Them Bullfrog Blues

Have you ever woke up with them bullfrogs on your mind?
I got the bullfrog blues and I can’t be satisfied,
Mexico got ‘em,
Japan got ‘em,
Cuba got ‘em,
Jamaica got ‘em too,
I got the bullfrog blues and I can’t be satisfied,
Italy got ‘em,
France got ‘em,
Brazil got ‘em,
the Netherlands got ‘em too
I got the bullfrog blues and I can’t be satisfied,
Canada got ‘em,
China got ‘em,
Brazil got ‘em,
And Columbia got ‘em too,
I got the bullfrog blues and I can’t be satisfied,
Uruguay got ‘em,
Venezuela got ‘em,
Argentina got ‘em,
And South Korea got ‘em too.


When the original version of this superb pre-war blues was penned by William Harris in 1928, bullfrogs had yet to be transplanted by humans outside of their native range in eastern North America. I have necessarily modified the lyrics accordingly, but the sentiment remains the same. Today, bullfrogs are found on every continent besides Antarctica, and thus it is not just the bluesmen of the Deep South that have bullfrogs on their minds. To me, the two pertinent questions are How has this happened? and Why does it matter? The first is simple, the second multifaceted, subject to personal bias, and can only be briefly touched upon in a single blog post*.

The short answer to the question of how this situation arose is: people. The slightly longer answer is: people… obviously. More informatively, bullfrogs have both been introduced accidentally, as stowaways or hitchhikers, and purposefully, with the intention of farming them for their meaty (and I’m told tasty) legs. Despite the mechanism, the end result is the same; an animal is now where it’s not supposed to be. The myriad consequences of these artificial dispersal events are still being quantified, but we have been witnessing the detrimental impacts of bullfrogs for decades and it is truly terrifying to extrapolate out from the irreparable damage caused by a single species to the carnage posed by biological invasions generally. We have only documented the tip of the iceberg when it comes to the impact of invasive species, but it is patently clear that if we do not divert our course, this thing is big enough to sink us.

Ecosystems are intricate interwoven fabrics, such that you cannot alter one aspect without having knock-on effects that percolate throughout the rest of the . Herein lies the destructive power of invasive species. Their impacts are often so far-reaching they are near-impossible to predict, and have only begun to be quantified. The most intuitive impacts of invasive organisms are those directly experienced by closely related species through competition; i.e. if bullfrogs are introduced, something has to make way. A loose ‘one in, one out’ policy was elegantly described by Bob MacArthur and E.O. Wilson studying island communities. This delicate balancing act however, assumes that the system in question exists in a serene, idyllic state of equilibrium. Invasive species are by definition a severe disruption to the equilibrium, and tend to exert a much more damaging influence on local fauna, typically leading to more of a ‘one in, many out’ situation. Indeed with bullfrogs, this is what we see the world over.

Native frog communities likely tell stories of bullfrogs akin to those yarns spun by Romans of Visigoths**. Standing 20cm long, weighing 1.5kg, they will eat anything that fits in their mouth, from spiders to sparrows. Their tadpoles are toxic, allowing them to breed in virtually any body of water, and circumventing any population regulation from predatory fish. They arrive, they establish, and they multiply. Unimpeded, they reek havoc on competitors and prey alike. Direct impacts such as these are well understood and receive the majority of attention from scientists studying invasive species. Interestingly in the case of bullfrogs however, and the role they play in global amphibian declines, indirect effects are of much greater concern.

American bullfrogs are thought to be responsible for the rapid spread of a deadly fungal pathogen that threatens % of all frog species with extinction. Chytrid routinely makes mainstream news and has already led to the decimation of frog populations in several continents. Bullfrogs are one of the few amphibian species that are seemingly immune to the fungus, but can still carry and transmit the pathogen. Perhaps then, our analogy should not be the Roman-Visigoth clashes of antiquity, but that of the European conquest of the Americas. Far more indigenous people died as a result of diseases brought over on Spanish and Portuguese ships than direct fighting with Conquistadors. Similarly, an indigenous frog does not even have to encounter the invader to suffer as a result of its arrival.

When I first introduce kids to the topics of evolution and natural selection, the concept of ‘survival of the fittest’ inevitably draws the same astute question: why is there so much diversity? This is an excellent question that, for the most part, baffles scientists. Explaining patterns of diversity is a major branch of the biological sciences, and answers to date are largely underwhelming in their explanatory power and often mindbogglingly complex. Critics have a much easier time of it. If what you say is true, the doubters assert, why isn’t there one species that out-competes all others? Why isn’t there just one frog, the fittest, most well-adapted frog that drives all others to extinction? Perhaps soon there will be.



*Especially one that is already running too long on account of the inclusion of made-up lyrics to a song that no-one but me has heard. To those who complain that this tune is too long and repetitive, I wholeheartedly agree. It is depressing that we have let the situation develop into such a sorry state of affairs. But I have bad news. The global invasion of bullfrogs across the globe is far from complete; by the end of the decade, we will almost certainly be able to add a couple more verses. Indeed, in my haste I forgot to include the UK, my homeland; we have them there too.

**The Visi- in Visigoths actually means west, so it is a fitting analogy to the amphibian invader in the western hemisphere terrorizing the longstanding civilizations of the Old World.