The contribution of the live vertebrate trade toward taxonomic homogenization

Background/Question/Methods
The process of taxonomic homogenization is accelerated by human-mediated transport, and can be expected to increase with global trade, as wildlife can be transported more quickly and easily now than in the past.  One aspect of this increased transport, the trade in live vertebrates, is known to be a major pathway for introductions and also is responsible for declines of wild populations due to overharvesting. The transport, introduction, and extinction of species have been shown to be distributed non-randomly among higher taxonomic groups. Use of data describing trade in live vertebrates, lists of species of global conservation concern, and records of successful vertebrate introductions provide a framework to explore the relationship between trade and species introductions and extinctions.  When examined across taxonomic groups, such data should highlight the relative role of vertebrate trade in the homogenization process and should allow conservation efforts to be concentrated on taxonomic groups that are particularly problematic.  In this study we used records of species transported to and from the United States as a proxy for the pool of species traded globally, and addressed the potential for taxonomic homogenization specifically through the lens of trade.  We assessed whether the observed number of species for three categories (number of species traded, successfully introduced, and IUCN-listed) are randomly distributed among six vertebrate groups (amphibians, turtles, lizards, snakes, birds, and mammals) and their nested sets of families.

Results/Conclusions
We found that the number of species in each category was not randomly-distributed among vertebrate groups, or the families within those groups.  The importance of each homogenizing mechanism (introduction or extinction) differs among each vertebrate group and the families within.  Vertebrate groups and families with both more successfully introduced and IUCN-listed species than expected are most susceptible to the homogenization process because they could be homogenized through both mechanisms.  Families with more successfully introduced or IUCN-listed species than expected contribute to the homogenization process though the respective mechanism.  The use of trade data allowed us to assess directly the impact of human influence on the homogenization process for all the vertebrate groups we analyzed, an important factor affecting the global and taxonomic patterns of homogenization.  When making conservation recommendations regarding restriction of trade due to its contribution to the homogenization process, specific attention should be given to those vertebrate groups and families that contain a disproportionate number of species that are traded, successfully introduced, and/or IUCN-listed.