United States Commerce in Live Vertebrates: Patterns and Contribution to Biological Invasions and Homogenization
Type of Degreedissertation
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The trade in live vertebrates is a threat to biodiversity, homogenizing distinct flora and fauna, introducing invasive species and parasites, and depleting wild populations. Because records of live vertebrates imported or exported by the United States are maintained by the US Fish and Wildlife Service (USFWS), a complete record of species in trade can be generated for this country. I obtained USFWS records for 6 taxonomic groups (amphibians, turtles, lizards, snakes, birds, and mammals) from 1968 – 2006 and used these trade data to quantify patterns in trade over time, and assess its importance as an invasion pathway and contribution to biological homogenization. The United States transported over 4200 species of terrestrial vertebrates during 1968 – 2006. Because trade in live vertebrates is dynamic, there have been changes in the species used for trade, quantities of individuals traded, and trading countries. I found that trade in live vertebrates contributes to both mechanisms of biological homogenization, extinctions and introductions. Based on Monte Carlo sampling, the number of species traded, established, and threatened with extinction were not randomly distributed among vertebrate families. Vertebrate families that were traded preferentially were also more likely to be established or threatened with extinction, compared to families that were not traded preferentially. I followed this research with additional work that focused solely on introductions. I used USFWS trade data to estimate the number of species that have transitioned successfully through the five stages of the invasion process, and compared those transition rates to those expected by the “tens rule”. I found that roughly 10% of all vertebrate species imported to the United States were introduced. Birds and snakes did not differ from what was expected by the “tens rule” for the establishment transition. Amphibians, lizards and snakes exhibited a high transition success at the establishment stage (~ 45%). All vertebrate species differed from the “tens rule” in the final spread stage, their transition success was approximately 40%. Finally, I used human influence variables (import pressure, previous invasion success elsewhere, monetary value, wild caught vs. captive bred) to assess their ability to predict introduction and establishment success among vertebrate species imported to the United States. Import pressure was measured as the average number of individuals imported and separated by time period into past and recent import pressure. Among the a priori models, those that included past import pressure were the best models consistently across all 6 vertebrate groups. For specific taxonomic groups, previous success elsewhere was the most important variable among the top models, and improved the prediction of introduction and establishment success.