- Four species of green treefrog live in the lowlands around the Yellow Sea in Asia.
- Three of the species inhabit traditionally managed rice paddies. Unfortunately, this substitute habitat is vanishing fast.
- Professor Amaël Borzée at Nanjing Forestry University, China, is working at speed to find out exactly where these treefrogs are and what help they will need to survive.
Treefrogs have a special place in many people’s hearts. Brightly coloured ones look like toys that have been whimsically placed in the vegetation by a passing toddler. We imagine them in complex foggy rainforests, but if you’re lucky – and prepared to head out at night – you might hear their calls in unexpected situations. Professor Amaël Borzée and his team have spent over a decade doing just that in and around the Yellow Sea Basin in Asia. The researchers have been following four species of treefrog living in this area particularly closely.
One of the species, Dryophytes japonicus, has become a relatively successful generalist that lives in several countries and in both natural and man-made habitats, but three more specialist species (Dryophytes immaculatus, D. suweonensis and D. flaviventris) have ended up in smaller niches. These specialist species are found almost entirely in lowland wetlands, and surprisingly, many of them live almost exclusively in rice paddies!
“Humankind significantly impacted treefrog populations when they transformed wetlands into rice farmland, starting about 10,000 years ago.”
Translocated and transformed
These treefrogs have travelled a long way in both distance and ecological preferences to arrive in this unlikely habitat. Their ancestors crossed the Bering pass from America about 17–22 million years ago. In Asia, humankind significantly impacted treefrog populations when they transformed wetlands into rice farmland, starting about 10,000 years ago. Rice became a popular crop, and traditional flooded paddy fields were well suited to the treefrogs’ ecology. This change in landscape may have boosted the size of some populations, while also restricting them to non-natural wetlands.
The perks of farm living
Dryophytes treefrogs need water to complete their tadpole life stages, but they climb out of the water to hibernate underground in damp spots among surrounding undergrowth in the winter. This means they are safely out of the way when farmers drain their fields in the colder months. The frogs then jump back into the paddies to call for mates and lay eggs when it is re-flooded the following spring. Some tadpole predators, such as fish and dragonfly larvae, remain in the water – so this winter draining is quite a convenient clear-out process for the treefrogs.
Unfortunately, this substitute niche is vanishing fast. Rice is now less popular, cities are growing, and even the rice paddies are not what they used to be from a treefrog perspective. Soft drainage ditches with undergrowth are being ‘tidied up’ and smoothed off with concrete that offers no shelter for hibernating frogs. Also, climate change means the rice that is cultivated grows faster, and increasingly the water is not there long enough for the tadpoles to metamorphose. Perhaps most distressingly, the remaining rice straw is sometimes illegally burnt at the end of the season, which either kills the frogs or at least dries the area out to the point where they struggle to survive.
Only tiny pockets left
The more land Professor Borzée surveys and the more he finds out about the threats these frogs face, the more worried he has become for their future.
D. immaculatus – commonly known as the Chinese Immaculate treefrog – is a species of treefrog found in China. In living memory, it was found in many locations along the Yangtze River basin and around Nanjing and Beijing, but now the Chinese Immaculate treefrog is functionally extinct in previous strongholds of Jiangsu and Shanghai. There are some small populations in Hefei in Anhui, and in Jiangsu, but work is needed to make these frogs less vulnerable to further losses.
Populations of the Suweon treefrog (D. suweonensis) are present along the west coast of the Korean Peninsula, but without help they are likely to become extinct in southern areas. The situation further north is likely to be safer since there are more natural wetlands and land use is less intensive, with lower rates of mechanisation and chemical inputs.
Lastly, D. flaviventris, the Yellow-bellied treefrog, is perhaps the most vulnerable of this group of three similar species and is recommended to be listed as ‘Critically Endangered’. Its range is restricted to a very small strip of land between the Mangyeong river and the Chilgap hills in Korea. The hills are an obvious barrier to this lowland species, but it is not exactly clear what prevents this species from expanding beyond the river.
