Toadly Screwed

Invasive amphibian could make Madagascan predators 'croak'.

An inspiring talk by Wolfgang Wüster at Bangor's Herpetological society explored the recent research publication on the effects of invasive Asian toads, Duttaphyrnus melanosticus on native fauna of Madagascar.

Background

Madagascar is a threatened biodiversity hotspot due to its unique island fauna. Most of the species on the island are maladapted to predation, competition, novel pathogen transmission and toxins from non-native species. Consequently, the predicted effects of the introducton of D. melanostictus could be devastating to local fauna.

Bufonid toads can produce toxins causing cardiac arrest and even death

Bufonid toads like D. melanostictus possess skin toxins, called bufadienolides, secreted from the paratoid gland (fig 1.). These toxins are called cardiac glycocides, which they cause heartbeat irregularities due to the blocking of sodium-potassium ion channels which leads to cardiac arrest and ultimately death. The presence of these bufadienolides on naive fauna could potentially be devastating. A classic example of this is from cane toad invasions in Australia where the introduction of cane toads to control local bug populations lead to a devastating decline in native fauna, including varanid lizards, colubrid snakes and mammals.

Figure 1. Duttaphyrnus melanostictus with paratoid gland, where toxins are secreted, highlighted.

Hypothesis

The importance of this study was to address potential impacts of these toads on the native fauna in order to promote any preventative measures to halt the introduction and spread of D. melanostictus. They predicted that there would likely be no resistance to the bufanolide toxins which would have a high impact on local fauna. To test their predictions, they measured resistance of various fauna to the bufanolide toxins by sequencing ion channel sites for the resistance to the toxin.

Their results proved quite disappointing; > 3 snake lineages that could have possibly possessed the toad resistance (boas, mimophis and pseudoxyrhophinae) proved SENSITIVE to toad toxins. >Amphibian and lizard groups were also SENSITIVE to toad toxins

>Birds, also SENSITIVE to toad toxin

Figure 2. Bufanolide toxin structure.

Is there a light at the end of the tunnel?

Two species of mammal were found to be RESISTANT to the toad toxin, the white-tailed ansangy and the brown rat. However... the ansangy is a herbivore, meaning it would never consume toad toxins regardless and the brown rat is mostly omnivorous with occasion to eat frogs if necessary, but would likely be tadpoles or juvenile frogs. They also found some groups that didn't show resistance but did not show NO resistance, which could suggest TOLERANCE to the toxins.

Summary

In summary, the research was rather inconclusive, with a lot of unanswered questions, including the difficulty to predict the true impact of the toads on local fauna, but we can likely suggest from previous examples (such as Australia) that some fauna will be negatively effected but hopefully through predator learning experiences in various taxa they can recover.

This talk particularly inspired me to explore avenues of research into vulnerable fauna and provided the opportunity and contacts to explore studies in the ontogeny of toxicity in toads to further understand the effects in Madagascar or further baseline studies ahead of invasion to understand the impacts on other populations.

Asian toad, Duttaphrynus melanostictus.

Ujvari, B., Casewell, N., Sunagar, K., Arbuckle, K., Wüster, W., Lo, N., O’Meally, D., Beckmann, C., King, G., Deplazes, E. and Madsen, T. (2015). Widespread convergence in toxin resistance by predictable molecular evolution. Proceedings of the National Academy of Sciences, 112(38), 11911-11916.