Australia's Most Destructive Creature

In the 1930s Australian farmers had a really big problem where their sugarcane crops were being decimated by a voracious grub the larvae of the cane beetle which destroys the plant’s roots where after failed attempts at chemically controlling the grubs officials sought a biological solution where after hearing about a measure to control a similar cane beetle in Hawaii with the voracious cane toad Australian officials jumped at the idea where after dutifully carrying out exactly zero studies of the potential impact on the Australian environment they transported a hundred toads from Hawaii and let them loose and with that single action the Australian ecosystem was never the same demonstrating how biological control introduction without environmental impact assessment created an irreversible ecological catastrophe.

One hundred toads quickly became one thousand and then ten thousand and then ten million and then a hundred million where they have rapidly spread over much of the continent where today cane toads properly known as Bufo marinus or buffo toads are the most notorious of all invasive species in Australia demonstrating how unchecked exponential population growth combined with rapid geographic expansion creates continent-scale biological invasions.

This lumpy guy is massive poisonous and seemingly unstoppable where the biggest cane toad ever recorded was almost 40 centimeters or 15 inches long and weighed 6 pounds or 2.75 kilograms where it eats anything it can fit into its mouth including insects trash dog food small reptiles small rodents and even birds demonstrating how extreme body size combined with dietary generalism creates highly invasive amphibians capable of exploiting diverse food resources.

The one thing it doesn’t eat though is the cane beetle where cane beetles perch too high off the ground for cane toads to reach and the toads certainly can’t get to the grubs underground where so you could say there are some regrets demonstrating how spatial niche mismatch between predator foraging height and pest microhabitat location caused complete biological control failure while ecological damage proceeded unchecked.

Since their introduction 87 years ago cane toads have been linked to the decline of several native species in the Northern Territory and Queensland including the Australian monitor lizard the northern blue tongued lizard the Australian water dragon the king brown snake the common death adder and the northern quoll where many species are at risk of total extinction all because of this toad demonstrating how invasive toxic prey creates cascading biodiversity loss through native predator poisoning across entire herpetofaunal and mammalian guilds.

A common misconception is that any species found living in its non-native habitat is an invasive species where this however is not the case where many species are introduced to new areas without it being a problem where most crops for example are non-native where a species only gets the invasive title when it causes harm to the environment displaces or out-competes native species hurts the economy of the area or becomes a danger to human animal or plant health demonstrating how invasive species designation requires demonstrated ecological economic or health impacts beyond mere non-native status.

Adult cane toads have venom secreting poison glands on each shoulder where a highly toxic milky substance is released when they are threatened where if an animal tries to bite at the toad the venom is secreted directly into the animal’s mouth and its effect is almost instant where the venom which is full of cardiac glycosides causes sodium to build up within the animal cells where when this happens in heart cells it completely alters the rhythm of the heart usually making it beat out of control where meanwhile potassium builds up between the cells which can cause paralysis where this then leads to difficulty breathing convulsions and soon after death where a cane toad’s poison can kill within minutes demonstrating how cardiac glycoside toxins disrupt ion homeostasis causing rapid cardiovascular collapse and neuromuscular failure.

In the cane toad’s natural habitat of South and Central America some predators like the broad-snouted caiman have evolved mechanisms to block the action of the poison but similar predators in Australia have not had time to keep up their end of the evolutionary arms race where freshwater crocodiles in Australia are experiencing mass die-offs due to the very same toad demonstrating how coevolutionary history provides toxin resistance while geographically naive predators remain lethally vulnerable.

A study conducted in 2008 saw 77% of freshwater crocodiles die in a period of three years directly following the inland progression of cane toads from the mouth of the Victoria River where the pace of their deaths exactly matched the pace of the toad’s invasion but all hope is not lost for the crocodiles where eventually the high death rate may naturally select for crocodiles that have a higher tolerance to the toad toxin demonstrating how invasion front mortality creates severe population bottlenecks with potential for evolutionary rescue through toxin resistance selection.

This evolutionary response is already being observed in some black snake populations that have been hit hard by the cane toads where natural selection is favoring individuals with higher tolerance to the toad toxin where this represents a parallel evolutionary trajectory to that predicted for freshwater crocodiles demonstrating how invasive toxic prey creates directional selection pressure for toxin resistance evolution in naive native predator populations.

