The outcomes of introductions to a given region are determined by several sets of features: (1) biological and ecological traits of the species; (2) dispersal possibilities and availability of suitable vectors; (3) resistance or vulnerability of recipient habitats; (4) historical circumstances, including the effect of residence time; and
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, etal. (2004) Alien plants in checklists and floras: Towards better communication between taxonomists and ecologists. Taxon 53: 131-143.
(5) geographical determinants such as the position of the target region, climate, or latitudinal patterns.
Several robust generalizations related to the geography of invasions have emerged recently. The number of naturalized species in temperate regions increases with temperature (and hence decreases with latitude), and their geographical ranges increase with latitude. Temperate mainland regions have more invasive species than tropical mainland regions. The high production ofbiomass ofnative species and rapid recovery of wet tropical vegetation after disturbances, rather than the high species diversity per se, probably accounts for the lower levels of invasions in tropical ecosystems. Tropical islands are, however, as invaded as temperate islands. Islands are generally more susceptible to invasions than mainlands. This is attributed to factors associated with their isolated evolutionary development, including low species diversity and absence of ecologically important groups of organisms. In the Galapagos Islands, over 3 million years of their history, only one new plant species arrived with birds or sea currents approximately every 10 000 years. Over the last 20 years, however, the human-assisted introduction rate has been about 10 species per year, or some 100 000 times the natural arrival rate.
At the global scale, the ecosystems most transformed by invasions of alien plants are: Mediterranean-climate areas (with exception of the Mediterranean Basin itself) in South Africa, California, Chile, and Australia; temperate grasslands in North America, South America, and Australia, that have been invaded by annual grasses mostly from Europe (e.g., B. tectorum); savannas and forests in humid and subhumid tropics and subtropics, especially in Central and South America, invaded by African C4 grasses such as Hyparrhenia rufa and Melinis minutiflora; tropical and subtropical habitats in Africa and Asia dominated by Neotropical woody plants like Ageratina adenophora and Lantana camara; and tropical wetlands and aquatic ecosystems on all continents. Undisturbed tropical forests, on the other hand, harbor only a very small number of alien plant species, most of which do not spread substantially beyond trails and gaps. Temperate agricultural or urban sites are the most invasible biomes, and the New World is more prone to invasion than Old World.
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