6. The Xenopus laevis Invasion in Portugal: An Improbable Connection of Science, Mediterranean Climate and River Neglect

The source populations of one of the most worldwide spread invasive amphibians—the African clawed frog, Xenopus laevis—inhabit the Mediterranean-type climate region of South Africa. Frogs captured here were exported to many world regions during the 20th century with scientific purposes. Soon after, established invasive populations were found in several places, the majority of them subject to Mediterranean-type climates. Here we reconstruct the historical contingencies that explain the transport, site of arrival, establishment, lag phase and expansion of this invasive frog in Portugal. Improbable events were associated with practically all stages of the invasion process. Individuals were probably brought to Portugal by a single researcher and their escape from her lab was facilitated by a disastrous flood. Despite the striking differences from native frogs, this species apparently lived undetected by biologists and authorities for more than 20 years in two streams in Lisbon metropolitan area, and the discovery of the invasive populations was only prompted by the beginning of stream restoration actions. Since 2010, this frog is being subject to a control and eradication program. We provide details on the progress of this program, as well as on the role that geographic and historical contingencies may have for its final success.