The biological effects, including lethal toxic ingestion, caused by Cane Toads (Bufo marinus) – 12 April 2005
Advice to the Minister for the Environment and Heritage from the Threatened Species Scientific Committee (TSSC) on Amendments to the List of Key Threatening Processes under the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act)
Name: 'The biological effects, including lethal toxic ingestion, caused by Cane Toads (Bufo marinus).'
Summary of the threatening process
Cane Toads are large ground dwelling amphibians with a dry warty skin. They have a bony head with bony ridges over their eyes, sit upright and move in short rapid hops. Their colour ranges from grey to olive brown and they have a pale belly with dark irregular spots. The average size of an adult is 10-15cm long but females have been recorded growing up to and over 23cm. They breed in still or slow-flowing water and can tolerate salinity levels up to 15%. Adult Cane Toads are active at night during the warm months of the year and can survive temperatures ranging from 5-40 degrees celsius. Cane Toad spawn occurs in long gelatinous strings with double rows of black eggs with females laying between 8-35 000 eggs at a time. Cane Toads have a life span of about five years, breed twice a year, and take between 6-18 months to reach sexual maturity.
Cane Toads can reach high densities in suitable habitat (over 2000 individuals per hectare). In recently colonised populations (less than two years old) they rapidly increase in number to levels far in excess of those in older established populations. Cane Toads move during the wet season, requiring access to water for rehydration and breeding, and during the dry season are known to seek refuge in damp areas near creeks.
The Cane Toad is a native of central and south America with a natural range extending from southern United States to tropical South America. Cane Toads have proven to be highly effective invaders of new ecosystems with their distribution now extending to over twenty new countries.
Cane Toads have been present in Australia for nearly seventy years. They were introduced to Australia in 1935, when approximately 100 individuals were imported from Hawaii in 1935. Around 3000 young toads were released at Gordonvale near Cairns, North Queensland, to control the sugar cane pests, French's Cane Beetle, and the Greyback Cane Beetle, Dermolepida albohirtum, whose larvae eat the roots of sugar cane and kill or stunt the plants. Cane Toads proved to be unsuccessful in controlling cane beetles.
Since its introduction to Australia, the Cane Toad has spread south and west across the continent and now occurs in Queensland, Northern Territory and New South Wales. From their introductory site near Cairns, the Cane Toad has spread throughout Queensland, being recorded in Brisbane in the 1940s, and was considered to occupy approximately 50% of the State by the late 1990s (Sutherst et al. 1995). During the early 1960s, the Cane Toad was recorded in north eastern New South Wales and is now considered to occur on the north coast of New South Wales as far south as the Clarence River/Yamba. The only confirmed breeding colony south of this area being at Lake Innes, near Port Macquarie.
Cane Toads crossed into the Northern Territory from Queensland in the 1980s, reached the southern end of Kakadu National Park in 2001, and have colonised Arnhem Land, the Gulf of Carpentaria lowlands and the Katherine River watershed.
Analysis of climatic variables and the adaptive abilities of the Cane Toad suggests that Cane Toads may eventually permanently inhabit the wet coastal areas of the eastern and northern parts of the continent at least as far south as Port Macquarie in the east of the continent and south to Broome in Western Australia. Temperature range analysis has indicated that Cane Toads, in warm years, could overwinter as far south as Bega near the Victorian border. Conversely, a drop of 2-2.5 degrees celsius in mean temperature would result in a shrinkage of the suitable overwintering area to the far north coast of NSW. It was estimated, based on this model, that Cane Toads will further increase their range primarily throughout coastal and near coastal regions of tropical Australia to encompass an area of approximately two million km2 (Sutherst et al. 1995). In addition, Cane Toads have now been recorded well inland, at least as far south as Dunmarra, Northern Territory.
Estimated rates of expansion vary from between 1–5 km per year in northern New South Wales to approximately 30 km per year in north western Queensland, and over 30 km per year in Kakadu National Park. One estimate in the Northern Territory, where Cane Toads are moving generally north west and downstream, is that the current rate of spread is about 60 km per year. Their advance has generally been rapid in the wet seasons and slower in the dry seasons.
Threats to native species
Cane Toads eat a wide variety of prey, breed opportunistically, have a far greater fecundity than native anurans and develop rapidly particularly in warmer waters. They are considered to be an extreme generalist with a tolerance for a broad range of environmental and climatic conditions and able to potentially occupy many habitats. There is considerable concern over the impact of the Cane Toad on native species and in particular, invertebrate communities, through predation and competition. Most significantly, they possess highly toxic chemical predator defences and many scientific and anecdotal reports exist of deaths of native predators that have attempted to consume Cane Toads. Historically, reports on the impacts of Cane Toads have been largely anecdotal with little quantitative data available since the Cane Toad's introduction in the 1930s. More recent studies have been undertaken to quantify the impacts of the Cane Toad and include:
- targeted surveys of the Northern Quoll, Kakadu National Park;
- monitoring of the calling activity of frogs in the Roper River Valley and in Kakadu National Park;
- broad scale fauna surveys undertaken in Kakadu National Park;
- research on the impacts of Cane Toads on native monitors; and
- studies on the susceptibility of reptiles and snakes to Cane Toad toxin.
Predation and Competition
Cane toads attain high population densities after colonisation and consume large volumes of invertebrates. While larval Cane Toads are algal, detritus and suspension feeders, metamorphling, juvenile and adult Cane Toads feed on a broad variety of small prey items, predominantly ground dwelling arthropods. Cane Toads are thought to consume approximately 200 food items per night, far more prey than most native frogs ingest in the same period. The bulk of the diet is usually ants, beetle and termites, although they can eat anything that fits in their mouth including a wide variety of insects, frogs, small reptiles, mammals and birds. In addition, Cane Toads have the potential to compete with native species for food and shelter sites. The ability of the Cane Toad to rapidly expand both its range and density and to consume relatively large numbers of a wide variety of prey has led to concern that the Cane Toad is a key factor in the decline in of many native species.
