Beastly Bullfrogs!

American Bullfrogs around Western North America are being labelled as one of the major causes of amphibian decline! 

The American Bullfrog, Rana catesbeiana, is becoming of great scientific and conservational interest as studies shine light on its impact on native populations of amphibians, especially frogs. With this new information, scientists are hoping to be able to manage this species and potentially eliminate the negative effects it has all together. 

Figure showing the worldwide distribution of the American Bullfrog 

So where have they managed to get to?

Over the years, this Invasive Alien Species has been introduced to over 40 different countries. The world map to the left shows the native distribution in grey and the non-native, introduced areas in black. As you can see, these famous frogs originate in Eastern North America and have managed to spread to areas of Western North America, South America, Europe and Asia due to human transportation and habitat modification.  

Why are they so destructive? 

American Bullfrogs are causing mayhem amongst native frog populations and Blaustein and Kiesecker repeatedly found that at sites where Red-Legged Frogs were absent, American Bull Frogs would be found close by. So what were the Bullfrogs doing that was so detrimental? 

Their studies showed that the American Bullfrog was basically eating the Red-Legged species. At both the tadpole and adult stage they recorded the Bullfrogs feeding on other amphibian species, especially the Red-Legged ones. The middle image to the left shows the scale of the American Bullfrog tadpole compared to the Red-Legged one on the image below that. 

However, this wasn't the only thing that was going on! Sometimes, as a response to the arrival of the American Bullfrog species, the Red-Legged ones would change their microhabitat, possibly by moving somewhere else in the water or changing their feeding habits. Unfortunately, in some cases this pushed the native frogs into an area where they were at increased risk of predation by fish, which in turn reduced the population numbers. 

Similar to this, Kiesecker and Blaustein found in another study that the time took for Red-Legged tadpoles to reach the metamorphosis stage increased and the body mass reached by the end of metamorphosis decreased when in the presence of American Bullfrogs. As a result the Red-Legged frogs that did survive were smaller and less competitive. 

But wait, there's still more!! American Bullfrogs are also causing damage through transmitting the fungus Batrachochytrium dendrobatidis which causes the infectious disease Chytridiomycosis. Greenspan et al explored how this disease does not always lead to death in all species. Unfortunately for the Red-Legged frogs, the disease is lethal and American Bullfrogs are unaffected by it and therefore they act as vectors that spread the disease to new populations they invade.   

   

Have humans had an impact?

Unsurprisingly, yes we have! Blaustein and Kiesecker found that habitat modifications made in Western North America actually led to more optimal conditions for the growth and success of the Invasive Alien American Bullfrogs. Converting large ephemeral wetlands into smaller permanent ponds, possibly though the development of infrastructure, creates prime breeding grounds of the Bullfrogs whilst removing the preferred spaces for other amphibian species. 

Also, American Bullfrogs only made it to places like Europe and Asia through the pet trade and accidental transport, a common way Invasive Alien Species are spread, as mentioned is last weeks post. 

To see The Simpsons experience with Invasive Alien Bullfrog species, click the link 

https://www.youtube.com/watch?v=g3V0wJ3q7l0