Cane toad ingestion deadly for dwarf crocs

It is well known that the toxins of the cane toad are often deadly when ingested by predators. Researchers from Charles Darwin University (CDU) have found that one such predator is the dwarf freshwater crocodile, whose population in northern Australia is diminishing as a result.

Dwarf freshwater crocodiles live upstream, in areas with limited resources; the lack of food means the growth of the animals is severely stunted. Thus, explains CDU’s Dr Adam Britton, cane toads provide them with a rare and plentiful meal. But not only does the ingestion of the toads cause the individual crocodiles to die, but their small size and lack of alternative food sources makes them a particularly vulnerable species.

To measure the impact of cane toads on the crocodiles, population surveys were conducted in three upstream creek systems, using day- and night-based survey methods, before the arrival of cane toads in the area. The surveys were repeated following the arrival of cane toads to compare the distribution and densities of freshwater crocodiles.

The researchers found “significant declines in crocodile density at two survey sites following the arrival of cane toads,” though the third site seemed unchanged. “There was a decline in mean density across all sites from 3 crocodiles km^-1 to 1.1 crocodiles km^-1,” they said. The discovery of dead crocodiles, and cane toad carcasses with crocodile bite marks, confirmed the reason for this decline.

According to Dr Britton, this presents a significant conservation issue not only for the dwarf crocodiles but also potentially for the entire upstream escarpment ecosystem.

The study did yield some positive results though. Dr Britton says the researchers have gained some insight into “why toads wipe out some populations but leave other ones largely unaffected.” The team also found possible evidence that the crocodiles could adapt their behaviour in the future by only eating the back legs of toads to avoid being poisoned.

They are now conducting further research into the genetics of dwarf freshwater crocodiles to better understand the long-term impacts of cane toads on their populations.

Their paper has been published at www.publish.csiro.au/paper/WR12215.htm.