A tale of two introductions


Native to Central and South America, cane toads have a permanent grumpy expression, and the chubby legs of a human toddler. They are large (a typical adult toad can measure up to 15-cm from snout to vent), and can be well over a kilogram in mass. In order to become big and strong, cane toads must consume an impressive number of calories. They achieve this by having a non-discriminatory diet, consuming: insects, other toads, plant matter, small mammals, and even birds.

The voracious appetite of the cane toad has made it a popular choice as a biological control agent. As toads hop through sugar plantations, they gobble up pest insects reducing the need for applying chemical insecticides. While early biological control programs augmented cane toad populations within their native range, scientists soon began to explore whether cane toads would be successful in new environments. After cane toad introductions to several Caribbean islands in the were successful in the 1930s,  a larger island, much further away began to show interest.

Across the South Pacific, Australia was struggling with a serious pest problem on sugar cane plantations. The cane beetle (Dermolepida albohirtum) – a small black and grey scarab native to Australia – was greatly reducing sugar yields. Having recently launched a successful biological control campaign to suppress the spread of prickly pear cactus, Australia gave the cane toads a shot – releasing thousands of young toads in the late 1930s.

Unfortunately, the toads did very little to control populations of cane beetles. The low levels of cover in sugar cane plantations reduced the foraging activity of cane toads. Additionally, adult beetles live near the top of plants and cane toads are ineffective climbers. Furthermore, the larvae of the cane beetles live deep in the soil feeding on sugar cane roots – far from the mouths of hungry toads. As the environment wasn’t right, and the food wasn’t accessible, the toads turned their bumpy backs on cane beetles. Instead they began consuming almost everything else, having devastating effects on Australian fauna.

The destructive impact that cane toads have had in Australia is not unique. The country has a long history of spectacularly invasive species which have been introduced either purposely or accidentally. Rabbits, camels, red foxes, feral goats, donkeys, and pigs are all thriving, and doing a number on the native flora and fauna. However, Australia is also the site of one of the most successful biological introductions to date: dung beetles.

In the 1960s, entomologist George Bornemissza realised that accumulating cattle dung was quickly blanketing pastures, where an estimated 2,000 km2  of land was lost to dung on an annual basis. While the immediate repercussion of  dung involves the limiting available grazing area, stagnant dung causes a myriad of other problems including: higher fluxes of greenhouse gases, increased rates of parasite transmission and provides ample habitat for pest flies to reproduce. The reason why this was such a problem in Australia was because a key group of insects were missing from the cow pats: dung beetles.

Australian dung beetles evolved to use the fibrous dry pellets of marsupials, and were unable to cope with the wet messy dung of bovines. In the late 1960s after careful planning and deliberation, the Australian government released 23 species of dung beetles from around the world at different points around Australia. These beetles had evolved to deal with dung of similar consistency, and went straight to work. Today, many species have successfully established and are thriving on the huge amounts of dung produced by the 29 million odd cattle in Austalia. While dung beetles free pasture from dung, their activity also improves soil fertility, removes habitat for the larvae of blood-sucking flies, and halts the spread of enteric parasites.

However, recent research from Australia demonstrates that these two introductions are interacting in an alarming way. Cane toads, which were imported to control a scarab beetle pest have instead switched their energy to controlling another type of scarab: dung beetles.  In a study published this week in the journal Ecosystems,  the profound impact our management choices have upon the surrounding ecosystems are clearly illustrated through the example of cane toads and dung beetles.

The study was conducted in the north of the Tarnami desert – which unsurprisingly is a hot and dry area. In order to raise cattle, you need to ensure that animals have access to plenty of water. As natural water sources aren’t terribly common, producers create water points through the use of boreholes and pumps. Water is driven from deep underground into two principal types of water sources : earthen dams where toads can easily access the water, or into large plastic or metal tanks which essentially removes toad access.