Lethal toxic ingestion
All stages of the Cane Toad's life cycle: eggs, tadpoles, toadlets and adult toads, are poisonous. Cane Toads have venom-secreting poison glands (known as parotoid glands) or swellings on each shoulder where poison is released when they are threatened. If ingested, this venom can cause rapid heartbeat, excessive salivation, convulsions and paralysis and can result in death for many native animals.
There are a number of anecdotal reports and more recently, experimental data, on the impacts of ingestion of the Cane Toad by native species. Studies have been, and are continuing to be, undertaken on the Northern Quoll, vertebrate fauna in Kakadu National Park, reptiles, snakes, and crocodiles to determine the impact of lethal toxic ingestion of the Cane Toad on these species.
Section 188(4) of the EPBC Act states:
A threatening process is eligible to be treated as a key threatening process if:
- it could cause a native species or an ecological community to become eligible for listing in any category, other than conservation dependent; or
- it could cause a listed threatened species or a listed threatened ecological community to become eligible to be listed in another category representing a higher degree of endangerment; or
- it adversely affects 2 or more listed threatened species (other than conservation dependent species) or 2 or more listed threatened ecological communities.
A. Could the threatening process cause a native species or an ecological community to become eligible for listing as Extinct, Extinct in the Wild, Critically Endangered, Endangered or Vulnerable?
Predation and Competition
Larval Cane Toads are algal, detritus and suspension feeders. While it has been shown that Cane Toad tadpoles readily prey upon anuran eggs, in laboratory tests in northern Queensland Cane Toad tadpoles were not found to be significant predators of native anuran eggs, hatchlings or tadpoles. This study suggested that native tadpoles are often likely to have a greater impact on the survival of early life history stages of native anurans via predation than are Cane Toad tadpoles (Crossland 1998).
A number of studies suggest that Cane Toad tadpoles may impact on the growth and survival of native species through competition for food. Cane Toad tadpoles appear to significantly affect tadpoles of the ornate burrowing frog, Limnodynastes ornatus, probably as a result of the superior competitive ability of Cane Toad tadpoles, rather than predation of early life stages of the species (Crossland 1997).
A study of native frog species from the Darling Downs area of southern Queensland that undertook competition trials found that Cane Toad tadpoles may affect the growth of native anuran tadpoles under some circumstances. Trials conducted in artificial ponds indicated that Cane Toad larvae reduced the growth of three native species, Limnodynastes tasmaniensis, Limnodynastes terraereginae, and Notaden bennettii (Williamson 1999).
It appears that Cane Toads are more likely to have an adverse impact on terrestrial ground frogs or larger species such as the Giant Frog (Cyclorana australis) and the Marbled Frog (Limnodynastes convexiusculus) due to competitive interaction and in some circumstances, high densities of Cane Toad tadpoles in isolated pools could out compete native aquatic vertebrates and invertebrates for food resources. However there is limited quantitative data to adequately support a conclusion on such impacts.
There is insufficient data on the diversity of Queensland's frog populations before Cane Toad invasion to determine if there has been a resulting change in frog diversity or density. In the Northern Territory, where the Cane Toad is still colonising, baselines studies have been undertaken to monitor the calling activity of native frogs to determine changes in frog communities in response to Cane Toad incursion. These studies, which commenced in 1996 in the Roper River Valley and in 1998 in Kakadu National Park are based on the identification of the calls of more than twenty species of frogs as well as Cane Toads. To date there is insufficient data from the Kakadu sites to support analysis. However, initial analysis of the Roper River Valley provides some indication of a decline in calling rates which correlates with the incursion of Cane Toads. In particular, the number of frog species calling per station declined markedly between the beginning of the study in 1997/98 and 2001/02.
There were statistically significant and substantial declines in the number of records for at least seven of the frog species monitored with possible slight increases in two species. Five frog species showed no clear evidence of a trend and five species had too little data to support a determination. Although initial analysis suggests that Cane Toads may have a detrimental effect on frogs, these declines cannot be unequivocally attributed to the impact of Cane Toads.
One study also found that Cane Toads caused a significant longer term reduction in the abundance of small lizards, possibly by reducing their invertebrate food supply (Catling et al. 1999).
Predation by Toads
Metamorphling, juvenile and adult Cane Toads feed on a broad variety of relatively small prey items, predominantly ground dwelling arthropods. The bulk of the diet is usually ants, beetle and termites, although they can eat anything that fits in their mouth including a wide variety of insects, frogs, small reptiles, mammals and birds.
One study undertaken in the western Gulf of Carpentaria, Northern Territory, reported that the abundance of beetles was significantly lower, in the short term, in sites colonised by Cane Toads compared to Cane Toad free sites (Catling et al. 1999). Apart from this study, there have been no studies to investigate the impact of Cane Toads on invertebrate prey communities in Australia or elsewhere.
A study on ground-nesting rainbow bee-eaters, Merops ornatus, found that Cane Toads were one of the most significant sources of mortality for bee-eater chicks. Cane Toads destroyed 33% of all nest attempts by blocking the nest entrance, or by eating fledglings and eggs. When Cane Toads were removed from the nests and placed up to 1200m away from the nest sites, 75% returned to the same burrow overnight or within 2 or 3 nights. Toads were less likely to return to the nest with increasing release distance. This behaviour may have implications for the Red-cheeked Dunnart which also has its young in ground level dens. If Cane Toads have a memory and use olfactory cues to locate prey as indicated in this study, there could be implications for peak breeding times in a number of ground nesting or denning animals (Boland 2004).
In northern Australia, competition between Cane Toads with native species for food and shelter sites is likely to be highest near permanent water bodies during the dry season. While predation and competition effects are suspected, due to the high densities and dietary ecology of Cane Toads, these are yet to be sufficiently confirmed, and it is difficult to adequately justify, at this stage, a case that these aspects of the threatening process could cause any native species or ecological community to become eligible for listing under the Act.
