Remarkable aggregation of squamates and caecilians associated with flood events during El Niño in southern Brazil

The effects of the El Niño-Southern Oscillation (ENSO) on biota remain poorly understood, despite the fact that this atmospheric-oceanic phenomenon influence both aquatic and terrestrial lifeforms worldwide. Herein, we report on a massive aggregation of thousands of squamata reptiles and caecilians which were drifted after a period of heavy rainfall. We performed a linear transect of 1,000 meters in length and 25 meters in width on a tourist beach of the Patos Lagoon (31°46’S, 52°13’O), in southern Brazil, during five consecutive days. A total of 2152 amphibians and reptiles were registered, with the majority 2002 (99.5%) belonging to the Pampean Water Snake species, Helicops infrataeniatus Jan, 1883. The occurrence of island species closely related to continental ones or even the presence of the same species in both environments is undoubtedly one of most exciting fields of biogeographic research worldwide. Dispersal by rafting on vegetation has been pointed as an important mechanism to promote the entrance of pulses of continental fauna into islands. However, there are few concrete evidences to support the hypothesis that viable populations of terrestrial vertebrates can be transported by water and settle in new environments. Our finding of a massive naturally elicited aggregation of amphibians and squamates at the shoreline of an estuary, accompanied by masses of plant materials, provide a piece of evidence for the extant probability of over-sea dispersal of animals surviving on rafts of vegetation. Colonization of new areas by amphibians and squamates (including fossorial forms) by such events appear more likely in view of our observations. Similar events of inundation with a massive drift of herpetofauna elements have repeatedly occurred in this region at least four times in the past decades, all associated with strong El Niño years.