As cane toad tadoples need a reliable water source to complete development, and adult cane toads require also require intermittent access to moisture – the team hypothesised that there would be higher densities of cane toads near earthen dams in comparison to tanks. Because these toads are known to prey on all sorts of insects, they hypothesised that higher densities of toads would cause lower densities of dung beetles.  Finally, the team hypothesised that lower densities of dung beetles would mean lower rates of dung removal. By sampling at 13 different watering points (five dams and eight tanks), the team found crystal clear answers to their questions.

Population densities of cane toads were five times higher in areas surrounding earthen dams, than surrounding tanks. Abundance of dung beetles was 12 times higher near tanks in comparison to earthen dams, thought to be caused by high predation rates. Finally, the increased population of dung beetles at tanks meant 13% higher rates of dung removal in comparison to rates at earthen dams.

By making the switch from earthen dams to tanks, cattle ranchers can play a significant role in reducing the spread of a problematic invasive species, while protecting a group of beneficial introduced species. As tanks are less prone to evaporation and seepage, less water is extracted, meaning fuel is saved. This is a more economically and environmentally sustainable option for cattle ranchers. Hopefully with time, more operations will make the switch to slow down the toads, thus supporting dung beetles and the myriad  of ecosystem services they provide.

Baillie, C. 2008. Assessment of evaporation losses and evaporation mitigation technologies for on farm water storages across Australia. Cooperative Research Centre for Irrigation Futures, Irrigation Matters Series, (05/08).

Feit, B., Dempster, T., Gibb, H., & Letnic, M. 2015. Invasive Cane Toads’ Predatory Impact on Dung Beetles is Mediated by Reservoir Type at Artificial Water Points. Ecosystems, 1-13.

Lever, C. 2001. The cane toad: the history and ecology of a successful colonist. Westbury Academic & Scientific Publishing.

Nichols, E., Spector, S., Louzada, J., Larsen, T., Amezquita, S., Favila, M. E., & Network, T. S. R. (2008). Ecological functions and ecosystem services provided by Scarabaeinae dung beetles. Biological conservation, 141(6), 1461-1474.

Penttilä, A., Slade, E. M., Simojoki, A., Riutta, T., Minkkinen, K., & Roslin, T. (2013). Quantifying beetle-mediated effects on gas fluxes from dung Pats. PloS one, 8(8), e71454.


4 thoughts on “A tale of two introductions

  1. TO: Paul Manning
    FROM: Eric Koperek
    SUBJECT: Unintended Consequences
    DATE: AM 3:29 Wednesday 22 April 2015

    I just finished reading “A Tale of Two Introductions”. This is the sort of “case study” that should be required reading at all agricultural colleges. We arrogant Humans are constantly tinkering with Mother Nature. We forget that Gaia was wise long before we apes evolved and that She plots deeply.

    A similar cautionary tale occurred in Indonesia before you were born. (Geezers like me have long memories).

    The Indonesian Government started spraying DDT to kill mosquitoes that transmitted malaria. Months later, tens of thousands of families were without shelter. The poison killed mosquitoes but not roaches. Local lizards ate the roaches then got sick = slow because of DDT toxicity. Local cats ate slow lizards and then died from DDT poisoning. Without cats to eat rats, vermin populations soared. Millions of rats swarmed over rice paddies and palm oil plantations. Rats carried fleas which transmitted Bubonic Plague. Then village roofs started caving in. Thatch beetles used to be controlled by lizards now dead by DDT accumulation in the food chain. Without any major predators, thatch beetles devoured every roof for miles around. The well-intentioned Government eradicated malaria only to replace it with 4 equally bad afflictions: Rats, fleas, bubonic plague, and famine. Chastened bureaucrats stopped spraying and parachuted thousands of cats onto affected islands. It took over 3 years for island ecologies to recover sufficiently for residents to re-thatch their roofs.

    Everything in nature is interconnected. Remove one key species and Bad Things Can Happen.

    So Endeth Eric’s Epistle for Today.


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