A market introduction

The Eastern Dwarf Tree Frog (Litoria fallax) is a small, bright green frog, often with brown markings on the legs and flanks, which inhabits swamps and ponds of lowland woodlands, forests and heathlands.  It is found, in Victoria, in suburban wetlands, associated with parks and quarries where it can be identified by its distinctive, high-pitched scratching call.  The only wild Victorian populations recorded in the Victorian Illustrated Fauna Information System are in wetlands of a park in the light-industrial suburb of Moorabbin, about 15 km south-west of Melbourne, as well as two sites further north near Fairfield and Warrandyte.

The Eastern Dwarf Tree Frog (EDTF) is native to near-coastal regions of Queensland and NSW, its introduction to Victoria is probably the result of transport in fruit and vegetable produce from Queensland.  This mode of translocation has become quite commonplace in recent years and the EDTF has sometimes been called the Banana Box Frog because of it often lives in banana plantations and becomes an unwitting 'contaminant' during packaging.  

The EDTF is not the only species that is transported with plant produce, the similar, Dainty Tree Frog (Litoria gracilenta) is also a common passenger as is the Northern Dwarf Tree Frog (Litoria bicolor).  The Victorian Frog Group estimates that over 6000 individual frogs may be introduced to Victoria in this manner each year and that these are usually released at or near the market delivery sites.  Clearly there is potential for a much greater number of wild populations being formed than are currently recorded and as the species of frog are not being checked before release it is possible that other small, Queensland species, related to the EDTF, such as the Cooloola Tree Frog (Litoria cooloolensis) and the Wallam Sedge Frog (Litoria olongburensis) could be among them.

The EDTF is tiny so it is not likely to become another environmental disaster like the Cane Toad (Bufo marinus) but there are reasons to be concerned about its introduction and the main one is the spread of disease.  In the past few decades there has been worldwide concern about the rapidly declining numbers of many frog species.  The reason for this decline is now thought to be a water-borne chytridiomycete fungus called Batrachochytrium dendrobatidis which generates a virulent skin infection from which the animals often do not recover.  The speed with which this disease has spread has been attributed, at least partly, to an international trade in frogs for medical and zoological research as well as for amateur animal husbandry.   As a consequence any movement of frogs from one place to another is now viewed with considerable caution by governments and biologists.

The Eastern Dwarf Tree Frog has been known to carry this fungus so that in itself is reason for serious attempts to restrict its establishment.  However, it is also known to be affected by another fungal disease called mucormycosis which is caused by the zygomycete fungus Mucor amphibium.   This disease causes lumps in the liver and other internal organs of frogs and may eventually kill them.  Queensland populations of at least one local frog species, Peron's Tree Frog (Litoria peronii) are known to be susceptible to infection but what is also worrying is that the same disease has been implicated in the deaths of Platypus in Tasmania.

The Tasmanian government has issued a number of documents which suggest that EDTF, introduced to that state through fruit and vegetable produce from Queensland, is the potential source of mucormycosis in Tasmanian Platypus.  The disease affects Platypus by the generation of sometimes large, open ulcers on the skin, which may develop from normally harmless small injuries. The animal's are thought to die as a result of secondary bacterial infections through the ulcers.  It is suggested that the Platypus is more susceptible to the disease than other mammals because its body temperature (32^o C) is normally lower than that of other species (36^o C) and that the infection cannot survive body temperatures of 36^o C or more.  This means that, theoretically at least, the Water Rat (Hydromys chrysogaster) is unlikely to be affected by the disease.

It is clear that the EDTF is not a native species to Victoria and that there is potential, should its numbers inrease, for it to become a biological hazard to native frogs and Platypus.  The Victorian Department of Natural Resources and Environment, however, views the likelihood of this threat as negligible and does not categorise the species as non-native in Victoria.  In addition, EDTF is listed as protected wildlife under the Wildlife Act and Wildlife Regulations, which means that a permit is required before any animal can be collected or moved.

© Paul Gullan, Viridans Biological Databases