Lethal Toxic Ingestion
The Northern Quoll
Historically, the Northern Quoll (Dasyurus hallucatus) was common across northern Australia occurring almost continuously from the Pilbara, Western Australia to near Brisbane, Queensland. This distribution has declined gradually over the last fifty years with the species' range contracting considerably in some areas with Northern Quolls disappearing from southern Queensland, the east Kimberley, the Gulf region and the drier southern extreme of their range in the Northern Territory. Over the last ten years, populations of Northern Quoll have been lost from most of the north eastern Top End of the Northern Territory, Cape York Peninsula and the Einasleigh Uplands of northern Queensland.
Some experts consider that a 75% reduction in the Northern Quoll's range has occurred between 1900-1990 and that, the Northern Quoll has been reduced to six major geographical centres: Drummond Range, central Queensland; wet tropics Northern Queensland; northern Cape York Peninsula; northern and western Top End Northern Territory; north Kimberley and Pilbara (Braithwaite and Griffiths 1994).
One cause postulated for this decline is predation of the species following the removal of ground cover by fire which increases the vulnerability of the Northern Quoll to predators. Broader changes in land use and land management (such as cattle grazing) are likely to have added to this process. Other factors which may have contributed include exotic disease, and poisoning from the lethal toxic ingestion of Cane Toads.
Any assessment of the Northern Quoll's eligibility for listing as a threatened species as a direct result of Cane Toad poisoning is complicated by the species having been in the process of decline prior to the arrival of the Cane Toad and the likelihood that other factors are also contributing to the decline. It is possible that the impact of the Cane Toad has accelerated a gradual decline, into a more sudden and catastrophic one.
Northern Quoll populations demonstrate a normal fluctuation in numbers involving a slight decline through the dry season. They eat several species of native frogs and are known to mouth Cane Toads causing the release of poison from the Cane Toad's parotoid glands (the swellings on each shoulder behind the eardrum). The poison is then ingested by Northern Quolls. Symptoms of death from Cane Toad toxin can include bright red lips and or gums and can also include a red roof of mouth or bright red nose and nose bleeds, red ears, bleeding from the ears, a red eye, red skin pouch, bright purple teats and faeces around the anus (Oakwood 2003).
The Northern Quoll in Northern Territory
Recent studies in Kakadu National Park have demonstrated that local extinction of Northern Quolls is occurring following Cane Toad invasion. Over the last two years this work has shown a significant decline in Northern Quoll populations with declines considered to be a direct result of lethal ingestion of Cane Toad toxin. Two recent studies are notable: one involving dedicated surveys of the Northern Quoll at two sites and another that undertook broad scale fauna surveys.
Targeted Quoll Surveys
Two study sites were chosen to study the effects of Cane Toads on the Northern Quoll, one in the southern region of Kakadu National Park (Mary River Ranger Station) and the other in the north of Kakadu National Park. A control site was located at East Alligator Ranger Station.
Radio tracking of Northern Quolls commenced at the Mary River Ranger Station, Northern Territory in 2002, shortly before the arrival of Cane Toads later that year. The Mary River population declined dramatically between October and December 2002 and by March 2003 appeared to be extinct with no Northern Quolls being caught in the subsequent May and July trapping trips. In contrast, Northern Quolls at the cane toad free control site at East Alligator were still abundant. Six subsequent trapping sessions at the Mary River site, southern Kakadu, have caught no Northern Quolls.
In the Mary River region of the Northern Territory, approximately 31% of Northern Quoll mortalities were considered to be the result of lethal toxic ingestion, there being no evidence of disease, heavy parasite infestation, or any other obvious changes at the site that could be responsible for the rapid decline.
The evidence from Kakadu National Park is compelling due to the precise timing of the population decline following the arrival of Cane Toads; the large proportion of dead Northern Quolls considered to die from lethal ingestion (about 31% recorded at Mary River); the fact that deaths were not consistent with any other known cause of mortality (post mortems indicated no internal injuries, no evidence of disease and no heavy parasite infestations); and that a closely monitored population in the north of Kakadu National Park over the same period, without Cane Toads present, showed no signs of decline.
Broad Fauna surveys
A series of broad scale fauna surveys were conducted in Kakadu at sites that had been surveyed up to 25 years previously. The monitoring and re-sampling of the terrestrial vertebrate fauna of Kakadu National Park was undertaken initially to provide some assessment of the short term impacts of colonising Cane Toads. The project sought to provide some assessment of the change in abundance of wildlife species in Kakadu National Park in the first year following the arrival of Cane Toads and was an assessment of short term responses only. Changes in fauna abundances were examined since the last surveys and again after the arrival of Cane Toads. This survey also used control sites, monitored over the same period to eliminate the influence of factors other than Cane Toads (e.g. fluctuations in Northern Quoll numbers between years due to levels of rainfall). While Northern Quolls declined to zero at the Cane Toad invaded sites, there was no decline at the control sites.
In November 2002, a preliminary report provided quantitative data which indicated that Northern Quoll numbers had declined dramatically with the arrival of Cane Toads. Two other species, Gilbert's Dragon and goannas also showed significant changes in abundance (Watson and Woinarski 2003a + b).
It is evident that a major decline of the Northern Quoll has occurred in Kakadu National Park, and will continue to occur, and that Northern Quolls may disappear from Kakadu National Park altogether within the foreseeable future due to the invasion of the Cane Toad.
Some experts have predicted that, within the next 10-20 years, Cane Toads will colonise almost the entire mainland range of the Northern Quoll and that a 95% reduction in the Northern Quoll's range is likely to occur between 1980-2010.