“Even a single well-managed site will prevent the extinction and loss of genetic distinction of the Suweon treefrog.”
The threat from hybridisation
The Suweon treefrog also faces a threat to its genetic identity. Males of the more numerous generalist species D. japonicus can intercept and mate with females as they arrive in the breeding grounds. The genetics of the two species are similar enough that the progenies of these matings are able to produce offspring. The bad news is that many of the observable features that are unique to the Suweon treefrog, such as its call and specific markings, are different in the hybrids.
What can be done?
The team is continuing their survey work to keep track of known populations of the rarer species of treefrog and try to discover new ones, but there is a lot of ground to cover and not much time. Dr Borzée and his team have demonstrated that even a single well-managed site could prevent the extinction and loss of genetic distinction of the Suweon treefrog. Because the rarer Dryophytes species share many similarities, the team believes that using a coordinated approach all around the Yellow Sea area is the most efficient way forward. Lessons learnt are often transferable, and we should aim to secure all three of these rare species for future generations to appreciate.
Let’s hope that the calls of these special frogs will continue to be heard in the night air all around the Yellow Sea.
What do you think are the immediate conservation actions that need to be taken?
If immediate conservation actions could be taken, I would recommend protecting the two sites with the highest conservation designation priority for the two Korean species as listed in our latest paper (Andersen et al, 2022). With these two sites protected, the probability of extinction over the next 100 years for these two species would drop close to zero. By ‘protection’, I mean maintaining rice agriculture as the treefrogs need the regulated hydroperiod that originates from rice plantation, and a single year without artificial flooding could result in local extirpation. For the species in China, we would need to do intense and large-scale surveys, possibly through citizen science, as there is still so much unknown about the distribution and population size of the species.
What is it like to head out at night in search of treefrogs?
Heading out to the rice paddies at night in spring is like heading home. I have spent so many nights surveying rice fields that it’s the place where I feel I should be, whatever the country and time of night. It does come with surprising questions from farmers, and a few times I have been challenged to show the owner of the fields that the treefrogs were indeed there – always successfully so far! A strong point of the surveys we do with my team is that we don’t need to walk in the fields, but only listen to the frogs so it doesn’t damage crops, and we stay friendly with farmers.
What was the most surprising place that you have found treefrogs?
There are two localities that come to mind directly. The first one is the city of Suwon, Korea, the eponymous city of the Suweon Treefrog, where I heard a male Suweon Treefrog calling the first year I started working on the species. It was the worse place ever for a treefrog to live, stuck in between apartment complexes. I did not know the importance of this lone calling male at the time, but it was the last time the species was heard calling in the city – until we reintroduced them. The second one was in Yeoncheon area in Korea, where we found a dozen males calling from a very small rice paddy stuck between train tracks and a national road. Such a sad encounter.
What would it mean to you to secure the future of all four Dryophytes species in this area?
It would mean peace of mind. This has become one of the goals driving most of my research projects – and free time – and I would feel like I can finally take a rest. It would also mean a lot of other things, such as national conservation plans for these species between regions that are not always in agreement, and these plans would indirectly protect a broad range of other species, including migrant birds that are declining incredibly quickly. So yes, achieving the protection of these treefrogs would mean that we live in a better world.
Are there treefrogs living in areas that have not been modified by human activities?
Very few – human activities have impacted most of the landscapes on Earth, and most species can cope with some level of habitat modifications. Some species like the treefrogs I study can cope with a lot of it, while others cannot be found in areas with even the smallest human settlements. So the answer to the question is ‘yes’, and you can find treefrogs in the rice paddies of south Asia or Africa, but also in the rice paddies of places where you would not expect them, such as southern France and southwestern USA. Treefrogs are adaptable, but to a certain point only, and if we let the landscapes cross this threshold, then it is not only treefrogs that become extinct.