Australian water rats have learned to flip cane toads over to carve out and eat their hearts with surgical precision avoiding any toxins where in turn this could have a positive effect for other native animals demonstrating how behavioral adaptation through learned specialized predation techniques enables exploitation of novel toxic prey while avoiding lethal poison exposure.

The life of a cane toad begins within an egg where one cane toad female can lay up to 40,000 eggs at a time where that’s a lot of eggs but usually when an animal has so many offspring they don’t expect many of them to survive where but the problem with the cane toads in Australia is that basically all of them do survive where even the eggs and the tadpoles are poisonous which if ingested by vertebrate predators like fish and turtles can be fatal demonstrating how extreme fecundity combined with life-stage toxicity and predator release creates unprecedented recruitment success.

Cane toads are now moving westward at a remarkably fast pace and the Australian government admits that it’s unlikely that they will ever be fully contained where since their introduction 87 years ago between the 1940s and 1960s cane toads expanded their range by about 10 kilometers a year where today toads are moving west at a rate of 50 kilometers a year where why are they so much faster now where to find out for sure researchers strapped tiny radio transmitters onto the toads and what they found shocked them where the toads were not hitchhiking but were themselves traveling at an alarming rate up to 1.8 kilometers a night demonstrating how accelerating invasion velocity through individual dispersal capability creates uncontainable range expansion.

We used to think vertebrates can’t really evolve on such short time scales but the cane toad is proving that they absolutely can if the conditions are right where for invading organisms whose populations explode very quickly this rapid evolution is all the more likely and all the more dangerous demonstrating how contemporary evolution challenges previous assumptions about vertebrate evolutionary timescales when selection pressures are intense and generation times are short.

Over a 60-year period Australian cane toad’s legs have grown over 25% and scientists soon realized that the modern cane toads with the largest legs are those on the western front the first toads to arrive in a new area where later waves of toads have shorter legs where the cane toads that are able to spread farther and faster have much less competition and thus there is an adaptive advantage in doing so where the cane toads were always spreading fast but now they are evolving to spread even faster demonstrating how spatial sorting and competitive release at invasion fronts drive rapid contemporary evolution of dispersal-enhancing morphology.

Researchers found that a handful of more developed tadpoles can devour 99 percent of the tiny hatchlings that emerge from the tens of thousands of eggs in a single clutch where this represents the fastest evolving cannibalism ever observed by scientists where the immense numbers and rapid evolution is creating a self-regulating population cap demonstrating how extreme intraspecific competition drives rapid behavioral evolution creating density-dependent mortality mechanisms in invasive populations.

The current guidelines on the most humane way to kill cane toads is to stick them in the freezer where the government community groups and frustrated individuals do their best to cull them using a big stick to hit them or air rifles or golf clubs or running them over in their cars where there are even toad busting militias that make it their mission to kill as many of the toads as possible but there are too many toads in Australia to eradicate by simply removing individuals demonstrating how mechanical control methods prove insufficient against established invasive populations with exponential reproductive capacity.

The ZW sex determination system is a chromosomal system that determines the sex of offspring of animals like birds some fish some reptiles and the cane toad where a Z and a W chromosome will give you a female and double Z chromosomes will give you a male where this is sort of the opposite of the mammal system where the heterozygous XY gives you a male and the homozygous XX a female and in the ZW system it’s the egg that determines the sex of the offspring unlike the XY system where it’s the sperm demonstrating how female heterogamety creates distinct sex determination mechanisms with important implications for genetic control strategies.

Researchers are trying to genetically develop a cane toad that can only produce male offspring using something called a W shredder gene drive where a W shredder toad is a toad who’s been gene edited so that all of its W bearing eggs are defective where that leaves just Z-bearing eggs so when the male adds his Z-chromosome to that you get only ZZ offspring meaning you get only male offspring and if the W-shredding gene is located on the Z chromosome then this shredding gene can be inherited where with enough gene shredder toads in the environment the number of females will be greatly reduced and a male dominated population would be incapable of effective procreation demonstrating how genetic biocontrol exploits sex determination systems to create sex ratio distortion for invasive species suppression.