The Northern Quoll in Queensland
In northern Queensland, the Northern Quoll appears to have undergone a decline during the past two decades. Anecdotal evidence exists suggesting severe population declines in almost immediate response to Cane Toad colonisation of their habitat in three widely distributed areas of northern Queensland (Burnett 1997). However, lack of systematic surveys in northern Queensland precludes any useful comparison between pre and post Cane Toad abundances of the species. Despite other potential impacts, including the impacts of a rapidly expanding agricultural industry and human population growth in north east Queensland, quoll species were considered to be at high risk from Cane Toads due to their habitat preferences, distribution and diet (Burnett 1997).
Cane Toads colonised Cape York between the mid 1980s to the mid 1990s and anecdotal information indicates that the Northern Quoll rapidly disappeared from two sites on Cape York Peninsula. There has been no sign of recovery in either of these sites. While there are claims that the Northern Quoll persists in some parts of Queensland despite the presence of Cane Toads, there appear to be no studies documenting a recovery of the Northern Quoll in northern Queensland. In particular, Northern Quolls appear not to have recovered in savannah areas, west of the eastern escarpment and from Cape York Peninsula.
Anecdotal evidence suggests that Queensland Northern Quoll populations have survived in small, high altitude areas associated with extremely rocky habitats. These populations would have survived Cane Toad invasion for over fifty years. However, there appear to be no studies documenting the size or extent of these populations or the factors that have led to the survival of these remnant populations following Cane Toad invasion. While Northern Quolls may still be present in a number of localised areas in Queensland in which Cane Toads have been present for many years, they do not appear, to date, to be recolonising their former locations and to date there is little evidence that any substantive recovery has occurred following Cane Toad invasion.
It is likely given the scientific data becoming available from the Northern Territory that the Northern Quoll in Queensland would have suffered a very high level of impact from the invasion of the Cane Toads and that this impact is likely to be ongoing and will limit the potential for recovery of the species.
Summary - Northern Quoll
Cane Toads are, at present, absent from Western Australia, and therefore have not contributed to a decline in numbers from the western part of the species' range. Northern Quoll populations demonstrate a normal fluctuation in numbers involving a slight decline through the dry season. It is considered that the Northern Quoll is likely to, and will continue to, decline over most of its mainland, and some of its island, range.
Over the last 10 years, the population has almost entirely been lost from the north east Top End, Northern Territory; Cape York Peninsula; and the Einasleigh Uplands of northern Queensland. These areas have been estimated to constitute approximately 30-40% of the Northern Quoll's pre-toad distribution. The viability of these remnant populations in the wild is, at this stage, unknown.
Over the next 10 years, the rest of the mainland Top End population is expected to also disappear, along with much of the Kimberley mainland population. These areas are estimated to constitute a further 30% of the species' pre-toad distribution. With the exception of some of its island locations, an almost total Cane Toad colonisation of the Northern Quolls range is expected.
While Northern Quolls are still present in a number of localised areas in Queensland in which Cane Toads have been present for many years, they do not appear, to date, to be recolonising their former locations and to date there is little evidence that any substantive recovery has occurred following Cane Toad invasion. More recent experimental evidence from the Northern Territory supports anecdotal reports from Cape York Peninsula and other areas of Queensland that Northern Quoll populations have been, and continue to be, severely affected by the presence of Cane Toads.
The Northern Quoll is likely to continue to disappear over most of its mainland, and some of its island, range. Based on evidence from Cape York Peninsula and more recently Kakadu National Park (and allowing for persistence in the existing pockets in Queensland and in some offshore islands), this reduction is estimated at about 95% of the range (and hence total population) as it was in 1980, by about 2010. Over the next 10 years, it is likely that the rest of the mainland Top End population will also disappear, as will much of the Kimberley mainland population, estimated to represent a further loss of approximately 30% of the species' pre-toad distribution.
Lethal ingestion of Cane Toads is thought to also pose a threat to many species of Australian reptiles. Both snakes and varanids prey upon frogs and have few options for prey manipulation. One study has estimated that 59% of 'agamids' (dragons), 85% of the 'varanids' (monitors) and all of Australia's crocodiles and freshwater turtles are potentially at risk from Cane Toads. This study reviewed published information on the distributions and dietary habits of Australian reptiles and tested the ability of eleven 'at risk' reptile taxa for their susceptibility to Cane Toad toxin (one python, one freshwater turtle, two crocodiles, two dragons and five monitor species) (Smith and Phillips in prep).
This work was based on ecoclimatic predictions of the likely eventual distribution of Cane Toads across Australia and identified those species that will come, or have come, into contact with Cane Toads. The percentage of the species' range currently encompassed by the Cane Toad was estimated along with the percentage of their range that is likely to become affected as Cane Toads reach their full extent.
The study concluded that 75 species are potentially at risk from the invasion of the Cane Toad. These represent both species of crocodile, all 14 species of tortoise, 37 of 63 species of agamid and 22 of 26 species of varanid. Of these 75 at risk species, 34 were considered likely to have their range totally encompassed by the Cane Toad (under predicted 2030 climate change models) and seven have already had their range totally encompassed.
Large bodied goannas are known to undergo initial population declines following the invasion of the Cane Toad. In Queensland, anecdotal evidence exists of severe population declines in Gould's goanna, Varanus gouldii; Merten's Monitor, Varanus mertensi, Varanus panoptes and the Spotted Tree Monitor, Varanus timorensis, in almost immediate response to Cane Toad colonisation (Burnett 1997). However a lack of systematic surveys of these species in northern Queensland precludes comparisons of pre and post Cane Toad abundances. Eight of the twenty monitor species whose distributions overlap potential Cane Toad distribution were considered to be at high risk from Cane Toads due to their micro habitat preferences, distribution and diet (Burnett 1997).
Other species thought to be impacted include the Mangrove Monitor, Varanus indicus, Northern Sand Goanna, Mitchell's Water Monitor, Varanus mitchelli and the Floodplain Monitor.
