In nature, each species functions in certain environmental conditions and plays a unique role in the ecosystem. This environmental and functional space within which a species operates is known as its ecological niche; the spatial extent of that niche defines its range. Changes in ecosystem structure or climatic conditions can alter a species’ range. Suitable conditions expand the range while unfavorable conditions shrink it.

Human activities have played an important role in altering these ranges as people moved species accidentally or intentionally, shaping ecological communities worldwide. For millennia, intentional species introductions were often driven by concerns for aesthetics, food, fiber, or recreation. Yet, the dramatic increase in the post-industrial-revolution volume of shipping and trade has transferred hundreds of species at unprecedented rates and to unprecedented distances. Global interconnectedness has also unintentionally resulted in some species expanding their ranges in ship ballast and transport containers.