Cane toads

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Predicting ecological impacts of cane toads – A preliminary risk assessment for Kakadu National Park


Rick van Dam & Dave Walden

Environmental Research Institute of the Supervising Scientist (eriss)

GPO Box 461 , Darwin NT 0801 Ph: 08 8920 1175 Ph: 08 8920 1171


Cane toads (Bufo marinus) entered the Northern Territory (NT) in 1980 from Queensland and by 2000 were rapidly approaching Kakadu National Park (KNP), having been reported in July of that year in the upper Mann River and Snowdrop Creek, approximately 15–30 km to the east of KNP .

Concern about the invasion of cane toads in KNP has been highlighted on a number of occasions, and in 1998 participants at a workshop on the potential impacts and control of cane toads in KNP conceded that a strategic approach for assessing and possibly minimising cane toad impacts should be developed. The first stage would be an ecological risk assessment to predict the likely extent of impacts of cane toads in KNP and identify key vulnerable habitats and species. This information could be used to develop new monitoring programs and assess existing ones.

The risk assessment was essentially a desktop and liaison exercise directed at collating, analysing, and making predictions based on all relevant information on cane toads in Australia and on the Kakadu environment. The impacts examined include: effects on predator species; effects on prey species; effects of resource competition; cultural effects; economic effects; and other potential effects. A

Using three criteria (definite, probable & possible) derived from information in the literature, a total of 151 species or species groups were identified as being susceptible to cane toads. These species cover a broad taxonomic range including aquatic invertebrates, fish, frogs, lizards, snakes, birds and mammals. Eleven species were considered definitely susceptible, comprising 5 lizard, 3 snake and 3 mammal species. Sixteen species or species groups were considered probably susceptible, while 124 species or species groups were considered possibly susceptible to cane toads. These species were further refined to a ranking of risk using exposure (ie available habitat overlap, feeding ecology, behaviour) and ecological/cultural importance status information. Four risk categories (likely, possible, uncertain & unlikely ) were defined.

Ten species were considered likely to be at risk of experiencing population level effects, with the northern quoll being assigned the highest priority. Twelve species or species groups were considered to be at possible risk although none were listed as endangered or vulnerable or thought to be notable. Thus, all species were assigned moderate priority status. Due to a lack of information, the risk of population level effects was considered to be uncertain for 98 species or species groups, although 21 of these were assigned high priority. A total of 31 species were considered unlikely to be at risk of experiencing population level effects and were assigned low priority.