Invasive species are species that occur beyond their accepted normal distribution as a result of human activities. Such species threaten agricultural, environmental and other resources by the damage they cause. They interfere by dominating a region or habitat by loss of a natural form of control. They may also be referred to as invasive exotics if they are non-indigenous in nature.
An introduced species might become invasive if it can out compete native species for resources.
Ex: yellow star-thistle and diffuse knapweed