The Most-Traveled Cane Toad

Anuran Populations

Questions answered (and bookmarked) on this page: 

Are cane toad populations stable? 

Controlling toad populations. 

Cane toad in my yard! 

What about other anurans? 

How can I help?

 

Is the population of cane toads stable?  Unlike many Anurans (and Bufo in particular), the cane toad population is stable.  The Australian population is 10 times greater than that of South America, but this would be expected from a situation in which a species essentially lacks predation. 

What attempts have been made to control exotic cane toads?  Governments have tried chemical, biological, and physical means of controlling cane toads.  The difficulties are not only cost and species selectivity,  but also efficacy.  When one female cane toad can lay upwards of 30,000 eggs, humans are unlikely to effect a long term population control of cane toads if even a few individuals remain.

Chemicals which alter reproductive capacity or result in death are likely to lack species specificity and thus affect other species. 

Biological means, such as introducing a natural predator, have the same potential disaster as did the introduction of a cane toad: the introduction of an exotic has unknown ramifications for any ecosystem. 

Some work has been done to sterilize males and then release them back into the environment.  Any females who then mate with such a male would then fail to successfully reproduce.  However, such efforts are all costly and ineffective, especially over large regions.  Additionally, cane toads are resistant to the effects of UV radiation, which in most species would result in male sterility. 

The use of pheremones is perhaps the most promising option.  Pheremones are scents, both attractant and repellant, that influence an animal’s behavior.  Repellants could be used to dissuade cane toads from entering new areas.  Attractants could be used to concentrate the toads so that collection or extermination might be more feasible. 

Other work has focused on introducing a host-specific disease or parasite that might wipe out cane toads, but again the unknown risk to other species exists.  Minomya elegans, a mosquito which feeds on cane toads, may be used as a vector for such parasites. 

Physical means have also been attempted.  Barriers such as fences that allow only animals smaller than cane toads to pass through have been used with some success on small scale projects, such as the protection of a single pond.  Barriers are only about 18 inches tall, enabling most critters to be unaffected by the barrier. 

Some state and local governments within Australia also offer programs in which people may pick up a toad trap from the local store, simply by placing a small deposit on the trap.  Toad roundups and “toad busts”, parties to search for, collect, and exterminate cane toad eggs, tadpoles, or metamorphosized toads are also common.  Residents are urged to remove standing water in their yards.

One problem common to all the attempted methods of toad control is the need for lots of funding.  Nothing is cheap when the affected regions are so large!  Piece-wise programs are sometimes all that's available. 

If a cane toad is in your yard, the Australian government urges residents to first remove pets and small children from the area.  Toads can be picked up (preferably with gloves) and placed in a container or bag for humane transport to a toad identification center, where they will be positively identified as cane toads and then humanely euthanized.  The government expresses concern that residents may kill native frogs which they incorrectly identify cane toad.  Furthermore, all animals – exotic and pesky or not – deserve humane deaths. 

 

Why are other frogs on the decline?

-Lifecycle-Because frogs live both in water and on land during their lifecycles, changes in either environment influence them.  The developmental stage is very vulnerable.

-Skin permeability- Amphibian skin is permeable to both gas and liquid, both during time spend in water and on land, and so the skin can absorb toxins all the time.  When frogs get really dehydrated, their skin is even less selective on what it absorbs. 

-UV exposure- Amphibians are greatly influenced by CFCs.  While CFCs are now mostly banned, they are still used in some products and in some parts of the world.  Ozone depletion enables greater UV penetration, which is very hazardous to amphibians during the lifecycle.  Frogs can and do get skin cancer.  Pigments in both frog skin and in frog eggs partially protect against UV radiation, as does their behaviors.  However, frogs attempt thermoregulation by basking in sunlight (especially at high elevations or in colder climates), and UV exposure is a considerable problem.

-Feeding habits-Collectively, frog feeding habits are like eating out of the trashcan -- they get both the good scraps and a lot of bad. Tadpoles are exposed to toxins both by gill respiration and by feeding.  They filter-feed on algae and plant particulates, both at the water surface and in the detritus at the bottom of the pond.  Oily substances congregate on top of the water, meaning that surfacing tadpoles are exposed to oil-soluble toxins as well as the water-soluble toxins through which they swim.  Chlorinated chemicals persist by adhering to sediments on the bottom of the pond.  Fat-soluble toxins can remain in the frog body for life.  As adults, frogs are exposed to contaminants on land and on the water surface, in the water, and in the pond bottom when they dive into water.  Adults also experience biomagnifications of toxins; they eat insects and other pests which may be contaminated with pesticides and herbicides. 

-Habitat destruction-When humans fractionate habitat, populations are isolated from each other and genetic diversity may decrease, making frogs more susceptible to pathogens or generally less successful in breeding.  Due to physiological constraints such as the need for water, frogs cannot easily migrate across a city or road to reach the rest of their population.

-Climatic swings-Changes in temperature and seasons influence the breeding habits and speed of maturation for tadpoles.  If the young do not metamorphosize before ponds evaporate, freeze over, or overheat, the next generation of frogs will be much smaller.

-Metamorphosis-Hormones drive the process of metamorphosis.  Hormone mimics and disruptors may therefore destroy the maturation process.  The process of metamorphosis itself is fraught with difficulties, as metamorphosizing frogs aren’t as agile or able to cope with changing conditions as are adult frogs.  As tadpoles essentially go through puberty, they are clumsy and less able to catch food.  They thus rely on fat stores for energy, but in the process of metabolizing fat also re-circulate toxins that were stored in that fat. 

-Reproduction-Male calling and female egg production is a huge energy demand, at a time when frogs are sitting by the ponds or mating instead of hunting for food.  They are forced to rely upon fat stores for energy, but in the process release and recirculate fat-soluble toxins.  Starving females may also resorb their eggs, which again re-circulates toxins.

 

What human activties harm frogs?  How can I help declining frogs?  You can help frogs by using less chemical fertilizer and pesticide, not trampling or otherwise destroying their habitat (eg, by taking souvenirs), by properly disposing of wastes, and driving slowly through frog habitats – especially during breeding season.  Do not release any pet frogs into the wild, as exotics such as cane toads alter ecosystems.  Promote forest and habitat preservation.  Do not buy wild-caught frogs from the pet trade.  Support healthy mining practices, as mining often releases gluts of heavy metals, which are toxic to life.  Do not eat frog species whose populations are in jeopardy, and if you do eat frog legs or eggs, be responsible and aware of how such foods were acquired.

 

Now that you understand a bit of the world crisis for frogs, and the story comes to an unhappy ending, learn my opinion on it!  Go to About me (and my opinions) next.