For many vertebrate groups, trade in live specimens is thought to be the most important pathway related to their introduction because millions of individuals are transported annually and many of these species have become established outside their ancestral ranges. Because importation records of live vertebrates that enter the United States are maintained by the US Fish and Wildlife Service (USFWS), an unusually complete record of species in trade that progress through all stages of the invasion process can be generated. We obtained importation data for 6 taxonomic groups (amphibians, turtles, lizards, snakes birds, and mammals) from USFWS declaration forms for available years between 1968 – 2005 either directly from the USFWS or from published compilations of these data.

Several hypotheses exist in the literature as to which factors best predict introduction and establishment. Some of these factors, such as introduction effort, can be modified to ask questions specific to the live vertebrate trade and its relationship to species introductions and establishment. We used results of previous studies to identify potentially important factors that affect the introduction and establishment of vertebrate species and could be modified for use as a trade-related variable. We created the following variables: average of number of individuals imported per year for two time periods (past and present) as a measure of import effort, monetary value, origin (captive or wild caught), and previous introduction or establishment success elsewhere. Accordingly, we used these variables to build an a priori set of candidate statistical models that reflect biological hypotheses and tested them among all taxonomic groups. All models were tested using logistic regression, with introduction or establishment as the binary response variable. We used Akaike's information criterion and weights to evaluate the amount of support in our data for each model in our candidate list.

Results/Conclusions

For most taxonomic groups, the best approximating models for introduction or establishment included a measure of past import effort and previous introduction or establishment success elsewhere, with past import effort making the highest contribution toward explaining the variation in the dependent variable. These results suggest that information related to trade can be used to predict species’ introduction and establishment. We suggest that import pressure be considered as an important predictor along with introduction effort, especially in cases where information on introduction effort is not available.