Population declines or evolutionary responses have been recorded for Varanus panoptes and Varanus gouldii. One expert noted that historically, such declines have been temporary, with recovery occurring in the following year and complete population recovery being noted in the third year post invasion and that many monitor species continue to survive following Cane Toad invasion.
In the Northern Territory, preliminary data from an intensive radio tracking study of goannas, initiated in September 2002, Kakadu National Park, has estimated an initial mortality rate of around 50-70% coincident with the arrival of Cane Toads. It was estimated that a 50% plus decline occurred in populations of Varanus panoptes within an approximately seven month period (late Oct 2003 to late May 2004) coincident with the arrival and build up of Cane Toad numbers (Holland 2004). There is evidence that most goannas died following attempted ingestion of Cane Toads. There is little quantitative data yet on whether or not recovery of goanna populations is likely to occur in the Northern Territory as it has in Queensland. Some species are known to persist in areas of Queensland that have had Cane Toads present from a few to many decades but there are no quantitative data on pre-toad population sizes.
Many native snakes prey on frogs using their mouths to capture and consume Cane Toads entirely and cannot avoid direct exposure to toxins in the Cane Toad's body. Frog eating snakes are known to be susceptible to impacts (e.g. Northern Death Adder, Acanthophis praelongus, King Brown or Mulga Snake, Pseudechis australis, Western Brown Snake, Pseudonaja nuchalis, Red-bellied Black Snake, Pseudechis porphyriacus).
Broad scale terrestrial vertebrate surveys undertaken in Kakadu National Park did not produce sufficient data to determine if snakes were impacted by Cane Toads. Snake capture rates in these surveys were considered to be too low for meaningful analysis. Researchers indicated that, in their opinion, 'elapid' snakes showed major declines in invaded areas, but there was insufficient quantitative information to adequately establish this fact (Watson and Woinarski 2003 a + b).
One study identified that 49 taxa of snake are potentially at risk from Cane Toads, based on their geographic distributions, dietary habits and ability to tolerate toad toxin. The lethal dose of toad toxins was assessed for ten of these species with 7 of the 10 being identified as easily capable of ingesting a fatal dose of toxin in a single meal. Of the 49 species identified, 26 were considered likely to have their range totally encompassed by the Cane Toad. The study showed that Cane Toads are likely to have a negative impact on greater than 30% of Australia's terrestrial snake fauna. Two groups (Tropidonophis and Stegonotus) showed a high resistance to Cane Toad toxin with Tropidonophis significantly more resistant than Stegonotus. With the exception of the Keelback, Tropidonophis mairii, which appeared to be extremely resistant to Cane Toad toxin, most snake species tested exhibited a low tolerance (Phillips et al. 2003).
Species of native snakes are also known to survive Cane Toad invasion. To what extent they are susceptible to Cane Toad toxin and whether observed impacts on individuals translates into population or species decline is uncertain. It is also not known whether these observed declines are short term or long term.
While there is limited quantitative information on the declines of many of these reptiles in response to cane toads, the vertebrate fauna surveys undertaken in Kakadu National Park do give an indication of potential impact on these species with substantial declines in Cane Toad invaded sites recorded for the Gilbert's Dragon and goannas (Watson and Woinarski 2003a + b).
Studies have demonstrated that Cane Toad eggs, hatchlings and tadpoles are toxic to many native aquatic predators. It has also been suggested that there are possible impacts from Cane Toads on a proteocephalid tapeworm of the python, Ataresia maculosus.
Other possible impacts have been suggested for ground feeding geckos, fish and bird species. In addition, there are reports of freshwater crocodiles, Crocodylus johnstoni, dying after ingestion of Cane Toad toxin in the Borroloola and Beswick communities and numerous anecdotal reports of freshwater crocodiles dying following Cane Toad consumption (Begg et al. 2000, Van Dam et al. 2002).
Broad scale fauna surveys undertaken in Kakadu National Park showed less substantial declines for a range of other species including the terrestrial gecko, Gehyra nana, and the pale field rat. There was little or no evidence of decline for some species (Northern Brown Bandicoot, Dingo, most frog species, Blue-winged Kookaburra, Kingfishers, Pheasant Coucal, Dollarbird, Grey Shrike-thrush, Magpie-lark and Butcherbirds). In addition, there was insufficient data for some species (small Dasyurids, raptors, goannas and elapid snakes) (Watson and Woinarski 2003 a + b).
Conclusion for Criterion A:
The Cane Toad is a highly invasive species which has colonised substantial areas of the Australian continent, occupying the habitats of many native species. As a result, a number of native species are considered to have experienced impacts from Cane Toads, most notably in relation to lethal ingestion of Cane Toad toxin. Much of the research to date remains inconclusive at a species and population level; is complicated by other factors of decline; and it is uncertain to what degree species may survive short term crashes into the longer term. More recent research on the impact of lethal ingestion of Cane Toad toxin is compelling, particularly in relation to recent population crashes of the Northern Quoll in Kakadu National Park, Northern Territory. Due to the symptoms and the absence of any other known cause of mortality, the deaths have been directly attributed to lethal toxic ingestion of Cane Toad poison. While the Northern Quoll is considered to have survived in isolated populations in Queensland, there is little information on how successful this persistence is and no evidence that the species is recovering, or likely to recover. It is estimated that the Cane Toad will, in the foreseeable future, colonise almost the entire mainland range of the Northern Quoll and the species will continue to decline as a result.
It is considered that the threatening process is eligible under this criterion as the process is likely to cause a native species, the Northern Quoll, Dasyurus hallucatus, to become eligible for listing as Extinct, Extinct in the Wild, Critically Endangered, Endangered or Vulnerable. It is highly likely that Cane Toads have had, and are continuing to have, a range of impacts on many other native species and it is possible that other species will become eligible for listing under the EPBC Act as a result. There is however a lack of verifiable field-based research at this stage to adequately demonstrate such impacts at a species level.
