Our tiny green hitchhiker: Citizen science reveals the frog popping up across eastern Australia

Thousands of people using the FrogID app have helped reveal just how much the Eastern Dwarf Tree Frog (Litoria fallax) is establishing populations outside its native range.

The Eastern Dwarf Tree Frog (Litoria fallax) is a commonly encountered tree frog, native to eastern Australia. Whilst only a couple of centimetres in body length, it’s a bright apple-green colour and sits out on vegetation during the day, so is often spotted. It also has a surprisingly loud advertisement call, and its unmistakable “screeeech-pip-pip” is a familiar sound from central-eastern Queensland in the north to the border of New South Wales and Victoria in the south.

The Eastern Dwarf Tree Frog (Litoria fallax) calling.

However, this little frog appears to be particularly good at hitchhiking around the country and establishing populations outside its native range. Far flung populations of the species were first reported in the 1990s, from around Melbourne, over 400km away from their native range. A decade later, populations in north-eastern Victoria, around 220km outside their native range, were reported.

This may seem like no big deal – after all, it’s just a small frog that is native to Australia. However, invasive species, even if they are native species, can have serious economic and ecological consequences. The most likely impacts are on the local frog species that now must deal with a new frog on the block (or rather, in the pond!), one that is likely to compete with them for food and other resources and that may also even spread disease. For species that are already in trouble (and unfortunately, there are many), this extra bit of pressure might just be the final straw.

With that in mind, we really need to get a handle on just where the Eastern Dwarf Tree Frog is. However, this is a huge logistical challenge. Conducting scientific surveys for frogs across thousands of kilometres, quickly, just isn’t possible. Or is it?

Thankfully, tens of thousands of people across Australia are already doing frog surveys across Australia, using the Australian Museum’s FrogID app! Simply by using the app to record calling frogs, they’ve already built an enormous database of frog records across the country! We used this amazing FrogID dataset to determine just where this frog was – were the already reported populations still there, and were they spreading? And are there any new places that this frog species popped up?

From late 2017 until mid-2022, almost 6,000 people submitted recordings of about 50,000 Eastern Dwarf Tree Frogs to FrogID, and 500 of these were outside their native range. These records confirm the persistence and likely spread of populations around the already-known populations in Melbourne and northern Victoria. The population in northern Victoria was also documented extending into NSW for the first time. The species was also detected in Canberra in the ACT, Griffith and Wagga Wagga in NSW, and Mirboo North in South Gippsland, Victoria – all outside their native range.

So how did this tiny frog travel hundreds of kilometres and set up new populations? Possibly by hitchhiking on horticultural products or fresh fruit. Their small body size, habit of sheltering in vegetation, often well away from water, and their relatively high tolerance of urbanisation makes this species particularly good at stowing away on things being moved from one place to another.

In these newly established populations, the Eastern Dwarf Tree Frog is now living alongside frog species that it never has before. We don’t know what the consequences of a new neighbour will be for the long-time residents such as the Endangered Sloane’s Froglet (Crinia sloanei), but the continued use of FrogID, alongside scientific surveys, will be vital in determining this.

By working together using FrogID, we’ve been able to gather an enormous amount of information on Australia’s frogs in a relatively short time. The project is giving us insight into the health of our frog populations and their environment and helping inform frog conservation on many fronts, including the detection of species outside their native range. Continued submission of FrogID recordings from citizen scientists across Australia will be critical in understanding how our frogs, and the ecosystems in which they live, are faring over time, and how best we can protect them.