Response to Ricciardi. Assessing species invasions as a cause of extinction

In our recent article in TREE [1], we considered the evidence for the common assumption that invaders are a major cause of species extinction. Ricciardi [2] criticized the focus of our paper, and challenged our statement that the role of zebra musselsDreissena polymorpha in unionid mussel extinctions is unclear. Like Ricciardi [2], we recognize and appreciate the importance of local population losses (extirpation). Risks to evolutionarily significant units (ESUs) rather than species have been the basis for managing endangered Pacific salmonids, and threatened populations are the focus for Endangered Species Act listing in the USA. However, our paper focused on the role of invasives in extinction, rather than in local losses, for several reasons. First, we believe strongly that better determination of the factors implicated in the decline of species threatened with global extinction is crucially important. Local losses might or might not be reparable, but, as the saying goes, ‘extinction is forever’. Second, Ricciardi argues that we should examine species that have undergone significant range contractions and population losses; we relied in part upon data from the IUCNRed List (http://www.redlist.org/) in making our argument, and many species suffering such contractions and local losses are on that list. Finally, as a practical matter, equivalent extensive validated lists for local populations are unavailable, even though they would be of great value if anyone was able to compile them. We questioned the role of zebra mussels in the extinction of unionids. Ricciardi argues that empirical modeling has suggested that zebra mussels have greatly accelerated the local extirpation of unionid species. The rapid rate of extinction of these bivalves was recognized long before zebra mussels invaded North America [3]. For these bivalves, the primary extinction threats are habitat loss and degradation (including construction of dams, development, pollution and toxic sediments [4–7]), harvest for the cultured pearl industry, and loss of native