B. Could the threatening process cause a listed threatened species or a listed threatened ecological community to become eligible to be listed in another category representing a higher degree of endangerment?
C. Does the threatening process adversely affect 2 or more listed threatened species (other than conservation dependent species) or 2 or more listed threatened ecological communities?
Two species listed as threatened under the Environment Protection and Biodiversity Conservation Act 1999 may be potentially affected by the Cane Toad. These species are the Ornamental Snake, Denisonia maculata, and the Green and Golden Bell Frog, Litoria aurea, both of which are listed as vulnerable under the Act.
The Ornamental Snake
The Ornamental Snake is confined to the Brigalow Belt, within the drainage system of the Fitzroy River, mid-eastern Queensland, where it occurs in open forest, woodland, and riparian habitat. The species is considered to be threatened by a combination of factors including: overgrazing, clearance of habitat for agriculture and grazing, pasture improvement, crop production, urban development and possibly poisoning by ingestion of Cane Toads. The diet of the Ornamental Snake is almost exclusively frogs, and Cane Toads are abundant within, and totally encompass, its range.
While this species is one of the 49 species considered to be potentially at risk from lethal ingestion of Cane Toad toxin (Phillips et al. 2003 in press), and it is likely that this is a factor in the decline of the species, there is lack of quantitative evidence that the species is declining, or has declined, as a direct result of lethal toxic ingestion of Cane Toad such that it would become eligible to be listed at a higher level of endangerment. Nor can it be established that the Cane Toad is having an adverse affect on the species. Additional research is needed to identify whether lethal toxic ingestion is contributing to a decline of the Ornamental Snake at a population or species level.
The Green and Golden Bell Frog
The Green and Golden Bell Frog has been recorded as being in decline since the 1960's. The factors leading to the decline are not well understand but are likely to include predation of the tadpoles by introduced fish such as Gambusia, loss of habitat through clearing and drainage, and diseases such as chytridiomycosis. The Cane Toad may also be a factor in the decline as the historical distribution of the Green and Golden Bell Frog has retracted to the south, such that its northern extent almost exactly correlates with the southern extent of the southern continuous distribution of the Cane Toad into New South Wales.
Whilst not likely to be the sole cause of the northern contraction in range of the Green and Golden Bell Frog, competition and predation interactions between the Cane Toad and the Green and Golden Bell Frog are likely to have operated in concert with other threatening processes such as Frog Chytrid infection, Gambusia predation, and habitat loss in the decline of the Green and Golden Bell Frog in the northern parts of its range. Despite these observations, at this stage there is insufficient quantitative data to indicate that Cane Toads are causing the Green and Golden Bell Frog to decline. such that it would become eligible to be listed at a higher level of endangerment or that it is having an adverse affect on the species.
Conclusion for criterion B and C:
It is considered that, at this stage, there is insufficient data to adequately quantify the exact level of impact that Cane Toads pose to any listed threatened species, including the Ornamental Snake and the Green and Golden Bell Frog. While it is likely that Cane Toads are having a detrimental effect, this effect is, to date, insufficiently quantified. There are also currently no data on the impact of the Cane Toad on a listed threatened ecological community. There is insufficient quantitative data to indicate that the threatening process could cause a listed threatened species or a listed threatened ecological community to become eligible to be listed in another category representing a higher degree of endangerment. In addition, it can not be adequately demonstrated that any listed threatened species or listed threatened ecological community is adversely affected by the threatening process.
TSSC considers that the threatening process is not eligible for listing under either Criterion B or Criterion C.
Conclusion - The threatening process does meet s188(4)(a), s188(4)(b) or s188(4)(c) of the EPBC Act. The threatening process could cause native species to become listed as threatened.
Although Cane Toads have been present in Australia since the 1930s, and are known to be highly invasive, there remains limited quantitative information on the impacts they have on many native species and ecosystems, particularly in the medium to longer term. Much information gathered to date is anecdotal or inconclusive at a population or species level.
There is, to date, insufficient data quantifying the impacts from Cane Toad predation (e.g. on aquatic invertebrates, insects, worms, snails etc) and the competition effects that may occur on native species through the presence of Cane Toads in their habitats. It is likely that Cane Toads compete with native species for food and shelter but little is known of these competition effects. While it is suspected that predation and competition may play significant roles in reducing biodiversity of native species, and altering ecosystems, data concerning such impacts is neither complete or comprehensive.
It is evident that many species continue to maintain populations throughout their range, even in the presence of the Cane Toad, and there are some anecdotal reports that the wildlife impacts of Cane Toads may diminish over time. Alternatively, a substantial body of anecdotal evidence, and more recent experimental evidence, suggests that some predator species undergo dramatic and potentially threatening declines in abundance as a result of fatal poisoning by Cane Toads, immediately following colonisation of their habitat.
To date, there is insufficient information to adequately quantify the likely extent of declines of many affected species. For some native species, Cane Toad impacts may be minor and inconsequential in the longer term, despite marked short term responses. Alternatively, species may experience little immediate response but be either directly or indirectly impacted on in the longer term. Most fauna populations undergo population fluctuations of varying magnitude and it is difficult to interpret changes from a baseline to a single subsequent re-sampling period. Longer term population trends can only be discerned from natural fluctuations by a series of monitoring periods. It is considered that further research on the precise nature of both short and longer term impacts of Cane Toads on native species is required, including the impacts that are likely to come from predation and competition.
