European toad versus cane toad: a study in contrasting research

Rivalled by only the rabbit and red fox as worst and most dangerous introduced species in Australia is the cane toad (Rhinella marina; historically and familiarly Bufo marinus).

Adult female cane toad with human hand for comparison

The toad is absolutely lethal to the unique marsupials of the genus Dasyurus (quolls) which have evolved for 150,000,000 years with zero exposure to toad toxins, and demonstrably cannot coexist with bufonids anywhere.  This is seen in the fact that toad invasion has throughout the monsoonal tropics caused 97 percent declines in Northern Quoll (Dasyurus hallucatus) populations within a couple of years without any later recoveries. Bufonids also extremely dangerous to a number of predatory reptiles, such as goannas (Varanus) and elapid snakes, though unlike quolls these species can undergo behavioral or morphological changes to permit them to avoid eating toads.

Northern Quoll (Dasyurus hallucatus) – went from “Least Concern” to Endangered in a decade due to the spread of cane toads

It is well known that the cane toad was introduced into Australia to control a number of species of native “cane beetles” that were killing sugarcane by feeding on the plant’s sweet roots. It is also well known that, besides extirpating quolls and goannas, the cane toad did not reduce the numbers of cane beetles. In fact, the year 1946 saw the worst outbreak on record although toads had been released a decade beforehand!

However, it is almost unknown that at the very time the cane toad was released into Queensland, the CSIR was experimenting with the European Common Toad (then Bufo vulgaris; now Bufo bufo) as a pest control agent for Oncoptera grass grubs that were eating pastures in southern Australia. Proposals to import Bufo vulgaris (as I will call it for the rest of this post) and also the natterjack toad Bufo calamita date back to Western Australia in 1897. They were never executed in the first third of the twentieth century, but with increasing pest problems in the 1930s, the CSIR imported several specimens of Bufo vulgaris for a thorough test as a biological control agent against various Oncoptera. The CSIR found that Bufo vulgaris devoured all stages of Oncoptera (except, perhaps, the eggs) but that it could not dig down to reach them in their burrows. Consequently, the CSIR did not release Bufo vulgaris into southern Australia.

European common toad (Bufo vulgaris; now Bufo bufo)

If Bufo vulgaris (or the natterjack toad) had been released, it would have been likely more disastrous than the cane toad. This is because those few Australian species able to handle toad poison are almost all (Podargus frogmouths being a possible exception) immigrants from west of Wallacea – a group of islands with neither toads nor those predators (e.g. quolls) most affected by bufonid poison. Such species stand less likely to migrate into the Eyrean and Bassian faunal regions of central and southern Australia.

Eastern Quoll (Dasyurus viverrinus) now extinct on the mainland due to cane toads, and “Endangered” even in toad-free Tasmania due to rapid anthropogenic climate change

Were either Bufo vulgaris or the natterjack released it is practically certain that the spotted-tailed quoll (Dasyurus maculatus) and eastern quoll (Dasyurus viverrinus) would have been extirpated extremely quickly except – perhaps – from small islands where the toads might not have been released. Most likely those two species would have gone “Extinct” before the middle 1970s when the first studies of the cane toad’s impact on native fauna was published by Mick Archer and snake specialist Jeanette Covacevich. Even the Western Quoll (Dasyurus geoffroii) could quite possibly have been extirpated by toads before 1974 – although the drastic (87 percent around Perth) enhanced greenhouse gases decline in streamflow over southwestern Australia since then might ironically have protected it from introduced Bufo vulgaris. I have also imagined that poison in Bufo vulgaris eggs and/or tadpoles would have been a threat to the iconic platypus (Ornithorhynchus anatinus) which hunts purely by touch and possesses as little exposure in its evolution to toad toxins as Dasyurus.

Spotted-tailed quoll Dasyurus maculatus – the largest Dasyurus. The CSIR’s prudence in not releasing Bufo vulgaris has so far saved this species from either extinction or being “Critically Endangered” and confined to small islands

In contrast to the CSIR’s serious study of Bufo vulgaris – although it quite naturally failed to test the toad’s toxicity toward those species probably saved thereby – Bufo marinus was released into Queensland with absolutely zero testing on the cane beetles it was supposed to control! Rather, the release of Bufo marinus was done as an act of faith that it would eat the beetles consuming sugar cane – a method used to sell toads to farmers and gardeners for several centuries before 1935.

Belief in the magical power of toads to control pests was dogma among the globe’s closely-knit sugar growing fraternity in the 1930s. It is highly plausible that this fraternity feared science for the same reason that large landowners in Catholic Europe did – that it would undermine their political power by providing justification for wealth redistribution from them. While the large landowners of Catholic Europe turned to stigmata stories and Marian apparitions as their means of countering class war, sugar planters held firmly onto beliefs about certain predators as effective pest control agents whether they worked or not. Thus, when the release of Bufo marinus in Puerto Rico coincided with reduced grub density and Raquel Dexter’s dissections showed Bufo marinus to eat beetles, it became dogma among the global sugar fraternity that toads would control beetles everywhere they be introduced. However, as demonstrated by Nigel Turvey in his Cane Toads: A Tale of Sugar, Politics and Flawed Science, the actual reason for the (temporary) decline in grubs in Puerto Rico was due to unusually wet rainy seasons pinching breeding by waterlogging soils.

The cost of this false belief to Australia’s unique native wildlife has been qualitatively different from other landmasses without native bufonids. Oceanic island predators – even when living on a landmass without toads – were chiefly predatory raptors that evolved on continents with toads. Thus, unlike quolls or Pseudechis snakes, other landmasses where Bufo marinus was introduced had predators who could effectively regulate its numbers – not to mention merely one-thousandth to one-hundredth the space to expand. Thus even with uniquely bad soils, uniquely low secondary productivity and uniquely unreliable runoff toads in Australia can reach higher densities than anywhere else in the world – further condemning those who introduced the cane toad based purely upon dogma.