Which frogs are best equipped to survive the human world?

We rank Australia’s frog species based on how likely they are to persist in human modified habitats – with some surprising results!

We live in a time when urban expansion outpaces human population growth, and when forests are being cleared faster than they can be gained. Human-driven habitat loss and modification is a major cause of global frog declines and if we are to conserve our amazing frog diversity, it is important that we identify which frog species are likely to be tolerant of habitat modification, and which are likely to be intolerant. This will allow conservation efforts to be directed towards the species that are most in need of our help.

Like us, frogs have preferences when it comes to where they live. While some species might be perfectly content living anywhere with water – in your backyard pond, down a drainpipe or even in your toilet – others have more specific (arguably, more refined) tastes, showering under the misty waterfalls of the rainforest, or riding fast-flowing streams.

But as the human population grows and the natural landscape becomes increasingly modified, these frogs are at risk of losing their homes, and finding a new one that supports their survival is no easy feat. The nearest suitable habitat may be too far away to reach and the journey can be treacherous. Anyone who has ever played Frogger or its various modern reincarnations would be able to tell you that as a frog, braving a road is a gamble on life.

To understand which species could persist or even thrive in a modified habitat, and which ones were vanishing, we created a ‘modification tolerance index’ that we could use to rank frog species from intolerant to tolerant. To create this tolerance index, we combined over 126,000 records of frogs submitted by people across Australia to the FrogID project, with a global measure of human habitat modification. This measure considers a broad range of human stressors including built-up areas, roads, railways, agriculture, energy production, mining, and night-time lights. We examined 87 frog species (those with 100 or more FrogID records) – more than a third of Australia’s frog species.

Alarmingly, we found that an overwhelming 70% of the frog species examined were intolerant of human modified environments.

The most intolerant species were habitat specialists (those with specific habitat requirements). This includes many frogs from the Pseudophryne genus such as the Red-crowned Toadlet (Pseudophryne australis), which resides exclusively in the Sydney Basin’s Hawkesbury sandstone regions, inhabiting open forests with temporary water bodies along drainage lines. Although their geographic range includes the more modified parts of Sydney, they are rarely found in people’s backyards, preferring remnant forests instead.

Frog species that laid their eggs on land were also amongst the most intolerant. This category, again, includes all the frogs from the Pseudophryne genus including the Crawling Toadlet (Pseudophryne guentheri), Bibron’s Toadlet (Pseudophryne bibronii) and Red-backed Toadlet (Pseudophryne coriacea). The resources they depend on (for example, moist leaf litter) are rarely present in human modified habitats. The future of these frogs will therefore depend on our ability to preserve their natural habitats.

There is some good news, however, for species that call from vegetation. These frogs tended to be the most modification tolerant, which suggests that creating greenspaces and ‘frog-friendly’ gardens with plenty of vegetation could help to improve frog diversity in modified areas, including our cities.

But when it comes to life in our cities, suburbia and farms, the standout and most tolerant species were the generalists. These species can use a diverse range of resources and can tolerate a wide variety of environmental conditions and habitats (including human modified habitats). Unfortunately, these tolerant species were few and far between. Only 3 in 10 frog species examined were tolerant of modified habitats and this was only considering the obvious elements of human modification. Our index did not account for other direct and indirect human impacts such as our influence on the climate, introduced species and water resource development. Factor those in, and the proportion of intolerant frogs are probably even higher!

Clearly, supporting the long-term persistence of frog populations requires greater consideration of the impacts of humans on the environment. We also need improved conservation measures. A good place to start would be to focus on habitat specialists and species we identified as intolerant of human modified habitats. Thankfully, with the help of thousands of citizen scientists distributed across the country, acquiring the information that scientists need to make informed conservation decisions is now easier than ever.

Read further for more information on Australia’s least and most tolerant frogs.

Coming in last on our list (the most intolerant of human modified habitats) was the Crawling Toadlet (Pseudophryne guentheri), a small ground-dwelling frog, no more than 4cm in body length, from southwest Western Australia. While this species was the least tolerant of the frogs that we studied, there were species that just didn’t have enough data for us to study. Most of these were habitat specialists, secretive species or species that live in very remote parts of Australia – those that are likely to be even more intolerant of habitat modification.

The second most intolerant species was the Bleating Froglet (Crinia pseudinsignifera), another small species (reaching 3cm in body length) from southwest Western Australia. This species lives in temporary swamps in granite areas.

The Ticking Frog (Geocrinia leai), another Western Australian frog, came in as the third most intolerant species. This small (no more than 3cm in length) frog can be found in the south west of the state in Jarrah forest. The males – the sex that makes advertisement calls – live up to their name, wooing females with a continuous ticking call. Females lay their eggs in a cluster on land under wet leaf litter, logs and waterside vegetation.

On the other hand, Australia’s most tolerant frog turned out to be the Striped Marsh Frog (Limnodynastes peronii). This species is likely a familiar one to the residents of Australia’s east coast, from south of Cape York to northern Tasmania. Males have a distinctive call that sound a lot like a dripping tap or a tennis ball being hit. In a chorus, they can sound like a mass of microwave popcorn. Not only does this species occupy human modified habitats, but it may even prefer them to more pristine habitats.

The second most tolerant species was the White-lipped Tree Frog (Litoria infrafrenata), a northern Queensland species and Australia’s largest frog, reaching 13.5cm in body length. This frog inhabits rainforest and Melaleuca swamps, but it is not unusual for them to appear on farms and in suburban gardens.

Third place for tolerance went to Western Australia’s Motorbike Frog (Litoria moorei). The frog doesn’t get its name for no reason. Its advertisement call resembles the rumble of a motorbike. If you had this species in your backyard, you would be forgiven for thinking that someone was doing burnouts outside your house.

Gracie Liu, FrogID validator, Herpetology, Australian Museum Research Institute; and, PhD candidate, University of New South Wales.