More recent studies on reptiles, frogs, and the vertebrate fauna of Kakadu National Park are instructive and in particular recent population crashes in Northern Quolls provides compelling evidence that the invasion of the Cane Toad into new environments can cause local populations of the Northern Quoll to decline catastrophically. Some local populations of the Northern Quoll persist in Queensland post the introduction of the Cane Toad, but little information exists concerning the numbers that persist, why they persist, and there is no indication that the species is recolonising its former range habitat, or that recovery is occurring or likely to occur in the presence of the Cane Toad. There is no evidence that Northern Quolls, which are generally solitary hunters, learn to live successfully with the Cane Toad. The lethal ingestion of Cane Toad toxin poses a significant risk to the Northern Quoll and is a threatening process which could cause this species to become eligible for listing as Extinct, Extinct in the Wild, Critically Endangered, Endangered or Vulnerable.
A combination of local control methods that can be developed quickly and longer term national initiatives have been suggested in order to control the various impacts that Cane Toads are considered to have on native species and ecosystems. In particular, a strategy has been proposed to coordinate joint national action. A number of activities attempting to address the threat posed by Cane Toads are currently underway and are instructive when considering the need for a national threat abatement plan:
- research into a biological control: significant resources, dating back to the early 1990s have been put into research on a suitable biological control. Initially, no methods were identified that would specifically target Cane Toads and enable broad scale control. While early research identified viruses from Venezuela that would control Cane Toads in Australia, laboratory trials showed that the same viruses also killed native Australian frogs. In 2000, a new research project began to investigate a mechanism to disrupt the development of tadpoles to sexual maturity. This research, undertaken by CSIRO, is ongoing, complex and may take at least a decade to complete with no guarantee of a feasible control method to control Cane Toads. To date, the project is progressing well with CSIRO advising of success in isolating possible genes and viruses that could be considered for use in preventing Cane Toad tadpoles from developing. While research on biological control progresses, other projects aimed at investigating alternative control methods, such as acoustic or chemical signalling; trapping; modification of habitat; and exclusion devices are also being undertaken.
- translocation of Northern Quolls from mainland Northern Territory (i.e. as part of the Island Arks program): approximately sixty Northern Quolls have been translocated to toad-free Astell and Pobasso islands from the mainland as a safeguard against advancing Cane Toads severely impacting mainland populations. This program is also attempting to improve the quarantining of islands to reduce the chance of Cane Toad invasion and to maintain the biodiversity present there;
- public awareness campaigns. In 2000, New South Wales launched a new campaign to; prevent Cane Toads establishing outside their existing range; raise community awareness of Cane Toads including impacts on biodiversity; increase community ownership and involvement of the Cane Toad issue; and raise community awareness of native frogs and their conservation requirements. The Northern Territory has also undertaken public awareness campaigns to alert residents and communities to the pending arrival of Cane Toads, notably around Darwin and rural communities. This campaign highlighted the biology and impact of Cane toads; their identification, and outlined safe methods of collection and disposal.
- a national approach to address Cane Toads is being developed by the Natural Resource Management Ministerial Council. It has directed the Vertebrate Pests Committee to review the current threat posed by Cane Toads; review the state of research to abate those threats; identify gaps in current approaches; and assess the costs and benefits of various options for national action;
- a Senate inquiry into the EPBC Act, 'Inquiry into the regulation, control and management of invasive species and the EPBC Amendment (Invasive Species) Bill 2002', has examined amendments to this legislation to better manage invasive species. Cane Toads were one of the species identified as part of this inquiry. It is expected to publish its finding by 25 November 2004; and
- in 2003, the Legislative Assembly of the Northern Territory held an inquiry into issues associated with the progressive entry of Cane Toads which recommended, amongst other things, that the Northern Territory Government develop and implement a plan of management for the control of Cane Toads. One resolution of the Inquiry was the establishment of a body to help coordinate research and management actions. This body is yet to be established.
Conclusion: Threat Abatement Plan
While data on the impact that Cane Toads have on many native species through predation and competition is inconclusive, it is considered that any national approach should also incorporate the wider impacts that Cane Toads are suspected of having. It is considered that a national approach is essential in providing coordination for current, and any future, activities related to controlling the impacts of Cane Toads. It is also noted that Cane Toads are recognised as pests in many other countries and a threat abatement program developed in Australia may have far wider benefits beyond Australia. This could involve continuing research, public education, quarantine measures; and local projects to minimise the impact of toads on areas of high conservation value. It is evident that research is lacking in relation to the precise nature and extent of a number of impacts.
Research sites have been established to the west of Kakadu National Park and it is hoped that these studies will provide information directly relevant to the management of Cane Toad impacts in Kakadu National Park as well as much of northern Australia. Coordination of these various studies to assess available results, identify gaps and provide support funding if required, and to consider their implications will require ongoing attention. To this end, a national approach to address Cane Toads is being developed by the Natural Resource Management Ministerial Council. It has directed the Vertebrate Pests Committee to review the current threat posed by Cane Toads; review the state of research to abate those threats; identify gaps in current approaches; and assess the costs and benefits of various options for national action.
TSSC considers that, given the above and other initiatives including means to coordinate a national response to the issue, the development of a national threat abatement plan is likely to be duplicative and inefficient. These initiatives are likely to involve stakeholder workshops and the preparation and implementation of a national strategy. The effectiveness of these initiatives should be considered before establishing another parallel process.
TSSC considers that priority actions that need to be continued in the short term include: assessing the feasibility of excluding Cane Toads from currently uninvaded islands or high conservation value areas; further examination of the practicality of physical barriers; and examination of other alternative control measures such as acoustic or chemical signalling and trapping. To this end, the TSSC will seek annual reports on the progress of these measures from the Department of the Environment and Heritage and reassess its recommendation to the Minister if required.
TSSC recommends that:
- The list referred to in section 183 of the EPBC Act be amended by including in the list as a key threatening process: 'The biological effects, including lethal toxic ingestion, caused by Cane Toads (Bufo marinus)'.
- Given the measures already in place, and being put in place, including means to coordinate a national response to the issue, the development of a national threat abatement plan is likely to be an inefficient way to approach the issue at this stage. It is recommended that the outcome of these initiatives be assessed before establishing any further processes. The Committee will seek annual progress reports from the Department and review the need for a national Threat Abatement Plan as required.
