Geographical Patterns

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 circum stances, including the effect of residence time; and (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 tropi cal ecosystems. Tropical islands are, however, as invaded as temperate islands. Islands are generally more susceptible to invasions than mainlands. This is attributed to factors asso ciated 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 grass lands 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 sub humid tropics and subtropics, especially in Central and South America, invaded by African C4 grasses such as Hyparrhenia rufa and Melinis minutiflora; tropical and sub tropical habitats in Africa and Asia dominated by Neotropical woody plants like Ageratina adenophora and Lantana camara; and tropical wetlands and aquatic ecosys tems 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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