eDNA reveals a link between alien bullfrogs and the occurrence of amphibian diseases
The American bullfrog is an non-native amphibian species that arrived in Belgium from North America. This invasive non-native species can carry pathogens that have little effect on it. Two of these pathogens, chytrid fungus and ranavirus, are responsible for the decline in amphibian populations worldwide. But are these pathogens also present in Flanders, and is there a link with bullfrogs?
In order to investigate this, we developed an environmental DNA (eDNA) protocol that can be used to detect the presence of both pathogens in water. This protocol was applied to eDNA samples from Flemish ponds in the distribution area of the bullfrog. We looked not only at whether the DNA of the pathogens was present, but also at its quantity and that of the bullfrog itself.
The chytrid fungus was present in 30% of the 157 ponds studied. Both the presence and quantity of the fungus appeared to be strongly linked to the presence and quantity of bullfrogs. The ranavirus, on the other hand, was found in only 6% of the ponds and showed no connection with the presence of bullfrogs.
The results indicate that ponds colonised by bullfrogs are more likely to harbour the chytrid fungus. Our native amphibians are more exposed to this pathogen in these ponds. The research therefore shows a link, but does not prove that bullfrogs themselves spread the pathogens. Further studies are needed to unravel their exact role in this story.
Teun Everts & Rein Brys
Image above: American bullfrog (Rollin Verlinde - Vildaphoto)