DO Toxic Invasive Prey Become a Toxin Source for Native Consumers?

Toxic organisms can become food that potentially harms consumers. When these organisms become invasive species, the harm often turns to a serious threat that disrupts native ecosystems. On the other hand, there are consumers that can exploit toxic organisms for food and sequester intact toxins from them for the consumers' own chemical defense. Therefore, it can be expected that toxic invasive prey can become a toxin source for native consumers. Here, we focused on the relationship between toads, which are one of the major toxic invasive organisms and possess bufadienolides (BDs), and Rhabdophis snakes, which sequester BDs from toads. On Sado Island, Japan, R. tigrinus is native, but no toads had inhabited this island until Bufo japonicus formosus was introduced as a domestic invasive species in 1963 and 1964. At present, invasive toads are distributed only in the southwestern part of the island. We collected a total of 25 and 24 R. tigrinus from areas allopatric and sympatric with toads, respectively. Then, we investigated the possession of BDs and the BD profile of these snakes. We found that only R. tigrinus sympatric with toads possessed BDs, whereas all snakes allopatric with toads lacked BDs. Based on the characteristics of the BD profile, the toxin source was identified as B. j. formosus. Our findings show that a new case of impact caused by toxic invasive species, i.e., "toxin supply to native consumers from invasive prey", could occur.