Terms and Definitions

Unfortunately, terminologies and criteria for defining alien plants with respect to their status have evolved differently in different parts of the world. Criteria for objective categorization are often complicated by complex human-value systems. Plants encroaching in habitats in which they were not present before can be assessed from an ecological point of view (and termed colonizers) or from the biogeographical (invaders, or alien plants in a more general sense) or anthropocentric (termed weeds, pests, etc.) point of view. The biogeographical approach is preferred.

The status of a plant taxon in a given region is determined by (1) whether it is native or alien to that region (origin status); (2) when was it introduced (residence status); and (3) its degree of naturalization/invasion (invasion status). Defining the invasion status is the most complicated because there is a continuum of states. A theoretical framework with precise definition has been established to which real situations can be related.

Human activity is a key driver of invasions. An alien taxon is one that would not be present in the area had it not been its translocation by people; analogically, native taxa are those that would be present without human intervention. The invasion process comprises a sequence of barriers that a species must overcome (Figure 1). The geographical barrier between the region of origin and a target region is overcome with the help of humans. The second key principle is the ability to reproduce in the invaded region without the assistance of humans (or despite various human factors that potentially thwart reproduction). Successful reproduction is crucial; it separates casual alien species from naturalized or invasive species. Dispersal is another crucial prerequisite for invasion. Three categories of invasion status are distinguished: (1) casual alien plants -those that do not form self-replacing populations in the invaded region and whose persistence depends on repeated introductions of propagules; (2) naturalized plants - those that sustain self-replacing populations for several life cycles

Status of taxa

Status of taxa

Figure 1 Schematic representation of the naturalization-invasion continuum. A species must firstly overcome major geographical barriers such as oceans, mountain ranges, deserts. If it also overcomes environmental barriers in the area of introduction (climate constraints, seed predation, etc.), it becomes a casual alien. To become naturalized, the species must also cope with reproductive barriers. Some species cannot reproduce because of absence of one sex (in the case of dioecious species, short intervals between disturbances that prevent them from producing ripe seeds, etc.). If the species is able to overcome dispersal barriers and environmental barriers from resident vegetation, it may become invasive. Naturalization, that is, the capability of forming self-sustaining populations without human nurturing, is a crucial step in the invasion process. Adapted from Richardson DM, Pysek P, Rejmanek M, et al. (2000) Naturalization and invasion of alien plants: Concepts and definitions. Diversity and Distributions 6: 93-107.

Figure 1 Schematic representation of the naturalization-invasion continuum. A species must firstly overcome major geographical barriers such as oceans, mountain ranges, deserts. If it also overcomes environmental barriers in the area of introduction (climate constraints, seed predation, etc.), it becomes a casual alien. To become naturalized, the species must also cope with reproductive barriers. Some species cannot reproduce because of absence of one sex (in the case of dioecious species, short intervals between disturbances that prevent them from producing ripe seeds, etc.). If the species is able to overcome dispersal barriers and environmental barriers from resident vegetation, it may become invasive. Naturalization, that is, the capability of forming self-sustaining populations without human nurturing, is a crucial step in the invasion process. Adapted from Richardson DM, Pysek P, Rejmanek M, et al. (2000) Naturalization and invasion of alien plants: Concepts and definitions. Diversity and Distributions 6: 93-107.

or a given period of time (say 10 years) without direct intervention by people; and (3) invasive plants - a subset of naturalized plants that produce reproductive offspring, often in very large numbers, at considerable distances from the parent plants and which have the potential to spread over large areas (Table 1).

If sufficient information is available, taxa can thus be labeled using reasonably objective criteria according to their position along the naturalization-invasion continuum (Figure 1). These definitions are based on measures of population growth and spread, and do not rely on the (usually) subjective assessment of impact. They capture ecological process that can be confirmed with simple measurements. Application of this scheme will lead to uniformity and progress in understanding the processes driving invasions.

Residence status defines how long an alien species has been present in the region. In Europe, alien species are traditionally classified as archeophytes (introduced <1500; approximately corresponding with the discovery of America) and neophytes (introduced >1500). The separation between natives and archeophytes relies on a combination of paleo-botanical, archeological, ecological, and historical evidence (archeophytes and neophytes are absent from the fossil record in the last glacial period, the late glacial, and the early post-glacial). Archeophytes are often known from archeological evidence to have been present in prehistoric times. In other parts of the world, for example, Australia, a distinction is sometimes made between taxa that arrived before or after European colonization. In Hawaii and other Pacific Islands, alien species are sometimes categorized according to whether they were introduced by Polynesians before Captain James Cook's voyage in 1778, or later.

The term 'invasion' should be used only with reference to the dynamics of alien plants. For changes in distribution ranges of (native) plants after the retreat of glaciation, terms 'migration', 'range expansion', or 'range extension' have been suggested to distinguish these processes from biological invasions. Species that increase their distribution and colonize new habitats in a geographical area where they are native are termed 'expansive' and the process 'expansion'.

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