A tiny frog with a big problem has sparked a mountain-top rescue mission in Far North Queensland.

Conservationists are working in dense rainforest near the summit of Mt Lewis, north of Cairns, to save the critically endangered mountain-top nursery-frog which is facing two challenges – feral pigs and a changing climate.

The frog is roughly the size of a thumbnail and it lays eggs under leaf litter. It is only found in high-altitude cloud forests in Mt Lewis National Park near Julatten.

Feral pigs a major threat

Terrain NRM’s Dr Andrew Dennis says feral pig numbers appear to be increasing above 1,100 metres, based on the amount of disturbance being seen.

“Over time they’ve slowly migrated upwards from the low altitude areas of the mountain,’’ he said.

“In mountain-top nursery frog breeding sites up near the top of Mt Lewis, pigs are now digging up the ground and leaving bare soil where there was leaf litter and tree seedlings, and we’ve observed them digging in the roots of the Linospadix apetiolatus palms.

“These tiny frogs breed in leaf litter. The males call from the palms, to project their voices to females. Eggs are laid in the leaf litter on the palm leaves or the ground. Where it’s deep, it also holds a lot of food for them. But with pig disturbances, they are losing both breeding and feeding zones.”

Installing pig-exclosure fencing

One of the answers is pig exclosure fencing. Terrain NRM has been working with Western Yalanji Traditional Owners and Gulf Savannah NRM to erect fences around 10 breeding sites in dense cloud forest near the top of the mountain. Now they’re also working with the Queensland Parks and Wildlife Service and James Cook University researchers, using frog-call monitors and regular habitat assessments to monitor these sites and rare frogs.

“Breeding season is generally from October-November to March,’’ Dr Dennis said.

“We have 10 fenced sites that exclude pigs and 10 control plots that aren’t fenced and they’re all about 60 metres apart so that calls from frogs at one plot shouldn’t be heard at another.

Audio recorders for population monitoring

“We have audio recorders at each site to monitor calling activity and we’re also regularly monitoring the habitat, measuring things like seedling and leaf litter density and any recovery of the Linospadix apetiolatus palms.

“Pigs are churning up significant areas of this fragile habitat. We are measuring this as part of the project. On a wider scale, they’re also a weed and water quality threat to a highly biodiverse area of the Wet Tropics at the headwaters of the Mitchell and Mossman River systems.”

Climate change a threat to frogs high up on Mt Lewis

At least five frog species have disappeared from the top of Mt Lewis in the past few decades, with three of them likely extinct worldwide. Dr Dennis said a fungal disease had been the main cause of their demise, but climate changes pose another threat to species at the high altitudes of ranges like Mt Lewis.

“I’ve been visiting this region for well over 50 years. Walking in the cloud forest and around the creeks, it used to be noisy with frog calls day and night. Now it’s noticeably a lot quieter.”

Frog’s trill tells a sad story

The mountain-top nursery frog’s trill is also diminishing.

“Calls were detected by researchers in 2003 at 1,100 metres. But 20 years later, in 2023, there were no calls at 1,100 metres. They were detected at 1,200 metres and above.

New fishways are helping fish species, including barramundi and mangrove jack, to breed up in North Queensland waterways.

Six fishways built between Tully and Crystal Creek near Paluma over the last year have removed barriers for juvenile fish. Together, they’ve opened up more than 100 kilometres of creek and river systems – and recent surveys at these sites have shown they are fish highways.

Fish highways – 16 native species including barra, tarpon

More than 1000 fish were recorded moving through one of the fishways – at Insulator Creek north of Townsville – in one day. And in more recent surveys, over 4500 fish were caught and released during monitoring work at four rock ramp fishways.

“Sixteen native species, including juvenile barramundi, empire gudgeon, rainbow fish, tarpon and long-finned eels, were recorded,’’ Terrain NRM’s Deb Bass said.

“Of the 80+ species of native fish we have in the Wet Tropics region, up to half need to move between salt and freshwater habitats to complete their lifecycles. Barriers like weirs, pipes, culvert crossings and weed chokes can stop these species from thriving.

“In recent surveys at our fishways, eight of the 16 native species recorded are species that need that movement – from their beginnings in saltwater to safer freshwater nursery grounds as juveniles and then back downstream to the saltwater to breed as adults. So we can see the fishways are providing access for these species, from barramundi and mangrove jack to shrimps.”

A series of underwater stairs for fish

Terrain NRM has been working with OzFish Unlimited, Catchment Solutions and Australasian Fish Passage Services on the ‘Fish Homes and Highways’ project, which is funded by the Australian Government’s Reef Trust. Fishways are a relatively new solution in the region – a series of underwater stairs for fish, with deep pools between each one for resting.

Catchment Solutions fishery ecologist Matt Moore said rocks were placed strategically, sometimes in S-bend and sometimes a straight line, to achieve the right gradient to create a slower flow while also being able to withstand floods.

“To pull in our monitoring traps and see all the fish – a lot goes into the design of these structures, so we love seeing a result like this,’’ he said.

New ‘nursery slot’ for super small juveniles 

A ‘nursery slot’ is a new feature, trialled in one of the fishways, to help super small juveniles move upstream.

“It needs 20mm or less of water going through it to create the right conditions for weaker-swimming juvenile fish,” Matt said. “We recently recorded the smallest snakehead gudgeon ever documented in Australian fishway studies travelling through the slot. It was 16mm long.”

