The word "weed" may come to mind
But that’s not exactly what we mean. 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 a matter of context. Many plants that are desirable natives in natural areas are weeds in cultivated fields, managed pastures, forest plantations, or, if human activity alters the original disturbance regime, even their original habitats. At the same time, useful crops or forage plants can be undesirable in native ecosystems. So, we’re calling the subjects of this document aliens — plants not native to Alberta in the recent past, prior to European settlement. 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 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. However this statement may simply be a tautology --- would we say "aggressive" if it were not so? It may well be that untangling other causes of disturbance from the properties of the invader is not possible. 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.
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.