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 cate gorization 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 biogeographi cal (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 deter mined 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 reproduc tion 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 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 mea surements. 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 ofAmerica) 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
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, Pyjsek P, Rejmanek M, et al. (2000) Naturalization and invasion of alien plants: Concepts and definitions. Diversity and Distributions 6: 93-107.
Native plants (synonym: indigenous) are taxa that have originated in a given area without human involvement or that have arrived there without intentional or unintentional intervention of humans from an area in which they are native. This definition excludes products of hybridization involving alien taxa since 'human involvement' in this case includes the introduction of an alien parent.
Alien plants (exotic; introduced; non-native; non-indigenous) are plant taxa in a given area whose presence there is due to intentional or unintentional human involvement, or which have arrived there without the help of people from an area in which they are alien. Taxa can be alien to any definable area, e.g. continents, islands, bio- or ecoregions, or any political entity (countries, states, provinces). Human involvement here does not include habitat changes, global warming, atmospheric nitrogen fertilization, acid rain, etc.
Cryptogenic species are those that are not demonstrably native or alien.
Casual alien plants are those that may flourish and even reproduce occasionally outside cultivation in an area, but that eventually die out because they do not form self-replacing populations, and rely on repeated introductions for their persistence.
Naturalized plants (established) are alien taxa that sustain self-replacing populations for at least 10 years without direct intervention by people (or in spite of human intervention) by recruitment from seed or ramets (tillers, tubers, bulbs, fragments, etc.) capable of independent growth. How long a species must persist to be considered naturalized is inevitably arbitrary hence affects how the definition should be used in practice. A 10-year period reasonably reflects possible negative effects of short-term 'catastrophic events' such as climatic extremes, outbreak of pests, and pathogens, etc. A species may form self-replacing populations for several years and then go extinct; such species should still be termed casual. Taxa persisting in sites where they were planted after cultivation has ceased represent a special category but they can be classified within the current scheme as either casual or naturalized.
Invasive plants are a subset of naturalized plants that produce reproductive offspring, often in very large numbers, at considerable distances from the parent plants and thus have the potential to spread over a large area. Approximate scales suggested for plants are >100 m in <50 years for taxa spreading by seeds and other propagules (for dioecious taxa that rely exclusively on seeds for reproduction, this applies only after the introduction of both sexes); > 6 m in 3yrs for taxa spreading by roots, rhizomes, stolons, or creeping stems. Organisms should be labelled 'invasive' with reference to a given geographic locality. Organisms that spread previously, but do not spread currently because the total range of suitable habitats and landscapes has been occupied, should still be termed invasive because local eradication will undoubtedly lead to re-invasion.
Transformers are subset of invasive plants that change the character, condition, form or nature of ecosystems over an area which is substantial relative to the extent of that ecosystem. The term is an ecological one; a plant can be a transformer without receiving human attention by way of economic concern or control efforts. See text for categories of transformers that may be distinguished and examples of species.
Pests (harmful species; problem species; noxious species - the last term is often used, particulary in USA, for a subset of taxa, those whose control/eradication is mandatory). Taxa (not necessarily alien) that grow or live in sites where they are not wanted and which have detectable economic or environmental impact or both. For plants, a special term 'weed' is used besides those given above. This term is anthropocentric and plant is considered a weed if it interferes with human objectives. The terms 'environmental weeds' or 'species of environmental concern' are used for alien plant taxa that invade natural vegetation, usually adversely affecting native biodiversity and/or ecosystem functioning.
Based on Richardson DM, Pysek P, Rejmanek M, et al. (2000) Naturalization and invasion of alien plants: Concepts and definitions. Diversity and Distribution 6: 93 107 and Pysek P, Richardson DM, Rejmanek M, et al. (2004) Alien plants in checklists and floras: Towards better communication between taxonomists and ecologists. Taxon 53: 131 143.
'migration', 'range expansion', or 'range extension' have been suggested to distinguish these processes from biological inva sions. 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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