When a species ends up in a new ecosystem, it is considered "introduced". Often, invasive species are spread by humans who do not realize that these plants, animals and insects are highly destructive.

This may happen, for example, when people plant garden ornamentals, range forage plants for cattle, or plants used for erosion control and habitat enhancement for wildlife. This can also occur when animals and insects are introduced to be used to control other organisms (particularly in agriculture).

Other species are introduced accidentally on imported nursery stock, fruits, and in ship ballast waters, on vehicles, in packing materials and shipping containers, through human-built canals, and from human travel. Dumping aquarium exotic fish and unwanted exotics into the water or wild are other common ways invasive species spread.