The Minister for the Environment, Heritage and the Arts has decided to approve the development of a threat abatement plan (TAP) to address the 2005 key threatening process - the biological effects, including lethal toxic ingestion, caused by Cane Toads (Bufo marinus)
Large scale research efforts to control toad populations, such as researching biological control solutions, have not yielded sufficiently encouraging results to justify their continuation or implementation. Similarly, efforts to stop or limit toad incursion by physical barriers or removal, despite having some success at a very local scale, do not provide a broad scale solution and protection for the key biodiversity assets impacted by the cane toad.
The Threatened Species Scientific Committee advised the Minister that a TAP which sets a nationally consistent approach to coordinated management and research is more likely to actively protect Australia's biodiversity and assets than broad scale, landscape-wide control methods. The Committee believes that a strategically targeted TAP under the EPBC Act would provide authoritative national leadership that would guide investment and action by other jurisdictions, research organisations and non-government organisations.
For more information about the decision see:
- Boland, C.R.J. (2004). Introduced cane toads are active nest predators and competitors of rainbow bee eaters: observational and experimental evidence. Biological Conservation, 120, 53-62.
- Braithwaite, R.W., and Griffiths, A. (1994). Demographic variation and range contraction in the northern quoll Dasyurus hallucatus (Marsupialia: Dasyuridae). Wildlife Research 21:203-17.
- Burnett, S. (1997). Colonising cane toads cause population declines in native predators: reliable anecdotal information and management implications. Pacific Conservation Biology 3:65-72.
- Catling, P.C., Hertog, A., Burt, R.J., Wombey, J.C., and Forrester, R.I. (1999). The short-term effect of cane toads (Bufo marinus) on native fauna in the Gulf Country of the Northern Territory. Wildlife Research 26:161-185.
- Crossland, M.R. (1997). Impact of the eggs, hatchlings and tadpoles of the introduced cane toad, Bufo marinus (Anura: Bufonidae) on native aquatic fauna in northern Queensland, Australia. PhD thesis, James Cook University, Townsville.
- Crossland, M.R. (1998). A comparison of cane toad and native tadpoles as predators of native anuran eggs, hatchlings and larvae. Wildlife Research 25: 373-81.
- Environment Australia (2003). Submission to the Legislative Assembly of the Northern Territory Sessional Committee on Environment and Sustainable Development Inquiry into issues Associated with the progressive entry into the Northern Territory of Cane Toads.
- Freeland, W.J. (1986). Populations of cane toad, Bufo marinus, in relation to time since colonization. Australian Wildlife Research 13, 321-329.
- Freeland, W.J. (2004). A review of the cane toad's (Bufo marinus Lineaus) impacts on the native Australian fauna.
- Grigg, G. Taylor, A., and McCallum, H. (2003). Impact of cane toads on native frogs, Roper River Valley and Kakadu National Park. Brief synopsis of study and results to date, May 2003.
- Holland, D.C., (2004). Interim report for study of impacts of cane toads, Bufo marinus, on two goanna species, Varanus panoptes and Varanus goudii.
- Legislative Assembly of the Northern Territory, Sessional Committee on Environment and Sustainable Development. Issues associated with the progressive entry into the Northern Territory of cane toads. Volume 1. Report No.1 October 2003.
- Oakwood, M. (2000). Reproduction and demography of the northern quoll, Dasyurus hallucatus, in the lowland savanna of northern Australia. Australian Journal of Zoology 48:519-539.
- Oakwood, M. (2003). The effect of cane toads on a marsupial carnivore, the northern quoll, Dasyurus hallucatus. Progress report to Parks Australia North, May 2004.
- Oakwood, M. (2003). Quolls decline with the advance of toads. Savanna Links (newsletter of the Cooperative Research Centre for Tropical Savannas Management) 26, 6, 12.
- Oakwood, M. (2004). Case of the Disappearing Spots. Nature Australia in press.
- Phillips, B.L., Brown, G.P., and Shine, R. (2003). Assessing the potential impact of cane toads Bufo marinus on Australian snakes. Conservation Biology, in press.
- Seabrook, W. (1991). Range expansion of the introduced cane toad Bufo marinus in New South Wales. Australian Zoologist 27, 58-62.
- Smith, J.G., and Phillips, B.L. (in prep). Toads in the tropics: predicting the effects of novel toxins on native taxa. Manuscript for consideration Biological Conservation.
- Straughan, I.R. (1966). The natural history of the cane toad in Queensland. Australian Natural History 15, 230-232.
- Sutherst, R.W., Floyd, R.B., and Maywald, G.F. (1995). The potential distribution of the cane toad, Bufo marinus L. in Australia. Conservation Biology 10, 294-299.
- van Dam, R.A., Walden, D.J., and Begg, G.W. (2002). A Preliminary Risk Assessment of Cane Toads in Kakadu National Park. Scientist Report 164, Supervising Scientist, Darwin, Northern Territory.
- Watson, M., and Woinarski, J. (2003). A preliminary assessment of impacts of cane toads on terrestrial vertebrate fauna in Kakadu National Park. Report to Kakadu Research Advisory Committee, November 2002.
- Watson, M., and Woinarski, J. (2003). Vertebrate monitoring and re-sampling in Kakadu National Park 2002. Project RS10, Report to Parks Australia: March 2003.
- Williamson, I. (1999). Competition between the larvae of the introduced cane toad Bufo marinus (Anura: Bufonidae) and native anurans from the Darling Downs area of southern Queensland. Australian Journal of Ecology 24:636-643.
- Woinarski, J.C.Z., Milne, D.J., and Wanganeen, G. (2001). Changes in mammal populations in relatively intact landscapes of Kakadu National Park, Northern Territory, Australia. Austral Ecology 26:360-370.