The word "weed" may come to mind
But that’s not exactly what we have in mind. Some of the plants listed are weeds by just about any definition. Others are valued for their agricultural uses — they are not weeds in the usual sense. Whether a plant is a weed or not is entirely a matter of context. Plants that are desirable natives in a natural area may be weeds in cultivated fields, managed pastures, or forests, or, if human activity alters the original disturbance regime, even in natural areas. At the same time, useful crops or forage plants can be undesirable in a native ecosystem. So, we’re calling the subjects of this document aliens — plants not originally native to Alberta. We do list some species that are only weeds in an agricultural context as far as we know — they are alien, and regarded as weeds for regulatory purposes, but not problematic in natural environments. Alien invaders are aliens that once introduced into native habitats can do very well for themselves without further help, thank you. They not only survive and reproduce in their new environment — they displace native plants, by competition for resources, lack of co-evolved predators and pathogens, or even by direct chemical antagonism (alleopathy) or parasitism (via mychorhizae, if not direct) against which native plants that did not co-evolve with them have no defense. Some are agronomic species, others are just fellow-travelers introduced by people from Europe more by accident than design. The most aggressive alien invaders reduce biodiversity, by forming near-monospecific stands in previously “undisturbed” sites, displacing a multi-species native ecosystem. Aliens often reduce productivity for uses like grazing. The ability of alien invaders to establish themselves in native habitats interferes with the objective of preserving those habitats, interrelationships among species, and evolved resiliency and adaptation to natural disturbance and climatic variation. When we speak of native plants, we mean taxa that evolved here and were present as components of the ecosystems that existed here prior to the onset of European settlement.
The picture is by no means simple, or fully understood. Not all introduced species that survive without cultivation are invasive. Not all species that survive and reproduce unattended in neglected urban corners get the urge to move to the country. Some aliens invade rapidly, others possibly so slowly (however irreversibly) that we have not noticed. It’s not always clear whether an alien species is an invader — capable of driving the process of displacement of native species in undisturbed habitat, or a camp-follower, riding on the coattails of distress. An alien may dominate an otherwise native community because it is aggressive, or because it is better adapted than the natives to reduced dispersal due to fragmentation, altered disturbance regimes, climate change, or other stressors. Disturbances like fire, to which native ecosystems are adapted, may trigger invasions if alien seed is present, although the alien in question may be unable to establish itself otherwise, or to invade later stages of succession of the native vegetation.
To complicate things even further, it’s not always clear if a taxon is native or alien (or a hybrid). Some invasive aliens are distinct species, unrelated to anything that occurred before their introduction. Others are of the same species as a native taxon, yet differ enough that their behavior in native habitat is different. As interbreeding occurs between native and alien groups within a species, intergradation may change the role of the entire species in native ecosystems.