INDICATIONS THAT SPECIES MAY BE INTRODUCED
Dr. James T. Carlton and John Chapman proposed the following criteria in 1991 for introduced marine isopod crustaceans and they have been used widely to distinguish introduced and native species. However, these criteria are synthetic and there are perhaps infinite other criteria for distinghishing introduced and native species. These criteria all derive from a single question: "How do we know something is native?" For the most part, if a species isn't native, it's introduced. No one species meets all criteria and not all criteria can be tested for every species. However, the more criteria that are applicable, the higher the probability that the species is introduced.
Criterion 1: Appearance in local regions where not found previously. (This criterion can be assessed if the regions in question have been sufficiently sampled prior to the identification of the species in question).
Criterion 2: Initial expansion of local range subsequent to introduction. (This criterion is applicable if there are sufficient historical surveys soon after the introduction.)
Criterion 3: Association with human mechanism(s) of dispersal. (Introduced species commonly remain associated with the human dispersal mechanisms on which they arrived, as discussed below.)
Criterion 4: Association or dependency on other introduced species. (Introduced species commonly occur with, or prey predominantly upon other taxa that are known to be introduced.)
Criterion 5: Prevalence on or restriction to new or artificial environments(s). Introduced species often predominate on or are restricted to human-created substrates, such as cement floats, piers, pilings, rock jetties, or boat-bottoms.)
Criterion 6: Restricted distribution relative to native species distributions. (Introduced species often have northern and southern range limits along a continuous continental margin that are unrelated to classical biogeographic boundaries of native species.) Thus introduced species may occur in some ports or harbors, but not in adjacent, apparently suitable bays, ports or harbors that are inhabited by ecologically similar native species.)
Criterion 7: Isolated population on different continents or in isolated oceans (widespread disjunctive geographic regions). (Few shallow water temperate marine invertebrates of the northern hemisphere, which have well-defined distributions and which are well known taxonomically, have been demonstrated to have naturally isolated intercontinental or interoceanic populations.)
Criterion 8: Insufficient active dispersal capabilities to account for the observed distribution of the species. (Introduced species do not have larval or other adult life history stages and adaptations that permit dispersal to their entire present-day distributions.)
Criterion 9: Insufficient passive dispersal mechanisms that could account for the observed distribution of the species. (Introduced species do not have adaptations for dispersing by passive mechanisms, such as floating on drifting wood or algae or being carried on migrating birds, to their entire present day distributions.)
Criterion 10: Exotic evolutionary origin. (Most introduced species populations have the closest morphologic and genetic affinities to species groups occurring elsewhere in the world.)