scope, particularly in the last 200 years. Although only a fraction of species introductions result in established, invasive populations, successful invasions can cause profound negative changes in recipient ecosystems and the human societies dependent upon those ecosystems. Ecologists, economists, resource managers, and policymakers devote considerable energy to studying and minimizing the negative impacts of species invasions.