Do you want to know more about our new project to reduce threats to the critically-endangeed mountain-top nursery frog? Terrain NRM project leader Dr Andrew Dennis spoke with ABC Far North recently about a project that’ll lead to pig-exclusion fencing around known frog-breeding sites, pig control measures and species monitoring work.

Two thousand square metres of high-altitude rainforest will be fenced off to protect the critically endangered mountain-top nursery frog from feral pigs at Mt Lewis, west of Port Douglas. This frog is one of the most range-restricted on the continent. Terrain NRM is partnering with traditional owners, researchers and other groups on this project and Terrain’s Dr Andrew Dennis is with us to explain how pigs are threatening this frog’s existence at altitudes you wouldn’t expect.

Can you describe what these mountain-top nursery frogs look like…
Well, first of all, they’re tiny. They are about 17mm long – so less than two centimetres in length. They are mottle-coloured, so they blend into the leaf litter really well. The only way you detect them is by hearing the calls of the males. They’ve got a really distinct rattling sort of call that they emit in the wet evenings on those high mountain tops.

They are critically endangered, which means we’re concerned they could become extinct. What makes them so susceptible to extinction?
Firstly, they have a tiny distribution. When they were first found, they were only on a few ridge tops, about 1100m high in the cloud forests – very damp, moist, dripping forests. So that small distribution means anything that happens in that little area could impact the species. Secondly, climate change and global warming are probably the biggest threat. Mountain-top nursery frog breeding sites used to be common at 1100m, but they’re becoming less common there, and they’re more common at 1300m now. As it warms up that little bit, it seems that they might be moving up, but there’s not much we can do immediately on the ground for that. The other thing that’s happening is feral pigs are moving in bigger numbers. Back in the 1990s, a researcher suggested that the pigs might be damaging some of this frog’s habitat and eating some of the adult frogs when they turned logs. In the last five years or so, pig numbers have really exploded and there’s a lot more damage going on in those forests.

We know that pigs are rampant through the lowland rainforest. Why do we think they’re becoming increasingly problematic through the upland or high-altitude rainforest as well?
The evidence is there. Normally when you walk in these high altitude, really wet forests, you’re walking on a spongy mulch of leaf litter and you don’t actually see much soil. But now the pig numbers have got to the point where pigs are turning it all upside-down, and there’s a lot of muddy patches and wallows and broken-up logs where the pigs have pulled them apart looking for beetles and beetle larvae. With a few good wet years up here in the Wet Tropics the pig numbers are increasing everywhere. In the lowlands they’re increasing and they’re also increasing up the mountains. They’re learning to tolerate those colder high-altitude conditions.

In some areas feral pigs are much easier to target and tackle than others. This is pretty inaccessible terrain on Mt Lewis. How will it be done?
It’s dense rainforest and really tricky terrain. We know where the frogs are breeding and where their main breeding sites are because of some work by James Cook University students and lecturer Conrad Hoskins. So we can map the area. But we can’t fence off the whole area that these frogs might breed in. What we are doing is setting up 20 plots. Each one is ten metres by ten metres. For a tiny, less than two-centimetre frog you can actually fit quite a lot of individuals into that space. We are keeping the pigs out of those. And what we’ll have to do is very carefully bring in fencing materials and star pickets and build these fences under the rainforest canopy. The fence has got to keep pigs out but it also needs to allow the native animals to move in and out.

The critically endangered mountain-top nursery frog has jumped into the spotlight as a new project starts, aimed at improving its chances of survival.

The mountain-top nursery frog is only found near Mt Lewis, west of Port Douglas.

Terrain NRM secured funding this week through the Australian Government’s Saving Native Species (Priority Species) Grants for a project focused on protecting the rare frog’s breeding sites at the heights of Mt Lewis National Park.

Reducing threats to mountain-top nursery frogs

The project aims to reduce threats to the survival of the mountain top nursery frog and will be delivered by Terrain in partnership with Western Yalanji traditional owners, James Cook University researchers, Gulf Savannah NRM and the Queensland Parks and Wildlife Service.

Project leader Dr Andrew Dennis said feral pigs were a major threat and pig exclusion fencing would be installed around known frog breeding sites. Pig control measures would also begin soon, as well as species monitoring work.

“These frogs are only found in dense, small-leafed, or microphyll rainforest at altitudes above 1100 metres – they’re considered to be one of the most range-restricted frog species in Australia,” Dr Dennis said. “Because of this they’re extremely vulnerable to the effects of climate change from warming to changes in rainfall and cloud distribution.

Focus on feral pigs

“Feral pigs are another big threat, and pigs are increasing in numbers at high altitudes and disturbing and in some cases destroying the small palms and woody debris that are critical habitat for the mountain top nursery frog.

“Western Yalanji traditional owners will help monitor exclosure fencing and with surveying the feral pig damage on Mt Lewis, and we’ll work with Queensland Parks and Wildlife rangers on the most effective control measures.

“About 2000 square metres of frog breeding habitat will be fenced off.

“We will also be partnering with James Cook University on research to monitor protected and unprotected habitat, looking at the breeding success of mountain top nursery frogs, the amount of woody debris and the numbers of small palm trees (Linospadix apetiolatus) that they live in and near.