The Tens Rule, proposed by Williamson and Brown, is a probabilistic assessment of the proportion of species that reach particular stages in the invasion process. It predicts that 10% of imported species escape to become casual, 10% of casuals become naturalized, and 10% of naturalized species become pests. Alien pests thus comprise very roughly only 1% of the introduced species found casual. It was derived from European plants, but the general principle that successful invasions are rare holds for other regions and across many taxa of plants and animals. However, the reasonably constant proportion of alien taxa that invade across a wide range of systems is to some extent a result of the similar residence times of alien species in different alien floras. The estimates of proportion of species that go through stages of invasion process are thus bound to change over time. The Tens Rule is a useful generalization that can be used as a benchmark to which real data can be related; deviations indicate taxa with higher or lower invasiveness and regions/habitats with lower or higher invasibility.
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